| 1. |
- Acosta Navarro, Juan Camilo, et al.
(författare)
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Amplification of Arctic warming by past air pollution reductions in Europe
- 2016
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Ingår i: Nature Geoscience. - : Nature Publishing Group. - 1752-0894 .- 1752-0908. ; 9:4, s. 277-
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Tidskriftsartikel (refereegranskat)abstract
- The Arctic region is warming considerably faster than the rest of the globe(1), with important consequences for the ecosystems(2) and human exploration of the region(3). However, the reasons behind this Arctic amplification are not entirely clear(4). As a result of measures to enhance air quality, anthropogenic emissions of particulate matter and its precursors have drastically decreased in parts of the Northern Hemisphere over the past three decades(5). Here we present simulations with an Earth system model with comprehensive aerosol physics and chemistry that show that the sulfate aerosol reductions in Europe since 1980 can potentially explain a significant fraction of Arctic warming over that period. Specifically, the Arctic region receives an additional 0.3Wm(-2) of energy, and warms by 0.5 degrees C on annual average in simulations with declining European sulfur emissions in line with historical observations, compared with a model simulation with fixed European emissions at 1980 levels. Arctic warming is amplified mainly in fall and winter, but the warming is initiated in summer by an increase in incoming solar radiation as well as an enhanced poleward oceanic and atmospheric heat transport. The simulated summertime energy surplus reduces sea-ice cover, which leads to a transfer of heat from the Arctic Ocean to the atmosphere. We conclude that air quality regulations in the Northern Hemisphere, the ocean and atmospheric circulation, and Arctic climate are inherently linked.
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| 2. |
- Acosta Navarro, Juan Camilo, 1983-
(författare)
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Anthropogenic influence on climate through changes in aerosol emissions from air pollution and land use change
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Particulate matter suspended in air (i.e. aerosol particles) exerts a substantial influence on the climate of our planet and is responsible for causing severe public health problems in many regions across the globe. Human activities have altered the natural and anthropogenic emissions of aerosol particles through direct emissions or indirectly by modifying natural sources. The climate effects of the latter have been largely overlooked. Humans have dramatically altered the land surface of the planet causing changes in natural aerosol emissions from vegetated areas. Regulation on anthropogenic and natural aerosol emissions have the potential to affect the climate on regional to global scales. Furthermore, the regional climate effects of aerosol particles could potentially be very different than the ones caused by other climate forcers (e.g. well mixed greenhouse gases). The main objective of this work was to investigate the climatic effects of land use and air pollution via aerosol changes.Using numerical model simulations it was found that land use changes in the past millennium have likely caused a positive radiative forcing via aerosol climate interactions. The forcing is an order of magnitude smaller and has an opposite sign than the radiative forcing caused by direct aerosol emissions changes from other human activities. The results also indicate that future reductions of fossil fuel aerosols via air quality regulations may lead to an additional warming of the planet by mid-21st century and could also cause an important Arctic amplification of the warming. In addition, the mean position of the intertropical convergence zone and the Asian monsoon appear to be sensitive to aerosol emission reductions from air quality regulations. For these reasons, climate mitigation policies should take into consideration aerosol air pollution, which has not received sufficient attention in the past.
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| 3. |
- Acosta Navarro, Juan C., et al.
(författare)
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Future response of temperature and precipitation to reduced aerosol emissions as compared with increased greenhouse gas concentrations
- 2017
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Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 30:3, s. 939-954
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Tidskriftsartikel (refereegranskat)abstract
- Experiments with a climate model (NorESM1) were performed to isolate the effects of aerosol particles and greenhouse gases on surface temperature and precipitation in simulations of future climate. The simulations show that by 2025-2049, a reduction of aerosol emissions from fossil fuels following a maximum technically feasible reduction (MFR) scenario could lead to a global and Arctic warming of 0.26 K and 0.84 K, respectively; as compared with a simulation with fixed aerosol emissions at the level of 2005. If fossil fuel emissions of aerosols follow a current legislation emissions (CLE) scenario, the NorESM1 model simulations yield a non-significant change in global and Arctic average surface temperature as compared with aerosol emissions fixed at year 2005. The corresponding greenhouse gas effect following the RCP4.5 emission scenario leads to a global and Arctic warming of 0.35 K and 0.94 K, respectively.The model yields a marked annual average northward shift in the inter-tropical convergence zone with decreasing aerosol emissions and subsequent warming of the northern hemisphere. The shift is most pronounced in the MFR scenario but also visible in the CLE scenario. The modeled temperature response to a change in greenhouse gas concentrations is relatively symmetric between the hemispheres and there is no marked shift in the annual average position of the inter-tropical convergence zone. The strong reduction in aerosol emissions in MFR also leads to a net southward cross-hemispheric energy transport anomaly both in the atmosphere and ocean, and enhanced monsoon circulation in Southeast and East Asia causing an increase in precipitation over a large part of this region.
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| 4. |
- Acosta Navarro, Juan Camilo, et al.
(författare)
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Global emissions of terpenoid VOCs from terrestrial vegetation in the last millennium
- 2014
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Ingår i: Journal of Geophysical Research - Atmospheres. - : Wiley-Blackwell. - 2169-897X .- 2169-8996. ; 119:11, s. 6867-6885
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the millennial variability (1000 A.D.-2000 A.D.) of global biogenic volatile organic compound (BVOC) emissions by using two independent numerical models: The Model of Emissions of Gases and Aerosols from Nature (MEGAN), for isoprene, monoterpene, and sesquiterpene, and Lund-Potsdam-Jena-General Ecosystem Simulator (LPJ-GUESS), for isoprene and monoterpenes. We found the millennial trends of global isoprene emissions to be mostly affected by land cover and atmospheric carbon dioxide changes, whereas monoterpene and sesquiterpene emission trends were dominated by temperature change. Isoprene emissions declined substantially in regions with large and rapid land cover change. In addition, isoprene emission sensitivity to drought proved to have significant short-term global effects. By the end of the past millennium MEGAN isoprene emissions were 634 TgC yr-1 (13% and 19% less than during 1750-1850 and 1000-1200, respectively), and LPJ-GUESS emissions were 323 TgC yr-1(15% and 20% less than during 1750-1850 and 1000-1200, respectively). Monoterpene emissions were 89 TgC yr-1(10% and 6% higher than during 1750-1850 and 1000-1200, respectively) in MEGAN, and 24 TgC yr-1 (2% higher and 5% less than during 1750-1850 and 1000-1200, respectively) in LPJ-GUESS. MEGAN sesquiterpene emissions were 36 TgC yr-1(10% and 4% higher than during 1750-1850 and 1000-1200, respectively). Although both models capture similar emission trends, the magnitude of the emissions are different. This highlights the importance of building better constraints on VOC emissions from terrestrial vegetation.
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| 5. |
- Acosta Navarro, Juan Camilo
(författare)
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Historical anthropogenic radiative forcing of changes in biogenic secondary organic aerosol
- 2015
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Human activities have lead to changes in the energy balance of the Earth and the global climate. Changes in atmospheric aerosols are the second largest contributor to climate change after greenhouse gases since 1750 A.D. Land-use practices and other environmental drivers have caused changes in the emission of biogenic volatile organic compounds (BVOCs) and secondary organic aerosol (SOA) well before 1750 A.D, possibly causing climate effects through aerosol-radiation and aerosol-cloud interactions. Two numerical emission models LPJ-GUESS and MEGAN were used to quantify the changes in aerosol forming BVOC emissions in the past millennium. A chemical transport model of the atmosphere (GEOS-Chem-TOMAS) was driven with those BVOC emissions to quantify the effects on radiation caused by millennial changes in SOA.The specific objectives of this licentiate thesis are: 1) to understand what drove the changes in aerosol-forming BVOC emissions (i.e. isoprene, monoterpenes and sesquiterpenes) and to quantify these changes; 2) to calculate for the first time the combined historical aerosol direct and aerosol-cloud albedo effects on radiation from changing BVOC emissions through SOA formation; 3) to investigate how important the biological climate feedback associated to BVOC emissions and SOA formation is from a global climate perspective.We find that global isoprene emissions decreased after 1800 A.D. by about 12% - 15%. This decrease was dominated by losses of natural vegetation, whereas monoterpene and sesquiterpene emissions increased by about 2% - 10%, driven mostly by rising surface air temperatures. From 1000 A.D. to 1800 A.D, isoprene, monoterpene and sesquiterpene emissions decline by 3% - 8% driven by both, natural vegetation losses, and the moderate global cooling between the medieval climate anomaly and the little ice age. The millennial reduction in BVOC emissions lead to a 0.5% to 2% reduction in climatically relevant aerosol particles (> 80 nm) and cause a direct radiative forcing between +0.02 W/m² and +0.07 W/m², and an indirect radiative forcing between -0.02 W/m² and +0.02 W/m². The suggested biological climate feedback seems to be too small to have observable consequences on the global climate in the recent past.
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| 6. |
- Andersson, Camilla, et al.
(författare)
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Achievements and experiences from science–policy interaction in the field of air pollution : Synthesising 20 years of research and outreach,thinking about future needs
- 2021
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- For 20 years, the Swedish Environmental Protection Agency together with the MISTRA research foundation have funded five air pollution research programmes with focus on producing knowledge that supports policy and emission control in national and international arenas. The research has been multidisciplinary and has included research on emissions, atmospheric transport and transformation processes, human health effects, ecosystem effects, and emission control strategies. Research has also been conducted on the interaction between air pollution and climate change.Over these years, the link between the research programmes and the development of emission control strategies and policies in Sweden, the EU, and the UNECE Air Convention has been of high importance. This report presents how the research programmes have created societal benefits through support for the development of air pollution policies and emission control measures. The report also identifies future research needs to ensure continued progress towards even better air quality for future generations.
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| 7. |
- Bardakov, Roman, et al.
(författare)
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A Novel Framework to Study Trace Gas Transport in Deep Convective Clouds
- 2020
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Ingår i: Journal of Advances in Modeling Earth Systems. - 1942-2466. ; 12:5
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Tidskriftsartikel (refereegranskat)abstract
- Deep convective clouds reach the upper troposphere (8-15 km height). In addition to moisture and aerosol particles, they can bring aerosol precursor gases and other reactive trace gases from the planetary boundary layer to the cloud top. In this paper, we present a method to estimate trace gas transport based on the analysis of individual air parcel trajectories. Large eddy simulation of an idealized deep convective cloud was used to provide realistic environmental input to a parcel model. For a buoyant parcel, we found that the trace gas transport approximately follows one out of three scenarios, determined by a combination of the equilibrium vapor pressure (containing information about water-solubility and pure component saturation vapor pressure) and the enthalpy of vaporization. In one extreme, the trace gas will eventually be completely removed by precipitation. In the other extreme, there is almost no vapor condensation on hydrometeors and most of the gas is transported to the top of the cloud. The scenario in between these two extremes is also characterized by strong gas condensation, but a small fraction of the trace gas may still be transported aloft. This approach confirms previously suggested patterns of inert trace gas behavior in deep convective clouds, agrees with observational data, and allows estimating transport in analytically simple and computationally efficient way compared to explicit cloud-resolving model calculations.
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| 8. |
- Bardakov, Roman, 1992-, et al.
(författare)
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The Role of Convective Up- and Downdrafts in the Transport of Trace Gases in the Amazon
- 2022
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Ingår i: Journal of Geophysical Research - Atmospheres. - : American Geophysical Union (AGU). - 2169-897X .- 2169-8996. ; 127:18
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Tidskriftsartikel (refereegranskat)abstract
- Deep convective clouds can redistribute gaseous species and particulate matter among different layers of the troposphere with important implications for atmospheric chemistry and climate. The large number of atmospheric trace gases of different volatility makes it challenging to predict their partitioning between hydrometeors and gas phase inside highly dynamic deep convective clouds. In this study, we use an ensemble of 51,200 trajectories simulated with a cloud-resolving model to characterize up- and downdrafts within Amazonian deep convective clouds. We also estimate the transport of a set of hypothetical non-reactive gases of different volatility, within the up- and downdrafts. We find that convective air parcels originating from the boundary layer (i.e., originating at 0.5 km altitude), can transport up to 25% of an intermediate volatility gas species (e.g., methyl hydrogen peroxide) and up to 60% of high volatility gas species (e.g., n-butane) to the cloud outflow above 10 km through the mean convective updraft. At the same time, the same type of gases can be transported to the boundary layer from the middle troposphere (i.e., originating at 5 km) within the mean convective downdraft with an efficiency close to 100%. Low volatility gases (e.g., nitric acid) are not efficiently transported, neither by the updrafts nor downdrafts, if the gas is assumed to be fully retained in a droplet upon freezing. The derived properties of the mean up- and downdraft can be used in future studies for investigating convective transport of a larger set of reactive trace gases.
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| 9. |
- Bardakov, Roman, 1992-
(författare)
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Transport and chemical processing of trace gases in deep convective clouds
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Deep convective clouds can efficiently transport trace gases from the planetary boundary layer to the upper troposphere. Once there, some gases will contribute to new particle formation and growth, eventually producing aerosols that are large enough to influence cloud properties, the radiative budget of the Earth, and climate. The magnitude and exact pathways of the convective transport of many organic and inorganic compounds are, however, still unclear. This dissertation presents a framework to study vertical transport of gas mixtures by deep convective clouds. The method consists of a chemical box model that is driven by cloud air parcel trajectory data generated by large-eddy simulation. This combination allows us to examine detailed gas-cloud interactions as well as complex systems of gas-phase chemical reactions. A large ensemble of simulated cloud trajectories was used to identify and characterize convective up- and downdrafts in the Amazon region. The analysis showed that air parcels starting close to the surface (at 0.5 km) experienced a substantially larger probability of reaching the upper troposphere (above 10 km) than parcels starting at the top of the boundary layer. Furthermore, the framework was used to estimate the vertical transport of isoprene, isoprene oxidation products, ammonia, and several non-reactive trace gases. We found that a typical Amazonian deep convective cloud can transport around 30% of the boundary layer isoprene to the cloud outflow if the efficiency of the gas uptake on ice is high and there is no lightning within the cloud. If the efficiency of gas uptake on ice is low and lightning within the cloud is extensive, all isoprene will be oxidized. Several low-volatility isoprene oxidation products will then have relatively high concentrations in the outflow, which potentially could lead to new particle formation and growth. Another result was that up to 10% of the boundary layer ammonia can reach the cloud outflow, where it in some environments can nucleate synergistically with nitric and sulfuric acid. A key uncertainty in our estimates is the efficiency of gas uptake by ice particles.
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| 10. |
- Bardakov, Roman, et al.
(författare)
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Transport and chemistry of isoprene and its oxidation products in deep convective clouds
- 2021
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Ingår i: Tellus. Series B, Chemical and physical meteorology. - : Stockholm University Press. - 0280-6509 .- 1600-0889. ; 73:1, s. 1-21
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Tidskriftsartikel (refereegranskat)abstract
- Deep convective clouds can transport trace gases from the planetary boundary layer into the upper troposphere where subsequent chemistry may impact aerosol particle formation and growth. In this modelling study, we investigate processes that affect isoprene and its oxidation products injected into the upper troposphere by an isolated deep convective cloud in the Amazon. We run a photochemical box model with coupled cloud microphysics along hundreds of individual air parcel trajectories sampled from a cloud-resolving model simulation of a convective event. The box model simulates gas-phase chemical reactions, gas scavenging by liquid and ice hydrometeors, and turbulent dilution inside a deep convective cloud. The results illustrate the potential importance of gas uptake to anvil ice in regulating the intensity of the isoprene oxidation and associated low volatility organic vapour concentrations in the outflow. Isoprene transport and fate also depends on the abundance of lightning-generated nitrogen oxide radicals (NOx = NO + NO2). If gas uptake on ice is efficient and lightning activity is low, around 30% of the boundary layer isoprene will survive to the cloud outflow after approximately one hour of transport, while all the low volatile oxidation products will be scavenged by the cloud hydrometeors. If lightning NOx is abundant and gas uptake by ice is inefficient, then all isoprene will be oxidised during transport or in the immediate outflow region, while several low volatility isoprene oxidation products will have elevated concentrations in the cloud outflow. Reducing uncertainties associated with the uptake of vapours on ice hydrometeors, especially HO2 and oxygenated organics, is essential to improve predictions of isoprene and its oxidation products in deep convective outflows and their potential contribution to new particle formation and growth.
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| 11. |
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| 12. |
- Baró Pérez, Alejandro, 1991-
(författare)
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Aerosol impacts on subtropical low-level clouds: a satellite and modelling perspective
- 2023
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Complex interactions between aerosols, clouds, and radiation impact Earth's climate. However, several aspects of these interactions remain uncertain, which has led to extensive research over the last decades. This thesis explores some unresolved aspects by focusing on subtropical low-level stratocumulus (Sc) clouds, which have a significant cooling effect on climate. The clouds are also sensitive to varying aerosol conditions, which can influence their formation, properties, and lifetime. Clouds over the South East Atlantic have been studied in detail, using both numerical modeling and satellite observations, to shed light on the interactions between aerosols, clouds, and radiation. This geographical region displays a large and semi-permanent Sc cloud deck and is also subjected to meteorological conditions that bring large amounts of light-absorbing aerosols from biomass fires over the African continent. The biomass-burning plumes also bring enhanced levels of moisture, and the individual influence of the aerosols and the moisture on the low-level cloud properties have been investigated.The analysis of satellite retrievals showed a radiative impact (sensitive to aerosol composition and aerosol optical depth) of moist aerosol layers in the free troposphere over the South East Atlantic; however, it was not possible to observe a clear influence of these humid aerosol layers on the underlying low-level clouds. Aerosol-radiation interactions were implemented in a large eddy simulation (LES) code that was used to model stratocumulus to cumulus transitions (SCT) in weather situations where moist absorbing aerosol layers were in contact with low-level clouds and mixed into the marine boundary layer (MBL). In these simulations, the heating by the absorbing aerosol within the MBL affected the persistence of the Sc clouds by accelerating the SCT, especially during daylight and broken cloud conditions. However, the humidity accompanying the absorbing aerosol was also found to be important -- it reduced the deepening of the MBL when located above the Sc deck and delayed the SCT when in contact with clouds. Furthermore, the additional moisture resulted in a radiative cooling effect that was comparable to the radiative cooling effect caused by the aerosol itself. The simulated SCTs were found to be mostly driven by increased sea surface temperatures, regardless of aerosol conditions. This result was different compared to two other LES models where the SCT was driven by drizzle under the same low aerosol conditions. On a larger scale, it was found that an explicit description of aerosol-cloud interactions in a climate model led to smaller differences between the simulated and mean observed values of the shortwave cloud radiative effect compared to when a non-interactive parameterization was used.
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| 13. |
- Baró Pérez, Alejandro, et al.
(författare)
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Comparing the simulated influence of biomass burning plumes on low-level clouds over the southeastern Atlantic under varying smoke conditions
- 2024
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Ingår i: Atmospheric Chemistry And Physics. - 1680-7316 .- 1680-7324. ; 24:8, s. 4591-4610
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Tidskriftsartikel (refereegranskat)abstract
- Biomass burning plumes are frequently transported over the southeast Atlantic (SEA) stratocumulus deck during the southern African fire season (June-October). The plumes bring large amounts of absorbing aerosols and enhanced moisture, which can trigger a rich set of aerosol-cloud-radiation interactions with climatic consequences that are still poorly understood. We use large-eddy simulation (LES) to explore and disentangle the individual impacts of aerosols and moisture on the underlying stratocumulus clouds, the marine boundary layer (MBL) evolution, and the stratocumulus-to-cumulus transition (SCT) for three different meteorological situations over the southeast Atlantic during August 2017. For all three cases, our LES shows that the SCT is driven by increased sea surface temperatures and cloud-top entrainment as the air is advected towards the Equator. In the LES model, aerosol indirect effects, including impacts on drizzle production, have a small influence on the modeled cloud evolution and SCT, even when aerosol concentrations are lowered to background concentrations. In contrast, local semi-direct effects, i.e., aerosol absorption of solar radiation in the MBL, cause a reduction in cloud cover that can lead to a speed-up of the SCT, in particular during the daytime and during broken cloud conditions, especially in highly polluted situations. The largest impact on the radiative budget comes from aerosol impacts on cloud albedo: the plume with absorbing aerosols produces a total average 3 d of simulations. We find that the moisture accompanying the aerosol plume produces an additional cooling effect that is about as large as the total aerosol radiative effect. Overall, there is still a large uncertainty associated with the radiative and cloud evolution effects of biomass burning aerosols. A comparison between different models in a common framework, combined with constraints from in situ observations, could help to reduce the uncertainty.
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| 14. |
- Baró Pérez, Alejandro, et al.
(författare)
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Impact of smoke and non-smoke aerosols on radiation and low-level clouds over the southeast Atlantic from co-located satellite observations
- 2021
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Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 21:8, s. 6053-6077
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Tidskriftsartikel (refereegranskat)abstract
- Data derived from instruments on board the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) and CloudSat satellites as well as meteorological parameters from reanalysis are used to explore situations when moist aerosol layers overlie stratocumulus clouds over the southeast Atlantic during the biomass burning season (June to October). To separate and quantify the impacts of aerosol loading, aerosol type, and humidity on the radiative fluxes (including cloud top cooling), the data are split into different levels of aerosol and moisture loadings. The aerosol classification available from the CALIPSO products is used to compare and contrast situations with pristine air, with smoke, and with other (non-smoke) types of aerosols. A substantial number of cases with non-smoke aerosols above clouds are found to occur under similar meteorological conditions to the smoke cases. In contrast, the meteorology is substantially different for the pristine situations, making a direct comparison with the aerosol cases ambiguous. The moisture content is enhanced within the aerosol layers, but the relative humidity does not always increase monotonously with increasing optical depth. Shortwave (SW) heating rates within the moist aerosol plumes increase with increasing aerosol loading and are higher in the smoke cases compared to the non-smoke cases. However, there is no clear correlation between moisture changes and SW absorption. Cloud top cooling rates do not show a clear correlation with moisture within the overlying aerosol layers due to the strong variability of the cooling rates caused by other meteorological factors (most notably cloud top temperature). No clear influence of aerosol type or loading on cloud top cooling rates is detected. Further, there is no correlation between aerosol loading and the thermodynamic structure of the atmosphere nor the cloud top height.
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| 15. |
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| 16. |
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| 17. |
- Bender, Frida A-M, 1978-
(författare)
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Earth's albedo in a changing climate
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The albedo is a key parameter in the radiative budget of the Earth and a primary determinant of the planetary temperature and is therefore also central to questions regarding climate stability, climate change and climate sensitivity. Climate models and satellite observations are essential for studying the albedo, and the parameters determining it, on large spatial and temporal scales. Although climate models are able to capture the large-scale characteristics of the albedo, a bias is found between modelled and observed global albedo estimates, and on a regional scale particular problematic regions can be identified. Cloud parameters, that are of great importance for determining the albedo, vary widely among models, but lack of observations makes constraining models, and even evaluating models, difficult. The freedom of variability for cloud parameters can be used to make models agree with observations of the better constrained radiative budget. It is shown that tuning a model to different radiative budget estimates by altering cloud parameters can influence the climate sensitivity of the model, but the effect seen is small, compared to the range of climate sensitivities estimated by different models. Despite their different parameterizations of clouds, aerosols etc., models do have fundamental features in common, which can further the understanding of the real climate system. For instance it is found that sensitivity to volcanic forcing is related to climate sensitivity in an ensemble of models. If this relation is valid for the real climate as well, observations of the volcanic sensitivity can help restrict the climate sensitivity. The range of climate sensitivity estimates in models can largely be attributed to variations in cloud response to forcing. It is found that in models with high climate sensitivity changes in cloud cover and cloud reflectivity enhance a positive radiative forcing due to increased CO2 concentrations, feeding back on the warming and in models with low climate sensitivity, cloud response counteracts the positive radiative forcing and warming induced by the same forcing. As a consequence the total albedo response to increased CO2 forcing is found to be stronger (more negative) in high sensitivity models and vice versa. Cloud albedo and its variation between different cloud regimes, is important in this regard, yet not well known. A method based on the relation between cloud fraction and albedo is presented, giving a way to estimate regional cloud albedo, primarily for homogeneous cloud regimes, but possibly also extended to a global scale.
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| 18. |
- Bender, Frida A. -M., et al.
(författare)
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Quantification of Monthly Mean Regional-Scale Albedo of Marine Stratiform Clouds in Satellite Observations and GCMs
- 2011
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Ingår i: Journal of Applied Meteorology and Climatology. - 1558-8424 .- 1558-8432. ; 50:10, s. 2139-2148
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Tidskriftsartikel (refereegranskat)abstract
- Planetary albedo the reflectivity for solar radiation is of singular importance in determining the amount of solar energy taken in by the Earth-atmosphere system. Modeling albedo, and specifically cloud albedo, correctly is crucial for realistic climate simulations. A method is presented herein by which regional cloud albedo can be quantified from the relation between total albedo and cloud fraction, which in observations is found to be approximately linear on a monthly mean scale. This analysis is based primarily on the combination of cloud fraction data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and albedo data from the Clouds and the Earth's Radiant Energy System (CERES), but the results presented are also supported by the combination of cloud fraction and proxy albedo data from satelliteborne lidar [Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CA LIPSO)]. These data are measured and derived completely independently from the CERES-MODIS data. Applied to low-level marine stratiform clouds in three regions (off the coasts of South America, Africa, and North America), the analysis reveals regionally uniform monthly mean cloud albedos, indicating that the variation in cloud shortwave radiative properties is small on this scale. A coherent picture of low effective cloud albedo emerges, in the range from 0.35 to 0.42, on the basis of data from CERES and MODIS. In its simplicity, the method presented appears to be useful as a diagnostic tool and as a constraint on climate models. To demonstrate this, the same method is applied to cloud fraction and albedo output from several current-generation climate models [from the Coupled Model Intercomparison Project, phase 3 (CMIP3), archive]. Although the multimodel mean cloud albedo estimates agree to within 20% with the satellite-based estimates for the three focus regions, model-based estimates of cloud albedo are found to display much larger variability than do the observations, within individual models as well as between models.
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| 19. |
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| 20. |
- Bender, Frida, et al.
(författare)
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Response to the eruption of Mount Pinatubo in relation to climate sensitivity in the CMIP3 models
- 2010
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Ingår i: Climate Dynamics. - : Springer. - 0930-7575 .- 1432-0894. ; 35:5, s. 875-886
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Tidskriftsartikel (refereegranskat)abstract
- The radiative flux perturbations and subsequent temperature responses in relation to the eruption of Mount Pinatubo in 1991 are studied in the ten general circulation models incorporated in the Coupled Model Intercomparison Project, phase 3 (CMIP3), that include a parameterization of volcanic aerosol. Models and observations show decreases in global mean temperature of up to 0.5 K, in response to radiative perturbations of up to 10 W m−2, averaged over the tropics. The time scale representing the delay between radiative perturbation and temperature response is determined by the slow ocean response, and is estimated to be centered around 4 months in the models. Although the magniude of the temperature response to a volcanic eruption has previously been used as an indicator of equilibrium climate sensitivity in models, we find these two quantities to be only weakly correlated. This may partly be due to the fact that the size of the volcano-induced radiative perturbation varies among the models. It is found that the magnitude of the modelled radiative perturbation increases with decreasing climate sensitivity, with the exception of one outlying model. Therefore, we scale the temperature perturbation by the radiative perturbation in each model, and use the ratio between the integrated temperature perturbation and the integrated radiative perturbation as a measure of sensitivity to volcanic forcing. This ratio is found to be well correlated with the model climate sensitivity, more sensitive models having a larger ratio. Further, if this correspondence between “volcanic sensitivity” and sensitivity to CO2 forcing is a feature not only among the models, but also of the real climate system, the alleged linear relation can be used to estimate the real climate sensitivity. The observational value of the ratio signifying volcanic sensitivity is hereby estimated to correspond to an equilibrium climate sensitivity, i.e. equilibrium temperature increase due to a doubling of the CO2 concentration, between 1.7 and 4.1 K. Several sources of uncertainty reside in the method applied, and it is pointed out that additional model output, related to ocean heat storage and radiative forcing, could refine the analysis, as could reduced uncertainty in the observational record, of temperature as well as forcing.
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| 21. |
- Berger, Marit, et al.
(författare)
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Pristine aerosol concentrations, cloud droplet size and early Holocene climate
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- This work investigates how the simulated early Holocene climate is influenced by the representation of aerosols and their effect on the climate. The representations of the direct and first indirect aerosol effects in the Community Earth System Model, version1 (CESM1) are modified in two sensitivity experiments.In the first sensitivity experiment (CESM 9k R14), the first indirect effect on the simulated climate is modified by setting the cloud droplet effective radius, (Reff ) in the model to a constant value. This value is chosen to be representative for pristine conditions. In the second sensitivity experiment (CESM 9k CAMO), the representation of both the direct and first indirect effects is modified. An atmosphere-only model with interactive aerosols is used to simulate the early Holocene aerosol loading and the change in Reff due to the decrease in atmospheric aerosols.The changes in aerosol effects introduced in the two sensitivity experiments differ both in magnitude and spatial pattern. We find that despite the difference in the spatial pattern of the changes in the aerosol effects, the warming patterns in the two sensitivity experiments are similar; the surface temperature increases in both simulations, with an enhanced warming in the Arctic region. The warming is approximately twice as large in the CESM 9k R14 simulation than in the CESM 9k CAMO simulation.
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| 22. |
- Bourgeois, Quentin, et al.
(författare)
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Aerosol transport over the Andes from the Amazon Basin to the remote Pacific Ocean : A multiyear CALIOP assessment
- 2015
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 120:16, s. 8411-8425
-
Tidskriftsartikel (refereegranskat)abstract
- Six years (200702012) of data from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite instrument were used to investigate the vertical distribution and transport of aerosols over the tropical South American continent and the southeast Pacific Ocean. The multiyear aerosol extinction assessment indicates that aerosols, mainly biomass burning particles emitted during the dry season in the Amazon Basin, are lifted in significant amounts over the Andes. The aerosols are mainly transported in the planetary boundary layer between the surface and 2 km altitude with an aerosol extinction maximum near the surface. During the transport toward the Andes, the aerosol extinction decreases at a rate of 0.02 km(-1) per kilometer of altitude likely due to dilution and deposition processes. Aerosols reaching the top of the Andes, at altitudes typically between 4 and 5 km, are entrained into the free troposphere (FT) over the southeast Pacific Ocean. A comparison between CALIOP observations and ERA-Interim reanalysis data indicates that during their long-range transport over the tropical Pacific Ocean, these aerosols are slowly transported toward the marine boundary layer by the large-scale subsidence at a rate of 0.4 cm s(-1). The observed vertical/horizontal transport ratio is 0.700.8 m km(-1) Continental aerosols linked to transport over the Andes can be traced on average over 4000 km away from the continent indicating an aerosol residence time of 809 days in the FT over the Pacific Ocean. The FT aerosol optical depth (AOD) above the Pacific Ocean near South American coast accounts on average for 6% and 25% of the total AOD during the season of low and high biomass burning, respectively. This result shows that, during the biomass burning season, continental aerosols largely influence the AOD over the remote southeast Pacific Ocean. Overall, FT AOD decrease exponentially with the distance to continental sources at a rate of about 10% per degree of longitude over the Pacific Ocean.
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| 23. |
- Bourgeois, Quentin, et al.
(författare)
-
How much of the global aerosol optical depth is found in the boundary layer and free troposphere?
- 2018
-
Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 18:10, s. 7709-7720
-
Tidskriftsartikel (refereegranskat)abstract
- The global aerosol extinction from the CALIOP space lidar was used to compute aerosol optical depth (AOD) over a 9-year period (2007-2015) and partitioned between the boundary layer (BL) and the free troposphere (FT) using BL heights obtained from the ERA-Interim archive. The results show that the vertical distribution of AOD does not follow the diurnal cycle of the BL but remains similar between day and night highlighting the presence of a residual layer during night. The BL and FT contribute 69 and 31 %, respectively, to the global tropospheric AOD during daytime in line with observations obtained in Aire sur l'Adour (France) using the Light Optical Aerosol Counter (LOAC) instrument. The FT AOD contribution is larger in the tropics than at mid-latitudes which indicates that convective transport largely controls the vertical profile of aerosols. Over oceans, the FT AOD contribution is mainly governed by long-range transport of aerosols from emission sources located within neighboring continents. According to the CALIOP aerosol classification, dust and smoke particles are the main aerosol types transported into the FT. Overall, the study shows that the fraction of AOD in the FT - and thus potentially located above low-level clouds - is substantial and deserves more attention when evaluating the radiative effect of aerosols in climate models. More generally, the results have implications for processes determining the overall budgets, sources, sinks and transport of aerosol particles and their description in atmospheric models.
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| 24. |
- Bourgeois, Quentin, et al.
(författare)
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Ubiquity and impact of thin mid-level clouds in the tropics
- 2016
-
Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 7
-
Tidskriftsartikel (refereegranskat)abstract
- Clouds are crucial for Earth's climate and radiation budget. Great attention has been paid to low, high and vertically thick tropospheric clouds such as stratus, cirrus and deep convective clouds. However, much less is known about tropospheric mid-level clouds as these clouds are challenging to observe in situ and difficult to detect by remote sensing techniques. Here we use Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite observations to show that thin mid-level clouds (TMLCs) are ubiquitous in the tropics. Supported by high-resolution regional model simulations, we find that TMLCs are formed by detrainment from convective clouds near the zero-degree isotherm. Calculations using a radiative transfer model indicate that tropical TMLCs have a cooling effect on climate that could be as large in magnitude as the warming effect of cirrus. We conclude that more effort has to be made to understand TMLCs, as their influence on cloud feedbacks, heat and moisture transport, and climate sensitivity could be substantial.
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| 25. |
- Boy, M., et al.
(författare)
-
Interactions between the atmosphere, cryosphere, and ecosystems at northern high latitudes
- 2019
-
Ingår i: Atmospheric Chemistry and Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 19:3, s. 2015-2061
-
Tidskriftsartikel (refereegranskat)abstract
- The Nordic Centre of Excellence CRAICC (Cryosphere-Atmosphere Interactions in a Changing Arctic Climate), funded by NordForsk in the years 2011-2016, is the largest joint Nordic research and innovation initiative to date, aiming to strengthen research and innovation regarding climate change issues in the Nordic region. CRAICC gathered more than 100 scientists from all Nordic countries in a virtual centre with the objectives of identifying and quantifying the major processes controlling Arctic warming and related feedback mechanisms, outlining strategies to mitigate Arctic warming, and developing Nordic Earth system modelling with a focus on short-lived climate forcers (SLCFs), including natural and anthropogenic aerosols. The outcome of CRAICC is reflected in more than 150 peer-reviewed scientific publications, most of which are in the CRAICC special issue of the journal Atmospheric Chemistry and Physics. This paper presents an overview of the main scientific topics investigated in the centre and provides the reader with a state-of-the-art comprehensive summary of what has been achieved in CRAICC with links to the particular publications for further detail. Faced with a vast amount of scientific discovery, we do not claim to completely summarize the results from CRAICC within this paper, but rather concentrate here on the main results which are related to feedback loops in climate change-cryosphere interactions that affect Arctic amplification.
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| 26. |
- Braun, Christoph, et al.
(författare)
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Controls on Subtropical Cloud Reflectivity during a Waterbelt Scenario for the Cryogenian Glaciations
- 2022
-
Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 35:21, s. 3457-3476
-
Tidskriftsartikel (refereegranskat)abstract
- Waterbelt climate states with an ice-free tropical ocean provide a straightforward explanation for the survival of advanced marine species during the Cryogenian glaciations (720–635 million years ago). Previous work revealed that stable waterbelt states require the presence of highly reflective low-level mixed-phase clouds with a high abundance of supercooled liquid in the subtropics. However, the high uncertainty associated with representing mixed-phase clouds in coarse-scale general circulation models (GCMs) that parameterize atmospheric convection has prohibited assessment of whether waterbelt states are a robust feature of Earth’s climate. Here we investigate whether resolving convective-scale motion at length scales of hectometers helps us to assess the plausibility of a waterbelt scenario. First, we show that substantial differences in cloud reflectivity among GCMs do not arise from the resolved atmospheric circulation. Second, we conduct a hierarchy of simulations using the Icosahedral Nonhydrostatic (ICON) modeling framework, ranging from coarse-scale GCM simulations with parameterized convection to large-eddy simulations that explicitly resolve atmospheric convective-scale motions. Our hierarchy of simulations supports the existence of highly reflective subtropical clouds if we apply moderate ice nucleating particle (INP) concentrations. Third, we test the sensitivity of cloud reflectivity to the INP concentration. In the presence of high but justifiable INP concentrations, cloud reflectivity is strongly reduced. Hence, the existence of stable waterbelt states is controlled by the abundance of INPs. We conclude that explicitly resolving convection can help to constrain Cryogenian cloud reflectivity, but limited knowledge concerning Cryogenian aerosol conditions hampers strong constraints. Thus, waterbelt states remain an uncertain feature of Earth’s climate.
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| 27. |
- Bulatovic, Ines, et al.
(författare)
-
Aerosol Indirect Effects in Marine Stratocumulus : The Importance of Explicitly Predicting Cloud Droplet Activation
- 2019
-
Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 46:6, s. 3473-3481
-
Tidskriftsartikel (refereegranskat)abstract
- Climate models generally simulate a unidirectional, positive liquid water path (LWP) response to increasing aerosol number concentration. However, satellite observations and large-eddy simulations show that the LWP may either increase or decrease with increasing aerosol concentration, influencing the overall magnitude of the aerosol indirect effect (AIE). We use large-eddy simulation to investigate the LWP response of a marine stratocumulus cloud and its dependence on different parameterizations for obtaining cloud droplet number concentration (CDNC). The simulations confirm that the LWP response is not always positiveregardless of CDNC treatment. However, the AIE simulated with the model version with prescribed CDNC is almost 3 times larger compared to the version with prognostic CDNC. The reason is that the CDNC in the prognostic scheme varies in time due to supersaturation fluctuations, collection, and other microphysical processes. A substantial spread in simulated AIE may thus arise simply due to the CDNC treatment. Plain Language Summary Our poor understanding of aerosol-cloud-radiation interactions (aerosol indirect effects) results in a major uncertainty in estimates of anthropogenic aerosol forcing. In climate models, the cloud water response to an increased aerosol number concentration may be especially uncertain as models simplify, or do not account for, processes that affect the cloud droplet number concentration and the total amount of cloud water. In this study, we employ large-eddy simulation to explore how different model descriptions for obtaining the number concentration of cloud droplets influences the cloud water response of a marine stratocumulus cloud and thus the simulated aerosol indirect effect. Our simulations show a qualitatively similar cloud water response regardless of model description: the total amount of cloud water increases first and then decreases with increasing aerosol concentration. However, the simulated aerosol indirect effect is almost 3 times as large when the number concentration of cloud droplets is prescribed compared to when it is dependent on the calculated supersaturation and other microphysical processes such as collisions between cloud droplets. Our findings show that a relatively simple difference in the treatment of the number concentration of cloud droplets in climate models may result in a significant spread in the simulated aerosol indirect effect.
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| 28. |
- Bulatovic, Ines
(författare)
-
Investigating aerosol effects on stratocumulus clouds through large-eddy simulation
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Clouds have a large impact on Earth’s radiative budget by reflecting, absorbing and re-emitting radiation. They thus play a critical role in the climate system. Nevertheless, cloud radiative effects in a changing climate are highly uncertain. Atmospheric aerosol particles are another factor affecting Earth’s climate but the magnitude of their influence is also associated with high uncertainty. Therefore, an accurate representation of aerosol-cloud interactions in models is critical for having confidence in future climate projections. This thesis investigates aerosol impacts on cloud microphysical and radiative properties through numerical modelling, more specifically large-eddy simulation (LES). Moreover, the thesis investigates how the simulated cloud response to changes in the aerosol population depends on the model description of different processes. Mixed-phase stratocumulus (MPS) clouds are especially problematic to simulate for models on all scales. These clouds consist of a mixture of supercooled water and ice in the same volume and are therefore potentially thermodynamically unstable. MPS clouds over the central (north of 80° N) Arctic Ocean are particularly sensitive to aerosol changes due to the relatively clean atmospheric conditions in this region. At the same time, the clouds also have an important impact on the Arctic surface radiative budget. Therefore, this thesis mostly focuses on Arctic MPS clouds.Simulations of a typical subtropical marine stratocumulus cloud showed that the aerosol-cloud forcing depends on the model treatment for calculating the cloud droplet number concentration (CDNC). The simulated change in the top of the atmosphere shortwave radiation due to increased aerosol number concentrations was almost three times as large when the CDNC was prescribed compared to when the CDNC was prognostic. Simulations of a central Arctic summertime low-level MPS cloud confirmed that the chemical composition and the size of aerosol particles both can play an important role in determining the efficiency of an aerosol to act as cloud condensation nuclei - and thus influence cloud properties. However, the hygroscopicity of the aerosol particle was only important if the particles were small in size (i.e., if they correspond to the Aitken mode size) or if they were close to hydrophobic. Further, it was also found that Aitken mode particles can significantly change microphysical and radiative properties of central Arctic MPS if the concentration of larger particles (i.e., corresponding to the accumulation mode) is less than approximately 10-20 cm-3. One of the most recent research expeditions in the central Arctic (in the summer of 2018) was characterized by a high occurrence of multiple cloud layers. Namely, the boundary layer structure consisted of two MPS, one located close to the surface and one at the top of the boundary layer. Large-eddy simulations of an observed case with this particular cloud structure showed that the two-layer boundary-layer clouds are persistent unless the aerosol number concentrations are low (< 5 cm-3) or the wind speed is high (≥ 8.5 m s-1). In the model, low aerosol numbers led to a dissipation of the upper cloud layer while the lower cloud layer dissipated if the wind speed was strong. Changes in the optical thickness and cloud emissivity of each individual cloud layer of the two-layer cloud structure were found to substantially impact the surface radiative fluxes.
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| 29. |
- Bulatovic, Ines, et al.
(författare)
-
Large-eddy simulation of a two-layer boundary-layer cloud system from the Arctic Ocean 2018 expedition
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Climate change is particularly noticeable in the Arctic. The most common type of cloud at these latitudes is mixed-phase stratocumulus. These clouds occur frequently and persistently during all seasons and play a critical role in the Arctic energy budget. Previous observations in the central (north of 80° N) Arctic have shown a high occurrence of prolonged periods of a shallow, single-layer mixed-phase stratocumulus at the top of the boundary layer (altitudes ~300-400m). However, recent observations from the summer of 2018 (during The Microbiology-Ocean-Cloud-Coupling in the High Arctic (MOCCHA) Arctic Ocean 2018 (AO2018) expedition) instead showed a prevalence of a two-layer boundary-layer cloud system. Here we use large-eddy simulation to examine the maintenance of one of the cloud systems observed during MOCCHA AO2018 as well as the sensitivity of the cloud layers to different micro- and macro-scale parameters. We find that the model generally reproduces the observed thermodynamic structure well, with two near-neutrally stratified layers in the boundary layer caused by a low cloud (located within the first few hundred meters) capped by a lower temperature inversion, and an upper cloud layer (based around one kilometer or slightly higher) capped by the main temperature inversion of the boundary layer. The investigated cloud structure is persistent unless there are low aerosol number concentrations (< 5 cm-3), which cause the upper cloud layer to dissipate, or high large-scale wind speeds (³ 8.5 m s-1), which erode the lower inversion and the related cloud layer. These types of changes in cloud structure lead to a substantial reduction of the incoming net longwave radiation at the surface due to a lower emissivity or higher altitude of the remaining cloud layer. The findings highlight the importance of better understanding and representing aerosol sources and sinks over the central Arctic Ocean. Furthermore, they underline the significance of meteorological parameters, such as the large-scale wind speed, for maintaining the two-layer boundary-layer cloud structure encountered in the lower atmosphere of the central Arctic.
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| 30. |
- Bulatovic, Ines, 1991-, et al.
(författare)
-
Large-eddy simulation of a two-layer boundary-layer cloud system from the Arctic Ocean 2018 expedition
- 2023
-
Ingår i: Atmospheric Chemistry And Physics. - 1680-7316 .- 1680-7324. ; 23:12, s. 7033-7055
-
Tidskriftsartikel (refereegranskat)abstract
- Climate change is particularly noticeable in the Arctic. The most common type of cloud at these latitudes is mixed-phase stratocumulus. These clouds occur frequently and persistently during all seasons and play a critical role in the Arctic energy budget. Previous observations in the central (north of 80∘ N) Arctic have shown a high occurrence of prolonged periods of a shallow, single-layer mixed-phase stratocumulus at the top of the boundary layer (BL; altitudes ∼ 300 to 400 m). However, recent observations from the summer of 2018 instead showed a prevalence of a two-layer boundary-layer cloud system. Here we use large-eddy simulation to examine the maintenance of one of the cloud systems observed in the summer of 2018 and the sensitivity of the cloud layers to different micro- and macro-scale parameters. We find that the model generally reproduces the observed thermodynamic structure well, with two near-neutrally stratified layers in the BL caused by a low cloud (located within the first few hundred meters) capped by a lower-altitude temperature inversion and an upper cloud layer (based around one kilometer or slightly higher) capped by the main temperature inversion of the BL. The simulated cloud structure is persistent unless there are low aerosol number concentrations (≤ 5 cm−3), which cause the upper cloud layer to dissipate, or high large-scale wind speeds (≥ 8.5 m s−1), which erode the lower inversion and the related cloud layer. The changes in cloud structure alter both the short- and longwave cloud radiative effect at the surface. This results in changes in the net radiative effect of the modeled cloud system, which can impact the surface melting or freezing. The findings highlight the importance of better understanding and representing aerosol sources and sinks over the central Arctic Ocean. Furthermore, they underline the significance of meteorological parameters, such as the large-scale wind speed, for maintaining the two-layer boundary-layer cloud structure encountered in the lower atmosphere of the central Arctic.
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| 31. |
- Bulatovic, Ines, et al.
(författare)
-
The importance of Aitken mode aerosol particles for cloud sustenance in the summertime high Arctic - a simulation study supported by observational data
- 2021
-
Ingår i: Atmospheric Chemistry And Physics. - : Copernicus GmbH. - 1680-7316 .- 1680-7324. ; 21:5, s. 3871-3897
-
Tidskriftsartikel (refereegranskat)abstract
- The potential importance of Aitken mode particles (diameters similar to 25-80 nm) for stratiform mixed-phase clouds in the summertime high Arctic (> 80 degrees N) has been investigated using two large-eddy simulation models. We find that, in both models, Aitken mode particles significantly affect the simulated microphysical and radiative properties of the cloud and can help sustain the cloud when accumulation mode concentrations are low (< 10-20 cm(-3)), even when the particles have low hygroscopicity (hygroscopicity parameter - kappa = 0.1). However, the influence of the Aitken mode decreases if the overall liquid water content of the cloud is low, either due to a higher ice fraction or due to low radiative cooling rates. An analysis of the simulated supersaturation (ss) statistics shows that the ss frequently reaches 0.5 % and sometimes even exceeds 1 %, which confirms that Aitken mode particles can be activated. The modelling results are in qualitative agreement with observations of the Hoppel minimum obtained from four different expeditions in the high Arctic. Our findings highlight the importance of better understanding Aitken mode particle formation, chemical properties and emissions, particularly in clean environments such as the high Arctic.
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| 32. |
- Böö, Sebastian, 1979-
(författare)
-
Transport of mineral dust into the Arctic : Evaluation of two reanalysis datasets of atmospheric composition
- 2023
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main purpose of this thesis is to examine the mineral dust aerosol transport into the Arctic. Two three-dimensional reanalysis datasets of atmospheric composition, the Copernicus Atmosphere Monitoring Service reanalysis (CAMSRA) and the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), are analyzed with regard to dust transport into the Arctic. The reanalyses agree on that the largest mass transport of dust into the Arctic occurs across western Russia during spring and early summer, although large dust transport events can occur across other geographical areas during all seasons. In several aspects, the reanalyses show substantial differences. The transport in CAMSRA is considerably smaller, more concentrated and occurs at lower altitudes. Furthermore, the transport in CAMSRA is to a larger extent than MERRA-2 driven by well-defined events of dust transport in space and time.The reanalysis data are compared with surface measurements of dust in the Arctic and dust extinction satellite retrievals from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP). The comparison indicates that CAMSRA underestimates the dust transport into the Arctic and that MERRA- 2 likely overestimates it. The discrepancy between CAMSRA and MERRA-2 can in part be explained by the assimilation process where too little dust is assimilated in CAMSRA while MERRA-2 overestimates the production of light particles, causing an excessive transport, and the assimilation process further increases the dust concentration in remote areas. Despite the clear differences between the reanalyses, this study provides new insights into the spatio-temporal distribution of the dust transport into the Arctic and the transported mass is estimated to be within the range 1.5–31 Tg yr-1.The thesis also briefly examines the aerosol transport of all five aerosol species carried by the reanalyses, that in addition to dust are black carbon, organic matter, sea-salt and sulfate. The annual aerosol mass transport to the Arctic in CAMSRA and MERRA-2 are 24 Tg and 50 Tg respectively. The reanalyses show substantial differences regarding the proportions of the different aerosol types — emphasizing that it is crucial that the aerosol module manages to simulate the correct aerosol mass fractions, as the assimilation of AOD alone cannot change the proportions between the aerosols.
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| 33. |
- Böö, Sebastian, 1979-, et al.
(författare)
-
Transport of Mineral Dust Into the Arctic in Two Reanalysis Datasets of Atmospheric Composition
- 2023
-
Ingår i: Tellus. Series B, Chemical and physical meteorology. - 0280-6509 .- 1600-0889. ; 75:1, s. 13-32
-
Tidskriftsartikel (refereegranskat)abstract
- Two three-dimensional reanalysis datasets of atmospheric composition, the Copernicus Atmosphere Monitoring Service reanalysis (CAMSRA) and the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), are analyzed for the years 2003-2018 with respect to dust transport into the Arctic. The reanalyses agree on that the largest mass transport of dust into the Arctic occurs across western Russia during spring and early summer, but substantial transport events occasionally also occur across other geographical areas during all seasons. In many aspects, however, the reanalyses show considerable differences: the mass transport in MERRA-2 is substantially larger, more spread out, and occurs at higher altitudes than in CAMSRA, while the transport in CAMSRA is to a higher degree focused to well-defined events in space and time; the integrated mass transport of the 10 most intense 36-hour dust events in CAMSRA constitutes 6 % of the total integrated dust transport 2003-2018, whereas the corresponding value for MERRA-2 is only 1 %.Furthermore, we compare the reanalyses with surface measurements of dust in the Arctic and dust extinction retrievals from the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) satellite data. This comparison indicates that CAMSRA underestimates the dust transport into the Arctic and that MERRA-2 likely overestimates it. The discrepancy between CAMSRA and MERRA-2 can partially be explained by the assimilation process where too little dust is assimilated in CAMSRA while in MERRA-2, the assimilation process increases the dust concentration in remote areas. Despite the profound differences between the reanalyses regarding dust transport into the Arctic, this study still brings new insights into the spatio-temporal distribution of the transport. We estimate the annual dust transport into the Arctic to be within the range 1.5-31 Tg, where the comparison with observations indicates that the lower end of the interval is less likely.
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| 34. |
- Chiacchio, Marc, et al.
(författare)
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On the links between meteorological variables, aerosols, and tropical cyclone frequency in individual ocean basins
- 2017
-
Ingår i: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 122:2, s. 802-822
-
Tidskriftsartikel (refereegranskat)abstract
- A generalized linear model based on Poisson regression has been used to assess the impact of environmental variables modulating tropical cyclone frequency in six main cyclone development areas: the East Pacific, West Pacific, North Atlantic, North Indian, South Indian, and South Pacific. The analysis covers the period 1980-2009 and focuses on widely used meteorological parameters including wind shear, sea surface temperature, and relative humidity from different reanalyses as well as aerosol optical depth for different compounds simulated by the Goddard Chemistry Aerosol Radiation and Transport model. Circulation indices are also included. Cyclone frequency is obtained from the International Best Track Archive for Climate Stewardship. A strong link is found between cyclone frequency and the relative sea surface temperature, Atlantic Meridional Mode, and wind shear with significant explained log likelihoods in the North Atlantic of 37%, 27%, and 28%, respectively. A significant impact of black carbon and organic aerosols on cyclone frequency is found over the North Indian Ocean, with explained log likelihoods of 27%. A weaker but still significant impact is found for observed dust aerosols in the North Atlantic with an explained log likelihood of 11%. Changes in lower stratospheric temperatures explain 28% of the log likelihood in the North Atlantic. Lower stratospheric temperatures from a subset of Coupled Model Intercomparison Project Phase 5 models properly simulate the warming and subsequent cooling of the lower stratosphere that follows a volcanic eruption but underestimates the cooling by about 0.5 degrees C.
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| 35. |
- Christiansen, Sigurd, et al.
(författare)
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Influence of Arctic Microlayers and Algal Cultures on Sea Spray Hygroscopicity and the Possible Implications for Mixed-Phase Clouds
- 2020
-
Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-8996 .- 2169-897X. ; 125:19
-
Tidskriftsartikel (refereegranskat)abstract
- As Arctic sea ice cover diminishes, sea spray aerosols (SSA) have a larger potential to be emitted into the Arctic atmosphere. Emitted SSA can contain organic material, but how it affects the ability of particles to act as cloud condensation nuclei (CCN) is still not well understood. Here we measure the CCN-derived hygroscopicity of three different types of aerosol particles: (1) Sea salt aerosols made from artificial seawater, (2) aerosol generated from artificial seawater spiked with diatom species cultured in the laboratory, and (3) aerosols made from samples of sea surface microlayer (SML) collected during field campaigns in the North Atlantic and Arctic Ocean. Samples are aerosolized using a sea spray simulation tank (plunging jet) or an atomizer. We show that SSA containing diatom and microlayer exhibit similar CCN activity to inorganic sea salt with a κ value of ∼1.0. Large-eddy simulation (LES) is then used to evaluate the general role of aerosol hygroscopicity in governing mixed-phase low-level cloud properties in the high Arctic. For accumulation mode aerosol, the simulated mixed-phase cloud properties do not depend strongly on κ, unless the values are lower than 0.4. For Aitken mode aerosol, the hygroscopicity is more important; the particles can sustain the cloud if the hygroscopicity is equal to or higher than 0.4, but not otherwise. The experimental and model results combined suggest that the internal mixing of biogenic organic components in SSA does not have a substantial impact on the cloud droplet activation process and the cloud lifetime in Arctic mixed-phase clouds.
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| 36. |
- Decremer, Damien, et al.
(författare)
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Which significance test performs the best in climate simulations?
- 2014
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 66:1, s. 23139-
-
Tidskriftsartikel (refereegranskat)abstract
- Climate change simulated with climate models needs a significance testing to establish the robustness of simulated climate change relative to model internal variability. Student's t-test has been the most popular significance testing technique despite more sophisticated techniques developed to address autocorrelation. We apply Student's t-test and four advanced techniques in establishing the significance of the average over 20 continuous-year simulations, and validate the performance of each technique using much longer (375-1000 yr) model simulations. We find that all the techniques tend to perform better in precipitation than in surface air temperature. A sizable performance gain using some of the advanced techniques is realised in the model Ts output portion with strong positive lag-1 yr autocorrelation (> +/- 0.6), but this gain disappears in precipitation. Furthermore, strong positive lag-1 yr autocorrelation is found to be very uncommon in climate model outputs. Thus, there is no reason to replace Student's t-test by the advanced techniques in most cases.
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| 37. |
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| 38. |
- Dimitrelos, Antonios, 1986-
(författare)
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A large-eddy simulation perspective on Arctic airmass transformation and low-level cloud evolution
- 2022
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Arctic is currently warming faster than other regions of the Earth. Many processes and feedbacks contribute to the enhanced warming. Among these are the radiative effects of clouds. Arctic mixed-phase clouds, which contain both liquid and ice condensate, have high longevity and can exert significant surface warming since the amount of solar radiation in the region is relatively low and the surface reflectivity often is high. In this thesis, we study these clouds utilizing a large-eddy model coupled with one-dimensional thermodynamic sea ice model. The main aim is to understand the interactions between cloud dynamics, microphysics, radiation, and turbulent processes and how these together govern the life cycle and surface warming of the clouds. By comparing a group of models with observations from the summertime high Arctic, we confirm the hypothesis that when aerosol concentrations are low, a small increase in their number concentration can increase the liquid water content of the cloud and in turn, the surface warming. Idealized simulations of moist intrusions into the Arctic show that the surface temperature may increase by more than 15o C if we allow clouds to form during a moist intrusion compared to if the atmosphere is cloud free. The simulations also show that the large-scale divergence rate strongly impacts the maintenance of the liquid layer at the top of these clouds. A main finding of the thesis is that the temperature of the cloud that forms during a moist intrusion is close to the initial dew point temperature. Thus, the surface warming induced by the clouds depends mostly on the initial humidity of the air mass rather than the initial temperature. In addition, the stability of the initial dew point temperature profile largely controls the turbulent state of the cloud. If the profile is unstable, then the cloud can transform from a thin, stable stratus to a deeper stratocumulus cloud, which also enhances the surface warming. Consequently, both the initial amount and the vertical structure of the initial moisture of the intrusion are important for the warming of the sea ice. A change in the number of cloud condensation nuclei does not affect the cloud evolution considerably provided that there is a continuous supply of these nuclei. However, if cloud condensation nuclei sources are absent then the cloud may remain in its stable state. Furthermore, a decrease in the cloud ice condensate, which may be caused by a lack of ice nucleation particles, may delay the transformation of the cloud into a stratocumulus. These results suggest that any future change in aerosol loading and atmospheric moisture transport into the Arctic may alter the surface longwave cloud radiative effect and cause changes in the sea ice evolution.
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| 39. |
- Dimitrelos, Antonios, et al.
(författare)
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A Sensitivity Study of Arctic Air-Mass Transformation Using Large Eddy Simulation
- 2020
-
Ingår i: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 125:6
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Tidskriftsartikel (refereegranskat)abstract
- Arctic air mass transformation is linked to the evolution of low-level mixed-phase clouds. These clouds can alter the structure of the boundary layer and modify the surface energy budget. In this study, we use three-dimensional large eddy simulation and a bulk sea ice model to examine the lifecycle of clouds formed during wintertime advection of moist and warm air over sea ice, following a Lagrangian perspective. We investigate the stages of cloud formation, evolution, and decay. The results show that radiative cooling at the surface gives rise to fog formation which subsequently rises and transforms into a mixed-phase cloud. In our baseline simulation, the cloud persists for about 5 days and increases the surface temperature by on average 17 degrees C. Sensitivity tests show that the lifetime of the cloud is sensitive to changes in the vapor supply at cloud top. This flux is mainly impacted by changes in the divergence rate; an imposed convergence decreases the lifetime to 2 days while an imposed large-scale divergence increases the lifetime to more than 6 days. The largest difference in cloud radiative properties is found in the experiment with increased ice crystal number concentrations. In this case, the lifetime of the cloud is similar compared to baseline but the amount of liquid water is clearly depleted throughout the whole cloud sequence and the surface temperature is on average 6 degrees C cooler. The cloud condensation nuclei concentration has a weaker effect on the radiative properties and lifetime of the cloud. Plain Language Summary Arctic air mass transformation is a process in which an air mass originating over the open ocean enters the high Arctic and cools. Low-altitude clouds form and are often very persistent. They can exist for several days and warm the surface by emitting infrared radiation towards the surface. In this study, we have investigated the effect of the cloud on the surface energy budget by conducting large eddy simulations. In the model code we have incorporated a module that considers the thermodynamics of the sea ice surface. Knowing the sensitivity of these clouds to different parameters and physical processes will make us capable of predicting the cloud lifetime and radiative properties, and thus the induced warming effect on the sea ice surface. We have found that an increased ice crystal number concentration leads to a tenuous form of the cloud that only weakly warms the surface. An imposed large-scale ascent or descent affects the cloud lifetime by more than a day. Increasing the number of cloud condensation nuclei enhances the warming effect of the cloud.
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| 40. |
- Dimitrelos, Antonios, 1986-, et al.
(författare)
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Controls on surface warming by winter Arctic moist intrusions in idealized large-eddy simulations
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The main energy input to the polar regions in winter is the advection of warm, moist air from lower latitudes. This makes the polar climate sensitive to the temperature and moisture of extra-polar air. Here, we study this sensitivity from an air-mass transformation perspective. We perform simulations of an idealized maritime air mass brought into contact with sea ice employing a three-dimensional large-eddy simulation model coupled to a one-dimensional multilayer sea ice model. We study the response of cloud dynamics and surface warming during the air-mass transformation process to varying initial temperature and humidity conditions of the air mass. We find in all cases that a mixed-phase cloud is formed, initially near the surface but rising continuously with time. Surface warming of the sea ice is driven by downward longwave surface fluxes, which are largerly controlled by the temperature and optical depth of the cloud. Cloud temperature, in turn, is robustly constarined by the initial dewpoint temperature of the air mass. Since dewpoint only depends on moisture, the overall result is that surface warming depends almost exclusively on initial humidity and is largerly independent of initial temperature. We discuss possible climate implications of this result, in particular for polar amplification of surface warming and the role played by atmospheric energy transport.
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| 41. |
- Dimitrelos, Antonios, et al.
(författare)
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Controls on Surface Warming by Winter Arctic Moist Intrusions in Idealized Large-Eddy Simulations
- 2023
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Ingår i: Journal of Climate. - 0894-8755 .- 1520-0442. ; 36:5, s. 1287-1300
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Tidskriftsartikel (refereegranskat)abstract
- The main energy input to the polar regions in winter is the advection of warm, moist air from lower lati-tudes. This makes the polar climate sensitive to the temperature and moisture of extrapolar air. Here, we study this sensi-tivity from an air-mass transformation perspective. We perform simulations of an idealized maritime air mass brought into contact with sea ice employing a three-dimensional large-eddy simulation model coupled to a one-dimensional multilayer sea ice model. We study the response of cloud dynamics and surface warming during the air-mass transformation process to varying initial temperature and humidity conditions of the air mass. We find in all cases that a mixed-phase cloud is formed, initially near the surface but rising continuously with time. Surface warming of the sea ice is driven by downward longwave surface fluxes, which are largely controlled by the temperature and optical depth of the cloud. Cloud tempera-ture, in turn, is robustly constrained by the initial dewpoint temperature of the air mass. Since dewpoint only depends on moisture, the overall result is that surface warming depends almost exclusively on initial humidity and is largely indepen-dent of initial temperature. We discuss possible climate implications of this result-in particular, for polar amplification of surface warming and the role played by atmospheric energy transports.
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| 42. |
- Dimitrelos, Antonios, 1986-, et al.
(författare)
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The turbulent state of Arctic mixed-phase clouds under different conditions of moisture, aerosols, and ice water content
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Previous studies have shown that low-level mixed-phase clouds (MPCs) that form during moist intrusions into the Arctic can exist in either a stable (stratus) or a convective (stratocumulus) state. Here, we examine the process and conditions that promote a transition between the two states through idealized simulations using a three-dimensional large-eddy simulation model coupled with a one-dimensional multilayer sea ice model. We find that the vertical distribution of the initial dew point temperature (Td) profile has a fundamental influence on whether a transition from stable to convective conditions occurs or not. If the initial moisture content of the advected airmass decreased rapidly with height, i.e. if the Td profile is unstable, then a turbulent transition is likely to occur and a stratocumulus cloud can form. However, the availability and properties of aerosols and the cloud ice content can delay or even prevent stratocumulus formation, regardless if the conditions in terms of the initial Td profile are favorable. If no suitable cloud condensation nuclei are available at the base of the cloud at the time of the transition, then no new droplets form, the buoyancy remains low and the cloud remains in its stable state. Furthermore, a decrease in cloud ice water content results in a more stably stratified cloud layer and a delay in the transition. The transition from the stable to the convective state has a substantial effect on the surface warming induced by the cloud in the model; simulations with a transition generally show larger surface warming than simulations without a transition. Our results suggest that the low-level mixed-phase cloud evolution and the thermodynamic transition of an airmass during a moist intrusion are closely linked to the aerosol processing by the cloud, i.e. a chemical transformation, and that the two processes should be considered simultaneously.
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| 43. |
- Dipu, Sudhakar, et al.
(författare)
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Exploring Satellite-Derived Relationships between Cloud Droplet Number Concentration and Liquid Water Path Using a Large-Domain Large-Eddy Simulation
- 2022
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Ingår i: Tellus. Series B, Chemical and physical meteorology. - : Stockholm University Press. - 0280-6509 .- 1600-0889. ; 74:1, s. 176-188
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Tidskriftsartikel (refereegranskat)abstract
- Important aspects of the adjustments to aerosol-cloud interactions can be examined using the relationship between cloud droplet number concentration (Nd) and liquid water path (LWP). Specifically, this relation can constrain the role of aerosols in leading to thicker or thinner clouds in response to adjustment mechanisms. This study investigates the satellite retrieved relationship between Nd and LWP for a selected case of mid-latitude continental clouds using high-resolution Large-eddy simulations (LES) over a large domain in weather prediction mode. Since the satellite retrieval uses the adiabatic assumption to derive the Nd, we have also considered adiabatic Nd (NAd) from the LES model for comparison. The joint histogram analysis shows that the NAd-LWP relationship in the LES model and the satellite is in approximate agreement. In both cases, the peak conditional probability (CP) is confined to lower NAd and LWP; the corresponding mean LWP (LWP) shows a weak relation with NAd. The CP shows a larger spread at higher NAd (>50 cm–3), and the LWP increases non-monotonically with increasing NAd in both cases. Nevertheless, both lack the negative NAd-LWP relationship at higher NAd, the entrainment effect on cloud droplets. In contrast, the model simulated Nd-LWP clearly illustrates a much more nonlinear (an increase in LWP with increasing Nd and a decrease in LWP at higher Nd) relationship, which clearly depicts the cloud lifetime and the entrainment effect. Additionally, our analysis demonstrates a regime dependency (marine and continental) in the NAd-LWP relation from the satellite retrievals. Comparing local vs large-scale statistics from satellite data shows that continental clouds exhibit only a weak nonlinear NAd-LWP relationship. Hence a regime-based Nd-LWP analysis is even more relevant when it comes to warm continental clouds and their comparison to satellite retrievals.
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| 44. |
- Ekman, Annica, et al.
(författare)
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Do organics contribute to new particle formation in the Amazonian upper troposphere?
- 2008
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Ingår i: Geophysical Research Letters. ; 35:L17810, s. 5-
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Tidskriftsartikel (refereegranskat)abstract
- 3-D cloud-resolving model simulations including explicit aerosol physics and chemistry are compared with observations of upper tropospheric (12 km) aerosol size distributions over the Amazon Basin. The model underestimates the aerosol number concentration for all modes, especially the nucleation mode (d< 18nm). We show that a boundary layer SO2 mixing ratio of approximately 5 ppb would be needed in order to reproduce the high nucleation mode number concentrations observed. This high SO2 mixing ratio is very unlikely for the pristine Amazon Basin at this time of the year. Hence, it is suggested that vapours other than H2SO4 participate in the formation and growth of small aerosols. Using activation nucleation theory together with a small (0.4-10%) secondary organic aerosol mass yield, we show that isoprene has the potential of substantially increasing the number of small particles formed as well as reducing the underestimate for the larger aerosol modes.
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| 45. |
- Ekman, Annica M. L., et al.
(författare)
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Can an influence of changing aerosol emissions be detected in thepattern of surface temperature change between 1970 and 2000?
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The general circulation model CAM-Oslo was used to examine the influence of varyingaerosol and greenhouse gas emissions on the pattern of surface temperature change betweenthe years 1970 and 2000, and whether the temperature response over different regions wasgoverned by local (due to changes in energy fluxes) or far-field (due to changes in large-scale circulation) processes. Circulation changes, originating from precipitation anomaliesmainly over the west/central Pacific and off the east coast of North America, influenced asubstantial part of the northern hemisphere temperature change pattern in CAM-Oslo, inparticular over southern North America, but also over Europe and Asia. The result highlightsthe importance of better understanding zonally asymmetric precipitation changes due todifferent forcing agents. A local response in surface temperature due to net surface radiativeflux (RF) anomalies could also be detected over Europe and Asia, where the differencein all-sky net surface RF was mainly driven by aerosol- or circulation-induced changes inliquid water path and cloud cover. A local anthropogenic aerosol effect on the cloud dropletsize and subsequent short-wave (SW) RF was found over Europe and Asia, but only whenexcluding a change in the greenhouse gas concentration. For clear skies, the SW RF patternwas well-correlated with the aerosol optical depth anomalies. However, this correlation wasat least partly governed by relative humidity fluctuations. Overall, the greenhouse andaerosol effects on surface temperature were in the simulations found to be non-linear with asignificant feedback on the aerosol population from a warming climate.
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| 46. |
- Ekman, Annica M. L.
(författare)
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Do sophisticated parameterizations of aerosol-cloud interactions in CMIP5 models improve the representation of recent observed temperature trends?
- 2014
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Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-897X. ; 119:2, s. 817-832
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Tidskriftsartikel (refereegranskat)abstract
- Model output from the Coupled Model Intercomparison Project phase 5 (CMIP5) archive was compared with the observed latitudinal distribution of surface temperature trends between the years 1965 and 2004. By comparing model simulations that only consider changes in greenhouse gas forcing (GHG) with simulations that also consider the time evolution of anthropogenic aerosol emissions (GHGAERO), the influence of aerosol forcing on modeled surface temperature trends, and the dependence of the forcing on the model representation of aerosols and aerosol indirect effects, was evaluated. One group of models include sophisticated parameterizations of aerosol activation into cloud droplets; viz., the cloud droplet number concentration (CDNC) is a function of the modeled supersaturation as well as the aerosol concentration. In these models, the temperature trend bias was reduced in GHGAERO compared to GHG in more regions than in the other models. The ratio between high- and low-latitude warming also improved compared to observations. In a second group of models, the CDNC is diagnosed using an empirical relationship between the CDNC and the aerosol concentration. In this group, the temperature trend bias was reduced in more regions than in the model group where no aerosol indirect effects are considered. No clear difference could be found between models that include an explicit aerosol module and the ones that utilize prescribed aerosol. There was also no clear difference between models that include aerosol effects on the precipitation formation rate and the ones that do not. The results indicate that the best representation of recent observed surface temperature trends is obtained if the modeled CDNC is a function of both the aerosol concentration and the supersaturation. Key Points CMIP5 GCMs disagree on late 20th century zonal average aerosol forcing Including aerosol indirect effects reduces the zonal mean temperature bias A more sophisticated parameterization of droplet activation is beneficial
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| 47. |
- Ekman, Annica M. L., et al.
(författare)
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Impact of Two-Way Aerosol-Cloud Interaction and Changes in Aerosol Size Distribution on Simulated Aerosol-Induced Deep Convective Cloud Sensitivity
- 2011
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 68:4, s. 685-698
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Tidskriftsartikel (refereegranskat)abstract
- Recent cloud-resolving model studies of single (isolated) deep convective clouds have shown contradicting results regarding the response of the deep convection to changes in the aerosol concentration. In the present study, a cloud-resolving model including explicit aerosol physics and chemistry is used to examine how the complexity of the aerosol model, the size of the aerosols, and the aerosol activation parameterization influence the aerosol-induced deep convective cloud sensitivity. Six sensitivity series are conducted. A significant difference in the aerosol-induced deep convective cloud sensitivity is found when using different complexities of the aerosol model and different aerosol activation parameterizations. In particular, graupel impaction scavenging of aerosols appears to be a crucial process because it efficiently may limit the number of cloud condensation nuclei (CCN) at a critical stage of cloud development and thereby dampen the convection. For the simulated case, a 100% increase in aerosol concentration results in a difference in average updraft between the various sensitivity series that is as large as the average updraft increase itself. The change in graupel and rain formation also differs significantly. The sign of the change in precipitation is not always directly proportional to the change in updraft velocity and several of the sensitivity series display a decrease of the rain amount with increasing updraft velocity. This result illustrates the need to account for changes in evaporation processes and subsequent cooling when assessing aerosol effects on deep convective strength. The model simulations also show that an increased number of aerosols in the Aitken mode (here defined by 23 <= d <= 100.0 nm) results in a larger impact on the convective strength compared to an increased number of aerosols in the accumulation mode (here defined by 100 <= d <= 900.0 nm). When accumulation mode aerosols are activated and grow at the beginning of the cloud cycle, the supersaturation near the cloud base is lowered, which to some extent limits further aerosol activation. The simulations indicate a need to better understand and represent the two-way interaction between aerosols and clouds when studying aerosol-induced deep convective cloud sensitivity.
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| 48. |
- Ekman, Annica M. L., 1972-, et al.
(författare)
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Influence of horizontal resolution and complexity of aerosol–cloud interactions on marine stratocumulus and stratocumulus-to-cumulus transition in HadGEM3-GC3.1
- 2023
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Ingår i: Quarterly Journal of the Royal Meteorological Society. - 0035-9009 .- 1477-870X. ; 755:149, s. 2049-2066
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Tidskriftsartikel (refereegranskat)abstract
- Stratocumulus (Sc) clouds and stratocumulus-to-cumulus transitions (SCTs) are challenging to represent in global models and they contribute to a large spread in modeled subtropical cloud feedbacks. We evaluate the impact of increasing the horizontal model resolution (∼135, 60 and 25 km, respectively) and increasing the complexity of the aerosol–cloud interaction parameterization (interactive versus non-interactive at medium resolution) on springtime subtropical marine Sc properties and SCTs in the atmosphere-only version of HadGEM3-GC3.1. No significant impact on the spatial location of the SCT could be found between the different model versions. Increasing horizontal resolution led to small but significant increases in liquid water content and a stronger (more negative) shortwave (SW) cloud radiative effect (CRE), in particular over the southern-hemisphere Sc regions. However, for two out of the four studied regions, the stronger SW CRE also brought the model outside the range of satellite-derived values of the SW CRE. Applying non-interactive aerosols instead of interactive aerosols also led to significantly higher liquid water content and a stronger SW CRE over the southern-hemisphere Sc regions, while over the northern-hemisphere Sc regions, a competition between a substantial increase in the cloud droplet number concentration and small changes in the liquid water content resulted in a weaker SW CRE or non-significant changes. In general, using interactive instead of non-interactive aerosol–cloud interactions brought the model closer to satellite-retrieved mean values of the SW CRE. Our results suggest that increasing the horizontal resolution or the complexity of the aerosol–cloud parameterization has a small but statistically significant effect on the SW CRE of marine Sc, in particular over regions with high liquid water content. For these regions, the effect of introducing non-interactive versus interactive aerosol–cloud interactions is about as large as increasing the horizontal resolution from medium to high.
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| 49. |
- Ekman, Annica M. L., et al.
(författare)
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Sub-micrometer aerosol particles in the upper troposphere/lowermost stratosphere as measured by CARIBIC and modeled using the MIT-CAM3 global climate model
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. D11202-
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we compare modeled (MIT-CAM3) and observed (CARIBIC) sub-micrometer nucleation (N4-12, 4 <= d <= 12 nm) and Aitken mode (N-12, d > 12 nm) particle number concentrations in the upper troposphere and lowermost stratosphere (UT/LMS). Modeled and observed global median N4-12 and N-12 agree fairly well (within a factor of two) indicating that the relatively simplified binary H2SO4-H2O nucleation parameterization applied in the model produces reasonable results in the UT/LMS. However, a comparison of the spatiotemporal distribution of sub-micrometer particles displays a number of discrepancies between MIT-CAM3 and CARIBIC data: N4-12 is underestimated by the model in the tropics and overestimated in the extra-topics. N-12 is in general overestimated by the model, in particular in the tropics and during summer months. The modeled seasonal variability of N4-12 is in poor agreement with CARIBIC data whereas it agrees rather well for N-12. Modeled particle frequency distributions are in general narrower than the observed ones. The model biases indicate an insufficient diffusive mixing in MIT-CAM3 and a too large vertical transport of carbonaceous aerosols. The overestimated transport is most likely caused by the constant supersaturation threshold applied in the model for the activation of particles into cloud droplets. The annually constant SO2 emissions in the model may also partly explain the poor representation of the N4-12 seasonal cycle. Comparing the MIT-CAM3 with CARIBIC data, it is also clear that care has to be taken regarding the representativeness of the measurement data and the time frequency of the model output.
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| 50. |
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