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1.
  • Acosta Navarro, Juan Camilo, et al. (author)
  • Global emissions of terpenoid VOCs from terrestrial vegetation in the last millennium
  • 2014
  • In: Journal of Geophysical Research - Atmospheres. - : Wiley-Blackwell. - 2169-897X .- 2169-8996. ; 119:11, s. 6867-6885
  • Journal article (peer-reviewed)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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2.
  • Addor, Nans, et al. (author)
  • Propagation of biases in climate models from the synoptic to the regional scale : Implications for bias adjustment
  • 2016
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 121:5, s. 2075-2089
  • Journal article (peer-reviewed)abstract
    • Bias adjustment methods usually do not account for the origins of biases in climate models and instead perform empirical adjustments. Biases in the synoptic circulation are for instance often overlooked when postprocessing regional climate model (RCM) simulations driven by general circulation models (GCMs). Yet considering atmospheric circulation helps to establish links between the synoptic and the regional scale, and thereby provides insights into the physical processes leading to RCM biases. Here we investigate how synoptic circulation biases impact regional climate simulations and influence our ability to mitigate biases in precipitation and temperature using quantile mapping. We considered 20 GCM-RCM combinations from the ENSEMBLES project and characterized the dominant atmospheric flow over the Alpine domain using circulation types. We report in particular a systematic overestimation of the frequency of westerly flow in winter. We show that it contributes to the generalized overestimation of winter precipitation over Switzerland, and this wet regional bias can be reduced by improving the simulation of synoptic circulation. We also demonstrate that statistical bias adjustment relying on quantile mapping is sensitive to circulation biases, which leads to residual errors in the postprocessed time series. Overall, decomposing GCM-RCM time series using circulation types reveals connections missed by analyses relying on monthly or seasonal values. Our results underscore the necessity to better diagnose process misrepresentation in climate models to progress with bias adjustment and impact modeling.
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3.
  • Aires, F., et al. (author)
  • Microwave hyperspectral measurements for temperature and humidity atmospheric profiling from satellite: The clear-sky case
  • 2015
  • In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202 .- 2169-897X .- 2169-8996. ; 120:21, s. 11334-11351
  • Journal article (peer-reviewed)abstract
    • This study investigates the benefits of a satellite HYperspectral Microwave Sensor (HYMS) for the retrieval of atmospheric temperature and humidity profiles, in the context of numerical weather prediction (NWP). In the infrared, hyperspectral instruments have already improved the accuracy of NWP forecasts. Microwave instruments so far only provide observations for a limited number of carefully selected channels. An information content analysis is conducted here to assess the impact of hyperspectral microwave measurements on the retrieval of temperature and water vapor profiles under clear-sky conditions. It uses radiative transfer simulations over a large variety of atmospheric situations. It accounts for realistic observation (instrument and radiative transfer) noise and for a priori information assumptions compatible with NWP practices. The estimated retrieval performance of the HYMS instrument is compared to those of the microwave instruments to be deployed on board the future generation of European operational meteorological satellites (MetOp-SG). The results confirm the positive impact of a HYMS instrument on the atmospheric profiling capabilities compared to MetOp-SG. Temperature retrieval uncertainty, compared to a priori information, is reduced by 2 to 10%, depending on the atmospheric height, and improvement rates are much higher than what will be obtained with MetOp-SG. For humidity sounding these improvements can reach 30%, a significant benefit as compared to MetOp-SG results especially below 250 hPa. The results are not very sensitive to the instrument noise, under our assumptions. The main impact provided by the hyperspectral information originates from the higher resolution in the O2 band around 60 GHz. The results are presented over ocean at nadir, but similar conclusions are obtained for other incidence angles and over land. Key Points A hyperspectral MW instrument could improve temperature & humidity retrieval compared to MetOp-SG The main impact from HYMS comes from higher resolution in the O2 band around 60 GHz Hyperspectral information is not really sensitive to instrument noise.
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4.
  • Akperov, Mirseid, et al. (author)
  • Cyclone Activity in the Arctic From an Ensemble of Regional Climate Models (Arctic CORDEX)
  • 2018
  • In: Journal of Geophysical Research: Atmospheres. - 2169-8996 .- 2169-897X. ; 123:5, s. 2537-2554
  • Journal article (peer-reviewed)abstract
    • The ability of state-of-the-art regional climate models to simulate cyclone activity in the Arctic is assessed based on an ensemble of 13 simulations from 11 models from the Arctic-CORDEX initiative. Some models employ large-scale spectral nudging techniques. Cyclone characteristics simulated by the ensemble are compared with the results forced by four reanalyses (ERA-Interim, National Centers for Environmental Prediction-Climate Forecast System Reanalysis, National Aeronautics and Space Administration-Modern-Era Retrospective analysis for Research and Applications Version 2, and Japan Meteorological Agency-Japanese 55-year reanalysis) in winter and summer for 1981-2010 period. In addition, we compare cyclone statistics between ERA-Interim and the Arctic System Reanalysis reanalyses for 2000-2010. Biases in cyclone frequency, intensity, and size over the Arctic are also quantified. Variations in cyclone frequency across the models are partly attributed to the differences in cyclone frequency over land. The variations across the models are largest for small and shallow cyclones for both seasons. A connection between biases in the zonal wind at 200 hPa and cyclone characteristics is found for both seasons. Most models underestimate zonal wind speed in both seasons, which likely leads to underestimation of cyclone mean depth and deep cyclone frequency in the Arctic. In general, the regional climate models are able to represent the spatial distribution of cyclone characteristics in the Arctic but models that employ large-scale spectral nudging show a better agreement with ERA-Interim reanalysis than the rest of the models. Trends also exhibit the benefits of nudging. Models with spectral nudging are able to reproduce the cyclone trends, whereas most of the nonnudged models fail to do so. However, the cyclone characteristics and trends are sensitive to the choice of nudged variables.
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5.
  • An, Wenling, et al. (author)
  • Enhanced Recent Local Moisture Recycling on the Northwestern Tibetan Plateau Deduced From Ice Core Deuterium Excess Records
  • 2017
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 122:23, s. 12541-12556
  • Journal article (peer-reviewed)abstract
    • Local moisture recycling plays an essential role in maintaining an active hydrological cycle of the Tibetan Plateau (TP). Previous studies were largely limited to the seasonal time scale due to short and sparse observations, especially for the northwestern TP. In this study, we used a two-component mixing model to estimate local moisture recycling over the past decades from the deuterium excess records of two ice cores (i.e., Chongce and Zangser Kangri) from the northwestern TP. The results show that on average almost half of the precipitation on the northwestern TP is provided by local moisture recycling. In addition, the local moisture recycling ratio has increased evidently on the northwestern TP, suggesting an enhanced hydrological cycle. This recent increase could be due to the climatic and environmental changes on the TP in the past decades. Rapid increases in temperature and precipitation have enhanced evaporation. Changes of land surface of plateau have significantly increased evapotranspiration. All of these have intensified local moisture recycling. However, the mixing model used in this study only includes a limited number of climate factors. Some of the extreme values of moisture recycling ratio could be caused by large-scale atmospheric circulation and other climatic and weather events. Moreover, the potential mechanisms for the increase in local recycling need to be further examined, since the numeric simulations from climate models did not reproduce the increased contribution of local moisture recycling in precipitation.
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6.
  • Andrews, Timothy, et al. (author)
  • On the Effect of Historical SST Patterns on Radiative Feedback
  • 2022
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 127:18
  • Journal article (peer-reviewed)abstract
    • We investigate the dependence of radiative feedback on the pattern of sea-surface temperature (SST) change in 14 Atmospheric General Circulation Models (AGCMs) forced with observed variations in SST and sea-ice over the historical record from 1871 to near-present. We find that over 1871–1980, the Earth warmed with feedbacks largely consistent and strongly correlated with long-term climate sensitivity feedbacks (diagnosed from corresponding atmosphere-ocean GCM abrupt-4xCO2 simulations). Post 1980, however, the Earth warmed with unusual trends in tropical Pacific SSTs (enhanced warming in the west, cooling in the east) and cooling in the Southern Ocean that drove climate feedback to be uncorrelated with—and indicating much lower climate sensitivity than—that expected for long-term CO2 increase. We show that these conclusions are not strongly dependent on the Atmospheric Model Intercomparison Project (AMIP) II SST data set used to force the AGCMs, though the magnitude of feedback post 1980 is generally smaller in nine AGCMs forced with alternative HadISST1 SST boundary conditions. We quantify a “pattern effect” (defined as the difference between historical and long-term CO2 feedback) equal to 0.48 ± 0.47 [5%–95%] W m−2 K−1 for the time-period 1871–2010 when the AGCMs are forced with HadISST1 SSTs, or 0.70 ± 0.47 [5%–95%] W m−2 K−1 when forced with AMIP II SSTs. Assessed changes in the Earth's historical energy budget agree with the AGCM feedback estimates. Furthermore satellite observations of changes in top-of-atmosphere radiative fluxes since 1985 suggest that the pattern effect was particularly strong over recent decades but may be waning post 2014.
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7.
  • Axelsson, Josefine, et al. (author)
  • A Precipitation Isotopic Response in 2014-2015 to Moisture Transport Changes in the Central Himalayas
  • 2023
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 128:13
  • Journal article (peer-reviewed)abstract
    • The impact of moisture transport and sources on precipitation stable isotopes (d(18)O and d-excess) in the central Himalayas are crucial to understanding the climatic archives. However, this is still unclear due to the lack of in-situ observations. Here we present measurements of stable isotopes in precipitation at two stations (Yadong and Pali) in the central Himalayas during 2014-2015. Combined with simulations from the dispersion model FLEXPART, we investigate effects on precipitation stable isotopes related to changes in moisture sources and convections in the region, and possible influence by El Nino. Our results suggest that the moisture supplies related to evaporation over northeastern India and moisture losses related to convective activities over the Bay of Bengal (BoB) and Bangladesh region play important roles in changes in d(18)O and d-excess in precipitation in the Yadong Valley. Outgoing longwave radiation and moisture flux divergence analysis further confirm that the contribution from continental evaporation dominates the moisture supply in the central Himalayas with a lesser contribution from convection over the BoB during the 2015 monsoon season compared with 2014. A change in the altitude effect is observed in 2015, which is more significant than the temperature and precipitation amount effect during the observation period. These findings provide valuable insights into climatic interpretations of paleo-isotopic archives with an isotopic response to changes in moisture transport to the central Himalayas.
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8.
  • Ayala, Ana I., et al. (author)
  • Climate Change Impacts on Surface Heat Fluxes in a Deep Monomictic Lake
  • 2023
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 128:11
  • Journal article (peer-reviewed)abstract
    • Turbulent and radiative energy exchanges between lakes and the atmosphere play an importantrole in determining the process of lake-mixing and stratification, including how lakes respond to climate andto climate change. Here we used a one-dimensional hydrodynamic lake model to assess seasonal impacts ofclimate change on individual surface heat flux components in Lough Feeagh, Ireland, a deep, monomictic lake.We drove the lake model with an ensemble of outputs from four climate models under three future greenhousegas scenarios from 1976 to 2099. In these experiments, the results showed significant increases in the radiativebudget that were largely counteracted by significant increases in the turbulent fluxes. The combined change inthe individual surface heat fluxes led to a change in the total surface heat flux that was small, but sufficient tolead to significant changes in the volume-weighted average lake temperature. The largest projected changes intotal surface heat fluxes were in spring and autumn. Both spring heating and autumnal cooling significantlydecreased under future climate conditions, while changes to total surface heat fluxes in winter and summerwere an order of magnitude lower. This led to counter-intuitive results that, in a warming world, there wouldbe less heat not more entering Lough Feeagh during the springtime, and little change in net heating over thesummer or winter compared to natural climate conditions, projected increases in the volume-weighted averagelake temperature were found to be largely due to reduced heat loss during autumn.
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9.
  • Bardakov, Roman, 1992-, et al. (author)
  • The Role of Convective Up- and Downdrafts in the Transport of Trace Gases in the Amazon
  • 2022
  • In: Journal of Geophysical Research - Atmospheres. - : American Geophysical Union (AGU). - 2169-897X .- 2169-8996. ; 127:18
  • Journal article (peer-reviewed)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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10.
  • Barkley, Michael P., et al. (author)
  • Top-down isoprene emissions over tropical South America inferred from SCIAMACHY and OMI formaldehyde columns
  • 2013
  • In: Journal of Geophysical Research: Atmospheres. - : American Geophysical Union (AGU). - 2169-8996 .- 2169-897X. ; 118:12, s. 6849-6868
  • Journal article (peer-reviewed)abstract
    • We use formaldehyde (HCHO) vertical column measurements from the Scanning Imaging Absorption spectrometer for Atmospheric Chartography (SCIAMACHY) and Ozone Monitoring Instrument (OMI), and a nested-grid version of the GEOS-Chem chemistry transport model, to infer an ensemble of top-down isoprene emission estimates from tropical South America during 2006, using different model configurations and assumptions in the HCHO air-mass factor (AMF) calculation. Scenes affected by biomass burning are removed on a daily basis using fire count observations, and we use the local model sensitivity to identify locations where the impact of spatial smearing is small, though this comprises spatial coverage over the region. We find that the use of the HCHO column data more tightly constrains the ensemble isoprene emission range from 27-61TgC to 31-38TgC for SCIAMACHY, and 45-104TgC to 28-38TgC for OMI. Median uncertainties of the top-down emissions are about 60-260% for SCIAMACHY, and 10-90% for OMI. We find that the inferred emissions are most sensitive to uncertainties in cloud fraction and cloud top pressure (differences of +/- 10%), the a priori isoprene emissions (+/- 20%), and the HCHO vertical column retrieval (+/- 30%). Construction of continuous top-down emission maps generally improves GEOS-Chem's simulation of HCHO columns over the region, with respect to both the SCIAMACHY and OMI data. However, if local time top-down emissions are scaled to monthly mean values, the annual emission inferred from SCIAMACHY are nearly twice those from OMI. This difference cannot be explained by the different sampling of the sensors or uncertainties in the AMF calculation.
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11.
  • Beck, Hylke E., et al. (author)
  • Global Fully Distributed Parameter Regionalization Based on Observed Streamflow From 4,229 Headwater Catchments
  • 2020
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 125:17
  • Journal article (peer-reviewed)abstract
    • All hydrological models need to be calibrated to obtain satisfactory streamflow simulations. Here we present a novel parameter regionalization approach that involves the optimization of transfer equations linking model parameters to climate and landscape characteristics. The optimization was performed in a fully spatially distributed fashion at high resolution (0.05 degrees), instead of at lumped catchment scale, using an unprecedented database of daily observed streamflow from 4,229 headwater catchments (<5,000 km(2)) worldwide. The optimized equations were subsequently applied globally to produce parameter maps for the entire land surface including ungauged regions. The approach was evaluated using the Kling-Gupta efficiency (KGE) and a gridded version of the hydrological model HBV. Tenfold cross validation was used to evaluate the generalizability of the approach and to obtain an ensemble of parameter maps. For the 4,229 independent validation catchments, the regionalized parameters yielded a median KGE of 0.46. The median KGE improvement (relative to uncalibrated parameters) was 0.29, and improvements were obtained for 88% of the independent validation catchments. These scores compare favorably to those from previous large catchment sample studies. The degree of performance improvement due to the regionalized parameters did not depend on climate or topography. Substantial improvements were obtained even for independent validation catchments located far from the catchments used for optimization, underscoring the value of the derived parameters for poorly gauged regions. The regionalized parameters-available via -should be useful for hydrological applications requiring accurate streamflow simulations.
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12.
  • Bibi, S., et al. (author)
  • Response of Groundwater Storage and Recharge in the Qaidam Basin (Tibetan Plateau) to Climate Variations From 2002 to 2016
  • 2019
  • In: Journal of Geophysical Research-Atmospheres. - : American Geophysical Union (AGU). - 2169-897X .- 2169-8996. ; 124:17-17, s. 9918-9934
  • Journal article (peer-reviewed)abstract
    • Groundwater (GW) and recharge as the main drivers of the water budget are challenging to quantify due to the complexity of hydrological processes and limited observations. Understanding these processes in relation to climate is crucial for evaluating future water availability of Tibetan Plateau. By computing storage changes in Gravity Recovery and Climate Experiment terrestrial water storage and Global Land Surface Data Assimilation System land surface state variables and water balance approach, we calculated GW storage changes and recharges. In the Qaidam Basin (northern Tibetan Plateau), terrestrial water storage from the Gravity Recovery and Climate Experiment revealed a significant increasing trend of 25.5 mm/year during 2002-2012. However, an obviously turning point was found around 2012 and terrestrial water storage revealed a significant decreasing rate of 37.9 mm/year during 2013-2016. Similarly, GW (recharge) had a significant increasing trend of 21.2 (4.5) mm/year before 2012 and a decreasing rate of 32.1 (10.9) mm/year after 2012. Domain-averaged difference (P-ET) between precipitation (P) and evapotranspiration (ET) also exhibited an increasing trend of 4.4 mm/year during 2002-2012 and a decreasing rate of 9.0 mm/year during 2013-2016. Precipitation followed dissimilar pattern with an increasing rate of 5.3 mm/year during 2002-2012 while no significant trend during 2013-2016. However, ET had a consistent increasing trend over the basin during the past 15 years (0.9 mm/year before 2012 and 9.0 mm/year thereafter). This study concluded that GW amount and distribution is mainly controlled by precipitation and ET. Decrease in precipitation at high elevations and increase in ET may impact future groundwater availability in this region.
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13.
  • Bikkina, Srinivas, et al. (author)
  • Carbon isotope-constrained seasonality of carbonaceous aerosol sources from an urban location (Kanpur) in the Indo-Gangetic Plain
  • 2017
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 122:9, s. 4903-4923
  • Journal article (peer-reviewed)abstract
    • The Indo-Gangetic Plain (IGP) in northern India, Pakistan, and Bangladesh is a major source of carbonaceous aerosols in South Asia. However, poorly constrained seasonality of their sources over the IGP leads to large uncertainty in climate and health effects. Here we present a first data set for year-round radiocarbon (C-14) and stable carbon (C-13)-based source apportionment of total carbon (TC) in ambient PM10 (n = 17) collected from an urban site (Kanpur: 26.5 degrees N, 80.3 degrees E) in the IGP during January 2007 to January 2008. The year-round C-14-based fraction biomass (f(bio-TC)) estimate at Kanpur averages 777% and emphasizes an impact of biomass burning emissions (BBEs). The highest f(bio-TC) (%) is observed in fall season (October-November, 856%) followed by winter (December-February, 804%) and spring (March-May, 758%), while lowest values are found in summer (June-September, 69 +/- 2%). Since biomass/coal combustion and vehicular emissions mostly contribute to carbonaceous aerosols over the IGP, we predict C-13(TC) (C-13(pred)) over Kanpur using known C-13 source signatures and the measured C-14 value of each sample. The seasonal variability of C-13(obs)-C-13(pred) versus C-14(TC) together with air mass back trajectories and Moderate Resolution Imaging Spectroradiometer fire count data reveal that carbonaceous aerosols in winter/fall are significantly influenced by atmospheric aging (downwind transport of crop residue burning/wood combustion emissions in the northern IGP), while local sources (wheat residue combustion/vehicular emissions) dominate in spring/summer. Given the large temporal and seasonal variability in sources and emission strength of TC over the IGP, C-14-based constraints are, thus, crucial for reducing their uncertainties in carbonaceous aerosol budgets in climate models.
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14.
  • Bikkina, Srinivas, et al. (author)
  • Dual carbon isotope characterization of total organic carbon in wintertime carbonaceous aerosols from northern India
  • 2016
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 121:9, s. 4797-4809
  • Journal article (peer-reviewed)abstract
    • Large-scale emissions of carbonaceous aerosols (CA) from South Asia impact both regional climate and air quality, yet their sources are not well constrained. Here we use source-diagnostic stable and radiocarbon isotopes (delta C-13 and Delta C-14) to characterize CA sources at a semiurban site (Hisar: 29.2 degrees N, 75.2 degrees E) in the NW Indo-Gangetic Plain (IGP) and a remote high-altitude location in the Himalayan foothills (Manora Peak: 29.4 degrees N, 79.5 degrees E, 1950 m above sea level) in northern India during winter. The Delta C-14 of total aerosol organic carbon (TOC) varied from -178% to -63% at Hisar and from -198% to -1% at Manora Peak. The absence of significant differences in the C-14-based fraction biomass of TOC between Hisar (0.81 +/- 0.03) and Manora Peak (0.82 +/- 0.07) reveals that biomass burning/biogenic emissions (BBEs) are the dominant sources of CA at both sites. Combining this information with d13C, other chemical tracers (K+/OC and SO42-/EC) and air mass back trajectory analyses indicate similar source regions in the IGP (e.g., Punjab and Haryana). These results highlight that CA from BBEs in the IGP are not only confined to the atmospheric boundary layer but also extend to higher elevations of the troposphere, where the synoptic-scale circulations could substantially influence their abundances both to the Himalayas and over the downwind oceanic regions such as the Indian Ocean. Given the vast emissions of CA from postharvest crop residue combustion practices in the IGP during early Northeast Monsoon, this information is important for both improved process and model understanding of climate and health effects, as well as in guiding policy decision aiming at reducing emissions.
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15.
  • Björkman, Mats P., 1978, et al. (author)
  • Nitrate postdeposition processes in Svalbard surface snow
  • 2014
  • In: Journal of Geophysical Research - Atmospheres. - 0148-0227 .- 2156-2202 .- 2169-897X .- 2169-8996. ; 119:22
  • Journal article (peer-reviewed)abstract
    • The snowpack acts as a sink for atmospheric reactive nitrogen, but several postdeposition pathways have been reported to alter the concentration and isotopic composition of snow nitrate with implications for atmospheric boundary layer chemistry, ice core records, and terrestrial ecology following snow melt. Careful daily sampling of surface snow during winter (11-15 February 2010) and springtime (9 April to 5 May 2010) near Ny-Ålesund, Svalbard reveals a complex pattern of processes within the snowpack. Dry deposition was found to dominate over postdeposition losses, with a net nitrate deposition rate of (0.6+/-0.2) (my) molm 2 d 1 to homogeneous surface snow. At Ny-Ålesund, such surface dry deposition can either solely result from long-range atmospheric transport of oxidized nitrogen or include the redeposition of photolytic/bacterial emission originating from deeper snow layers. Our data further confirm that polar basin air masses bring 15 N-depleted nitrate to Svalbard, while high nitrate (delta) (18O) values only occur in connection with ozone-depleted air, and show that these signatures are reflected in the deposited nitrate. Such ozone-depleted air is attributed to active halogen chemistry in the air masses advected to the site. However, here the Ny-Ålesund surface snow was shown to have an active role in the halogen dynamics for this region, as indicated by declining bromide concentrations and increasing nitrate (delta) (18O), during high BrO (low-ozone) events. The data also indicate that the snowpack BrO-NO x cycling continued in postevent periods, when ambient ozone and BrO levels recovered.
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16.
  • Bosch, Carme, et al. (author)
  • Source-diagnostic dual-isotope composition and optical properties of water-soluble organic carbon and elemental carbon in the South Asian outflow intercepted over the Indian Ocean
  • 2014
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 119:20, s. 11743-11759
  • Journal article (peer-reviewed)abstract
    • The dual carbon isotope signatures and optical properties of carbonaceous aerosols have been investigated simultaneously for the first time in the South Asian outflow during an intensive campaign at the Maldives Climate Observatory on Hanimaadhoo (MCOH) (February and March 2012). As one component of the Cloud Aerosol Radiative Forcing Dynamics Experiment, this paper reports on the sources and the atmospheric processing of elemental carbon (EC) and water-soluble organic carbon (WSOC) as examined by a dual carbon isotope approach. The radiocarbon (C-14) data show that WSOC has a significantly higher biomass/biogenic contribution (865%) compared to EC (594%). The more C-13-enriched signature of MCOH-WSOC (-20.80.7) compared to MCOH-EC (-25.8 +/- 0.3 parts per thousand) and megacity Delhi WSOC (-24.1 +/- 0.9 parts per thousand) suggests that WSOC is significantly more affected by aging during long-range transport than EC. The C-13-C-14 signal suggests that the wintertime WSOC intercepted over the Indian Ocean largely represents aged primary biomass burning aerosols. Since light-absorbing organic carbon aerosols (Brown Carbon (BrC)) have recently been identified as potential contributors to positive radiative forcing, optical properties of WSOC were also investigated. The mass absorption cross section of WSOC (MAC(365)) was 0.5 +/- 0.2 m(2)g(-1) which is lower than what has been observed at near-source sites, indicating a net decrease of WSOC light-absorption character during long-range transport. Near-surface WSOC at MCOH accounted for similar to 1% of the total direct solar absorbance relative to EC, which is lower than the BrC absorption inferred from solar spectral observations of ambient aerosols, suggesting that a significant portion of BrC might be included in the water-insoluble portion of organic aerosols.
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17.
  • Bossioli, Elissavet, et al. (author)
  • Modeling Extreme Warm-Air Advection in the Arctic During Summer : The Effect of Mid-Latitude Pollution Inflow on Cloud Properties
  • 2021
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 126:7
  • Journal article (peer-reviewed)abstract
    • Usually the Arctic is relatively free of anthropogenic influence in summer, which means that particles from natural sources can be the most significant nuclei for cloud droplets. However, this is not the case during anomalously warm-air intrusions when the air origin comes from lower latitudes. In this modeling study, we investigate the effect of mid-latitude pollution inflow (anthropogenic and biomass burning [BB]) on the aerosol-cloud-radiation interactions during an episode of extreme warm-air advection. This particular episode resulted in anomalously high air temperatures over the East Siberian Sea and has accelerated sea-ice melting. The impact of different emission sources on aerosol vertical distribution, chemical composition, cloud formation, and radiation budget is examined using the Weather Research and Forecasting model, fully coupled with chemistry. Elevated turbulent clouds that occurred at the beginning of the episode are found to be more sensitive to aerosol variations and their negative feedback on supersaturation, compared to stably stratified fog layers that were dominant during the core period. Omission of either anthropogenic or BB source results in decreased cloud liquid water and cloud droplet concentrations; however, these changes are not substantially large to significantly modify the net surface radiation budget. Significant reduction of the net surface radiation is only observed if both anthropogenic and BB transported pollution reaches the area of interest.
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18.
  • Bourgeois, Quentin, et al. (author)
  • Aerosol transport over the Andes from the Amazon Basin to the remote Pacific Ocean : A multiyear CALIOP assessment
  • 2015
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 120:16, s. 8411-8425
  • Journal article (peer-reviewed)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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19.
  • Brooks, Ian M., et al. (author)
  • The Turbulent Structure of the Arctic Summer Boundary Layer During The Arctic Summer Cloud-Ocean Study
  • 2017
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 122:18, s. 9685-9704
  • Journal article (peer-reviewed)abstract
    • The mostly ice covered Arctic Ocean is dominated by low-level liquid-or mixed-phase clouds. Turbulence within stratocumulus is primarily driven by cloud top cooling that induces convective instability. Using a suite of in situ and remote sensing instruments we characterize turbulent mixing in Arctic stratocumulus, and for the first time we estimate profiles of the gradient Richardson number at relatively high resolution in both time (10 min) and altitude (10 m). It is found that the mixing occurs both within the cloud, as expected, and by wind shear instability near the surface. About 75% of the time these two layers are separated by a stably stratified inversion at 100-200 m altitude. Exceptions are associated with low cloud bases that allow the cloud-driven turbulence to reach the surface. The results imply that turbulent coupling between the surface and the cloud is sporadic or intermittent.Plain Language Summary: The lower atmosphere over the summertime Arctic Ocean often consists of two well-mixed layers-a surface mixed layer and a cloud mixed layer-that are separated by a weak decoupling layer at about 100 to 300 m above the surface. In these cases, the cloud cannot interact directly with the surface. Large-scale forecast and climate models consistently fail to reproduce this observed structure and may thus fail to correctly reproduce the cloud properties and the amount of energy absorbed by or emitted from the surface as solar and infrared radiation. This contributes to errors in reproducing changes in sea ice concentration over time. Here we use measurements made in the central Arctic to study the processes controlling whether or not the cloud is coupled to the surface. The effect of wind at the surface is found not to be a controlling factor. The depth of the cloud mixed layer is critical, but the multiple processes influencing it cannot be separated using the data available here. However, cooling at cloud top by infrared radiation is key, as is the extension of cloud into the temperature inversion-a unique feature of Arctic clouds.
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20.
  • Budhavant, Krishnakant, et al. (author)
  • Enhanced Light-Absorption of Black Carbon in Rainwater Compared With Aerosols Over the Northern Indian Ocean
  • 2020
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 125:2
  • Journal article (peer-reviewed)abstract
    • Black carbon (BC) aerosols affect climate, especially in high aerosol loading regions such as South Asia. A key uncertainty for the climate effects of BC is the evolution of light-absorbing properties in the atmosphere. Here, we present a year-round comparison of the mass absorption cross section (MAC; 678 nm) of BC in air (PM10) and rain, for samples collected at the Maldives Climate Observatory at Hanimaadhoo. We develop a filter-loading correction scheme for estimating BC absorption on filters used in high-volume samplers. The year-round average MAC(678) of BC in the rain is almost twice (13.3 +/- 4.2 m(2)/g) compared to the PM10 aerosol (7.2 +/- 2.6 m(2)/g). A possible explanation is the elevated ratio of organic carbon (OC) to BC observed in rain particulate matter (9.4 +/- 6.3) compared to in the aerosols (OC/BC 2.6 +/- 1.4 and water-insoluble organic carbon/BC 1.2 +/- 0.8), indicating a coating-enhancement effect. In addition to BC, we also investigated the MAC(365) of water-soluble brown carbon in PM10 (0.4 +/- 0.4 m(2)/g, at 365 nm). In contrast to BC, MAC(365)brown carbon relates to air mass history, showing higher values for samples from air originating over the South Asian landmass. Furthermore, calculated washout ratios are much lower for BC compared to OC and inorganic ions such as sulfate, implying a longer atmospheric lifetime for BC. The wet deposition flux for BC during the high loading winter was 3 times higher than during the wet summer, despite much less precipitation in the winter.
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21.
  • Chen, Bing, et al. (author)
  • Reconciling modeling with observations of radiative absorption of black carbon aerosols
  • 2017
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 122:11, s. 5932-5942
  • Journal article (peer-reviewed)abstract
    • The physical treatment of internal mixing and aging of black carbon (BC) aerosols that allow for enhanced solar absorption of the BC is an important parameterization in climate models. Many climate models predict a factor of 2-3 lower aerosol absorption optical depth (AAOD) than the atmospheric columnar absorption observed from ground-based networks such as AERONET, likely because these models do not parameterize properly the BC absorption enhancement (E-MAC). Models that are configured with an internal mixing have predicted large variations of E-MAC, which are poorly constrained from ambient measurements. We determined the BC E-MAC from aerosol coatings with a two-step solvent experiment to remove both organic and inorganic coatings in ambient fine particulate matter (PM2.5). Observations in a rural North China site showed that the E-MAC varied from 1.4 to 3. The E-MAC increases simultaneously with SO42-/EC ratios, suggesting the photochemical production of sulfate coatings enhanced BC absorption. A global climate model, parameterized to account for these observational constraints, verifies that sulfates are primary drivers of the BC absorption enhancement in severely polluted area in China. This magnification of the radiative forcing of coated BC is stronger by a factor of similar to 2 than predicted by the standard parameterization (external mixing) in the climate model and is in better agreement with AERONET observations of AAOD. This result would be useful for testing the representation of solar absorption by BC-containing particles in the newer generation of climate models. Plain Language Summary Atmospheric black carbon (BC) or soot in fine particulate matter (PM2.5) is emitted from incomplete combustion of fossil fuel or biomass/biofuel. The BC is an important pollutant for both air quality and Earth's energy balance, and the BC radiative forcing maybe second only to that of CO2. The photochemical production of nonabsorbing secondary aerosols may create a coating on BC and may thereby act as a lens which may enhance the light absorption. However, this absorption enhancement is poorly constrained by ambient measurements, and thus the estimates of BC climate forcing remain highly uncertain. To this end, an aerosol filter dissolution-filtration (AFD) with two-step solvent dissolution protocol was employed to remove both organic and inorganic coatings and then investigate their effects on BC light absorption. The observations and model simulation showed that the BC warming effect likely doubled due to lens effect from secondary aerosols.
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22.
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23.
  • Chen, Hans W., et al. (author)
  • Evaluation of Regional CO2 Mole Fractions in the ECMWF CAMS Real-Time Atmospheric Analysis and NOAA CarbonTracker Near-Real-Time Reanalysis With Airborne Observations From ACT-America Field Campaigns
  • 2019
  • In: Journal of Geophysical Research: Atmospheres. - 2169-8996 .- 2169-897X. ; 124:14, s. 8119-8133
  • Journal article (peer-reviewed)abstract
    • This study systematically examines the regional uncertainties and biases in carbon dioxide (CO2) mole fractions from two of the state-of-the-art global CO2 analysis products, namely, the Copernicus Atmosphere Monitoring Service (CAMS) real-time atmospheric analysis from the European Centre for Medium-Range Weather Forecasts (ECMWF) and the CarbonTracker Near-Real-Time (CT-NRT) reanalysis from the National Oceanic and Atmospheric Administration (NOAA), by evaluation against hundreds of hours of airborne in situ measurements from the summer 2016 and winter 2017 Atmospheric Carbon and Transport (ACT)-America field campaigns. Both the CAMS and CT-NRT analyses agree reasonably well with the independent ACT-America airborne CO2 measurements in the free troposphere, with root-mean-square deviations (RMSDs) between analyses and observations generally between 1 and 2 ppm but show considerably larger uncertainties in the atmospheric boundary layer where the RMSDs exceed 8 ppm in the lowermost 1 km of the troposphere in summer. There are strong variations in accuracy and bias between seasons, and across three different subregions in the United States (Mid-Atlantic, Midwest, and South), with the largest uncertainties in the Mid-Atlantic region in summer. Overall, the RMSDs of the CAMS and CT-NRT analyses against airborne data are comparable to each other and largely consistent with the differences between the two analyses. The current study provides uncertainty estimates for both analysis products over North America and suggests that these two independent estimates can be used to approximate regional CO2 analysis uncertainties. Both statistics are important in future studies in quantifying the uncertainties in regional CO2 mole fraction and flux estimates.
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24.
  • Chiacchio, Marc, et al. (author)
  • Evaluation of the radiation budget with a regional climate model over Europe and inspection of dimming and brightening
  • 2015
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 120:5, s. 1951-1971
  • Journal article (peer-reviewed)abstract
    • Shortwave (SW) and longwave (LW) components of the radiation budget at the surface and top of atmosphere (TOA) are evaluated in the regional climate model RegCM version 4 driven by European Centre for Medium-Range Weather Forecasts Reanalysis over Europe. The simulated radiative components were evaluated with those from satellite-based products and reanalysis. At the surface the model overestimated the absorbed solar radiation but was compensated by a greater loss of thermal energy while both SW and LW TOA net fluxes were underestimated representing too little solar energy absorbed and too little outgoing thermal energy. Averaged biases in radiative parameters were generally within 25 Wm(-2), were dependent on differences by as much as 0.2 in cloud fraction, surface, and planetary albedo and less dependent on surface temperature associated with the surface longwave parameters, and are in line with other studies. Clear-sky fluxes showed better results when cloud cover differences had no influence. We also found a clear distinction between land versus water with smaller biases over land at the surface and over water at the TOA due to differences in cloud fraction and albedo. Finally, we inspected dimming and brightening for the period 1979-2010 with an indication for dimming early in the time series (i.e., 1979-1987) and brightening after, which agrees with surface-based observations. After 2000, however, a decrease in the brightening by more than 1 order of magnitude was evident which is in contrast to the continued brightening found in surface records and satellite-derived estimates.
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25.
  • Chiacchio, Marc, et al. (author)
  • On the links between meteorological variables, aerosols, and tropical cyclone frequency in individual ocean basins
  • 2017
  • In: Journal of Geophysical Research. - : American Geophysical Union (AGU). - 0148-0227 .- 2156-2202. ; 122:2, s. 802-822
  • Journal article (peer-reviewed)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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26.
  • Christiansen, Sigurd, et al. (author)
  • Influence of Arctic Microlayers and Algal Cultures on Sea Spray Hygroscopicity and the Possible Implications for Mixed-Phase Clouds
  • 2020
  • In: Journal of Geophysical Research: Atmospheres. - 2169-8996 .- 2169-897X. ; 125:19
  • Journal article (peer-reviewed)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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27.
  • Chylek, Petr, et al. (author)
  • High Values of the Arctic Amplification in the Early Decades of the 21st Century: Causes of Discrepancy by CMIP6 Models Between Observation and Simulation
  • 2023
  • In: JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES. - 2169-897X .- 2169-8996. ; 128:23
  • Journal article (peer-reviewed)abstract
    • Arctic Amplification (AA) in the first decade of the 21st century has reached values between 4 and 5, with a subsequent decrease to current values of about 3.6, while the value was from 2 to 3 during the twentieth century. The ensemble mean of the CMIP6 models has difficulty in reproducing the recently observed high values of the AA. In this report, we identify the main reason for this difficulty to be the CMIP6 models overestimate of the mean global temperature trend since about 1990. The largest values of the AA are observed in winter and spring. A sharp AA peak in 1987 spring was caused by a peak in the Arctic temperature trend occurring at the same time as a dip in the trend of mean global temperature. The winter AA has increased almost monotonically since 1990. Dividing the AA between the Arctic land and ocean areas shows that the ocean area makes a larger contribution to the AA. Our future projection of the AA suggests an increasing AA for about the next decade, followed by a slow decrease to about 3.5 in the 2050s. The Arctic is warming faster than the average warming of the whole earth. The Arctic Amplification (AA) is defined as the ratio of the Arctic to global mean warming rates. Thus, the AA increases when the rate of Arctic warming increases, when the rate of global warming decreases, or when both happen at the same time. For most of the twentieth century, the AA was between 2 and 3. However, during the first few years of the 21st century, the AA has reached over four. The current climate models are not able to reproduce the observed early 21st-century high values of AA. We find that the main reason for this difficulty is the models' overestimate of the global warming rates after 1990. The early 21st century high values of the AA are caused by a higher temperature trend over the Arctic and a lower global temperature trendThe CMIP6 models' difficulty in reproducing the observed AA is due to the models' overestimate of the rate of mean global warming after 1990The future projection of the AA suggests an increasing AA for about the next decade with a slowly decreasing trend after that
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28.
  • Clifton, O. E., et al. (author)
  • Influence of Dynamic Ozone Dry Deposition on Ozone Pollution
  • 2020
  • In: Journal of Geophysical Research-Atmospheres. - : American Geophysical Union (AGU). - 2169-897X .- 2169-8996. ; 125:8
  • Journal article (peer-reviewed)abstract
    • Identifying the contributions of chemistry and transport to observed ozone pollution using regional-to-global models relies on accurate representation of ozone dry deposition. We use a recently developed configuration of the NOAA GFDL chemistry-climate model - in which the atmosphere and land are coupled through dry deposition-to investigate the influence of ozone dry deposition on ozone pollution over northern midlatitudes. In our model, deposition pathways are tied to dynamic terrestrial processes, such as photosynthesis and water cycling through the canopy and soil. Small increases in winter deposition due to more process-based representation of snow and deposition to surfaces reduce hemispheric-scale ozone throughout the lower troposphere by 5-12 ppb, improving agreement with observations relative to a simulation with the standard configuration for ozone dry deposition. Declining snow cover by the end of the 21st-century tempers the previously identified influence of rising methane on winter ozone. Dynamic dry deposition changes summer surface ozone by -4 to +7 ppb. While previous studies emphasize the importance of uptake by plant stomata, new diagnostic tracking of depositional pathways reveals a widespread impact of nonstomatal deposition on ozone pollution. Daily variability in both stomatal and nonstomatal deposition contribute to daily variability in ozone pollution. Twenty-first century changes in summer deposition result from a balance among changes in individual pathways, reflecting differing responses to both high carbon dioxide (through plant physiology versus biomass accumulation) and water availability. Our findings highlight a need for constraints on the processes driving ozone dry deposition to test representation in regional-to-global models.
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29.
  • Dimitrelos, Antonios, et al. (author)
  • A Sensitivity Study of Arctic Air-Mass Transformation Using Large Eddy Simulation
  • 2020
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 125:6
  • Journal article (peer-reviewed)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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30.
  • Duplessis, P., et al. (author)
  • Highly Hygroscopic Aerosols Facilitate Summer and Early-Autumn Cloud Formation at Extremely Low Concentrations Over the Central Arctic Ocean
  • 2024
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 129:2
  • Journal article (peer-reviewed)abstract
    • Arctic clouds are sensitive to atmospheric particles since these are sometimes in such low concentrations that clouds cannot always form under supersaturated water vapor conditions. This is especially true in the late summer, when aerosol concentrations are generally very low in the high Arctic. The environment changes rapidly around freeze-up as the open waters close and snow starts accumulating on ice. We investigated droplet formation during eight significant fog events in the central Arctic Ocean, north of 80 degrees, from August 12 to 19 September 2018 during the Arctic Ocean 2018 expedition onboard the icebreaker Oden. Calculated hygroscopicity parameters (kappa) for the entire study were very high (up to kappa = 0.85 +/- 0.13), notably after freeze-up, suggesting that atmospheric particles were very cloud condensation nuclei (CCN)-active. At least one of the events showed that surface clouds were able to form and persist for at least a couple hours at aerosol concentrations less than 10 cm-3, which was previously suggested to be the minimum for cloud formation. Among these events that were considered limited in CCN, effective radii were generally larger than in the high CCN cases. In some of the fog events, droplet residuals particles did not reactivate under supersaturations up to 0.95%, suggesting either in-droplet reactions decreased hygroscopicity, or an ambient supersaturation above 1%. These results provide insight into droplet formation during the clean late-summer and fall of the high Arctic with limited influence from continental sources. The Arctic atmosphere can be very clean in the summer, to the point that clouds cannot form because there are insufficient particles present for the water vapor to condense upon. This has important implications for the radiation budget, which is highly dependent on clouds. As part of the Arctic Ocean 2018 expedition in the central Arctic Ocean near the North Pole, we investigated the ability of particles to turn into droplets throughout the whole cruise (August 12 to 19 September 2018), and during eight significant fog events. Overall, we found that after the sea ice started to freeze, the particles were more capable of turning into cloud droplets. During one fog event, we observed fog droplets forming when the particle concentrations were lower than the limit that past studies had suggested that fog/cloud could be sustained. During several fog events, the dried fog droplets did not always re-form droplets when exposed to cloud-like conditions, which suggests that the original droplets must have formed under extreme conditions. Our results show that in the summer/fall in the high Arctic, liquid droplets sometimes form under unusual circumstances that are likely not always considered in models. Aerosol hygroscopicity was greater after surface water freeze-up than beforeHygroscopicity of Aitken mode particles was generally greater than accumulation mode particlesCloud droplet effective radii during aerosol-limited periods were larger generally than periods with higher aerosol concentrations
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31.
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32.
  • Eliasson, Salomon, et al. (author)
  • Systematic and random errors between collocated satellite ice water path observations
  • 2013
  • In: Journal of Geophysical Research - Atmospheres. - : John Wiley & Sons. - 2169-897X .- 2169-8996. ; 118:6, s. 2629-2642
  • Journal article (peer-reviewed)abstract
    • There remains large disagreement between IWP in observational datasets, largely because the sensors observe different parts of the ice particle size distribution. A detailed comparison of retrieved IWP from satellite observations in the Tropics ({plus minus}30{degree sign} latitude) in 2007 is made using collocated measurements. The DARDAR IWP dataset, based on combined Radar/Lidar measurements, is used as a reference as it provides arguably the best estimate of the total column IWP. For each dataset, usable IWP dynamic ranges are inferred from this comparison. IWP retrievals based on solar reflectance measurements, MODIS, and AVHRR-based CMSAF, and PATMOS-x, were found to be correlated with DARDAR over a large IWP range (~20-7000 g/m-2;). The random errors of the collocated datasets have a close to log-normal distribution, and the combined random error of MODIS and DARDAR is less than a factor of 2, which also sets the upper limit for MODIS alone. In the same way the upper limit for the random error of all considered datasets is determined. Datasets based on passive microwave measurements,MSPPS, MiRS, and CMO, are largely correlated with DARDAR for IWP values larger than approximately 700 g/m². The combined uncertainty between these datasets and DARDAR in this range is slightly less MODIS-DARDAR, but the systematic bias is nearly an order of magnitude.
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33.
  • Elvidge, A. D., et al. (author)
  • Surface Heat and Moisture Exchange in the Marginal Ice Zone : Observations and a New Parameterization Scheme for Weather and Climate Models
  • 2021
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 126:17
  • Journal article (peer-reviewed)abstract
    • Aircraft observations from two Arctic field campaigns are used to characterize and model surface heat and moisture exchange over the marginal ice zone (MIZ). We show that the surface roughness lengths for heat and moisture over uninterrupted sea ice vary with roughness Reynolds number (R*; itself a function of the roughness length for momentum, z0, and surface wind stress), with a peak at the transition between aerodynamically smooth (R*<0.135) and aerodynamically rough (R*>2.5) regimes. A pre-existing theoretical model based on surface-renewal theory accurately reproduces this peak, in contrast to the simple parameterizations currently employed in two state-of-the-art numerical weather prediction models, which are insensitive to R*. We propose a new, simple parameterization for surface exchange over the MIZ that blends this theoretical model for sea ice with surface exchange over water as a function of sea ice concentration. In offline tests, this new scheme performs much better than the existing schemes for the rough conditions observed during the 'Iceland Greenland Seas Project' field campaign. The bias in total turbulent heat flux across the MIZ is reduced to only 13 W m(-2) for the new scheme, from 48 and 80 W m(-2) for the Met Office Unified Model and ECMWF Integrated Forecast System schemes, respectively. It also performs marginally better for the comparatively smooth conditions observed during the 'Aerosol-Cloud Coupling and Climate Interactions in the Arctic' field campaign. The new surface exchange scheme has the benefit of being physically-motivated, comparatively accurate and straightforward to implement, although to reap the full benefits an improvement to the representation of sea ice topography via z0 is required.
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34.
  • Fang, Wenzheng, et al. (author)
  • Dual-Isotope Constraints on Seasonally Resolved Source Fingerprinting of Black Carbon Aerosols in Sites of the Four Emission Hot Spot Regions of China
  • 2018
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 123:20, s. 11735-11747
  • Journal article (peer-reviewed)abstract
    • Despite much recent efforts, the emission sources of black carbon (BC) aerosols. central input to understanding and predicting environmental and climate impact. remain highly uncertain. Here we present observational delta C-13/Delta C-14-based constraints on the sources of BC aerosols over the four seasons in each of the four key hot spot emission regions of China: Beijing-Tianjin-Hebei (BTH-Wuqing; where Wuqing is the sampling location), Yangtze River Delta (YRD-Haining), Pearl River Delta (PRD-Zhongshan), and Sichuan Basin (SC-Deyang). Overall, BC loadings were highest in winter, yet elevated loadings were also observed in other seasons, for example, spring at SC-Deyang and fall at PRD-Zhongshan. Annually, the dominant BC sources were coal (50 +/- 20%) for BTH-Wuqing, liquid fossil for YRD-Haining (46 +/- 8%) and PRD-Zhongshan (48 +/- 18%), whereas liquid fossil (42 +/- 17%) and biomass burning (41 +/- 14%) equally affected SC-Deyang. There is also different but distinct seasonalities in BC sources for the different sites. As an example, for BTH-Wuqing coal burning increased from summer to winter, while summer and spring BTH-Wuqing were more influenced by liquid fossil. In contrast, for YRD-Haining, the relative importance of emission sources was more constant over the year. These quantitative observational constraints on source-seasonality of BC aerosols in receptor sites located in China's four key economic zones highlight that regulatory control on BC aerosol emissions from different fuels should consider both seasonal and regional variations. Our results also suggest that models on estimates of BC-induced climate and air quality should consider variations over both regional and seasonal scales.
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35.
  • Gao, Yang, et al. (author)
  • The Influence of Increased CO2 Concentrations on AMOC Interdecadal Variability Under the LGM Background
  • 2024
  • In: JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES. - 2169-897X .- 2169-8996. ; 129:3
  • Journal article (peer-reviewed)abstract
    • This study explores the impact of rising CO2 levels on the Atlantic meridional overturning circulation's (AMOC) interdecadal variability within the context of the Last Glacial Maximum (LGM) background climate. Under heightened CO2 concentrations, the AMOC interdecadal variability intensifies dramatically, which is very different from the future warming case that shows a weakening of AMOC interdecadal variability in response to increased CO2 concentration. This unexpected phenomenon primarily results from the extensive retreat of sea ice, which exposes a larger portion of the ocean surface to efficiently feel the heat flux fluctuations from atmospheric processes. These findings underscore the significance of background climate conditions in shaping AMOC responses to increased CO2 and emphasize the necessity of considering these nuances to develop a more accurate understanding of AMOC dynamics in an evolving climate. Plain Language Summary The Atlantic meridional overturning circulation (AMOC) is an important component of the Earth system, and its interdecadal variability is predicted to be significantly weakened under future warming scenarios. In this paper, we analyze the response of AMOC interdecadal variability to rising CO2 levels under the background of the Last Glacial Maximum (LGM) and find that the AMOC interdecadal variability is intensified under increased CO2 , which is totally different from its response at the background of modern climate. Analyses suggest that this unexpected result is mainly caused by dramatic sea ice retreat, which exposes much seawater to efficiently receive large fluctuations of heat flux from atmospheric forcing. The findings reveal that the response of AMOC to increased CO2 and relevant dominant mechanism differs significantly under different climate conditions.
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36.
  • Geng, Xiaofei, et al. (author)
  • Year-Round Measurements of Dissolved Black Carbon in Coastal Southeast Asia Aerosols : Rethinking Its Atmospheric Deposition in the Ocean
  • 2021
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 126:18
  • Journal article (peer-reviewed)abstract
    • Dissolved black carbon (DBC) is an important recalcitrant fraction of marine dissolved organic matter. Riverine discharge is the largest known source of oceanic DBC; however, the significance of atmospheric deposition as a source of oceanic DBC remains poorly understood. In this study, year-round aerosol sampling was carried out at a rural coastal site in Southeast Asia for DBC analysis using the benzene polycarboxylic acid (BPCA) method. The results revealed the uncertainty of an earlier estimate of the atmospheric deposition flux of DBC to the global ocean (FDBC), which assumed a linear correlation between DBC and water-soluble organic carbon (WSOC). The correlation between DBC and WSOC depended on the sources of carbonaceous aerosols. The DBC/WSOC ratios were higher for the biomass burning aerosols. DBC was linearly correlated with black carbon (BC) for biomass or fossil fuel combustion aerosols. However, the DBC/BC ratios were higher for biomass burning aerosols (0.41 ± 0.22), whereas lower for fossil fuel combustion aerosols (0.04 ± 0.03). FDBC was revisited based on the relationship between DBC and BC. FDBC is primarily contributed by biomass burning aerosols and maybe previously underestimated. In this study, the DBC in aerosols had less condensed aromatic structures than the DBC present in the major rivers of the world, as shown by the BPCA compositions. This indicated that oceanic DBC sourced from atmospheric deposition was less likely to be removed by photodegradation and sedimentation, as compared to the DBC sourced from riverine discharge.
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37.
  • Giamarelou, Maria, et al. (author)
  • Indirect evidence of the composition of nucleation mode atmospheric particles in the high Arctic
  • 2016
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 121:2, s. 965-975
  • Journal article (peer-reviewed)abstract
    • Previous long-term observations have shown that nanoparticle formation events are common in the summer-time high Arctic and linked to local photochemical activity. However, current knowledge is limited with respect to the chemical precursors of resulting nanoparticles and the compounds involved in their subsequent growth. Here we report case-study measurements during new particle formation (NPF) events of the particle size distribution (diameter>7nm) and for the first time the volatility of monodisperse particles having diameter 40nm, providing indirect information about their composition. Volatility measurements provide indirect evidence that a predominant fraction of the 12nm particle population is ammoniated sulfates in the summertime high Arctic. Our observations further suggest that the majority of the sub-40nm particle population during NPF events does not exist in the form of sulfuric acid but rather as partly or fully neutralized ammoniated sulfates.
  •  
38.
  • Glantz, Paul, et al. (author)
  • Remote sensing of aerosols in the Arctic for an evaluation of global climate model simulations
  • 2014
  • In: Journal of Geophysical Research - Atmospheres. - : Wiley-Blackwell. - 2169-897X .- 2169-8996. ; 119:13, s. 8169-8188
  • Journal article (peer-reviewed)abstract
    • In this study Moderate Resolution Imaging Spectroradiometer (MODIS) Aqua retrievals of aerosol optical thickness (AOT) at 555 nm are compared to Sun photometer measurements from Svalbard for a period of 9 years. For the 642 daily coincident measurements that were obtained, MODIS AOT generally varies within the predicted uncertainty of the retrieval over ocean (Delta AOT = +/- 0.03 +/- 0.05 . AOT). The results from the remote sensing have been used to examine the accuracy in estimates of aerosol optical properties in the Arctic, generated by global climate models and from in situ measurements at the Zeppelin station, Svalbard. AOT simulated with the Norwegian Earth System Model/Community Atmosphere Model version 4 Oslo global climate model does not reproduce the observed seasonal variability of the Arctic aerosol. The model overestimates clear-sky AOT by nearly a factor of 2 for the background summer season, while tending to underestimate the values in the spring season. Furthermore, large differences in all-sky AOT of up to 1 order of magnitude are found for the Coupled Model Intercomparison Project phase 5 model ensemble for the spring and summer seasons. Large differences between satellite/ground-based remote sensing of AOT and AOT estimated from dry and humidified scattering coefficients are found for the subarctic marine boundary layer in summer.
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39.
  • Glantz, Paul, et al. (author)
  • Unmasking the Effects of Aerosols on Greenhouse Warming Over Europe
  • 2022
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 127:22
  • Journal article (peer-reviewed)abstract
    • Aerosol optical thickness (AOT) has decreased substantially in Europe in the summer half year (April–September) since 1980, with almost a 50% reduction in Central and Eastern Europe, according to Modern-Era Retrospective analysis for Research and Applications, Version 2 (MERRA-2) reanalysis. At the same time, strong positive trends in ERA5 reanalysis surface solar radiation downward for all-sky and clear-sky conditions (SSRD and SSRDc, respectively) and temperature at 2 m are found for Europe in summer during the period 1979–2020. The GEBA observations show as well strong increases in SSRD during the latest four decades. Estimations of changes in SSRDc, using the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model, show similarly strong increases when fed by MERRA-2 AOT. The estimates of warming in this study, caused by increases in SSRD and SSRDc, are based on energy budget approximations and the Stefan Boltzmann law. The increases in near surface temperature, estimated both for clear-sky and all-sky conditions, are up to about 1°C for Central and Eastern Europe. The total warming over large parts of this region for clear-sky conditions is however nearly double the global mean temperature increase of 1.1°C, while somewhat less for all-sky conditions. The effects of aerosols on warming over the southerly Iberian Peninsula are weaker compared to countries further north. The rapid total warming over the Iberian Peninsula is probably caused by greenhouse warming, drier surface conditions, and to some degree decline in aerosols. Reduced cloud cover is found for large parts of Europe in summer during the latest four decades.
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40.
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41.
  • Groenholm, T., et al. (author)
  • Aerosol particle dry deposition to canopy and forest floor measured by two-layer eddy covariance system
  • 2009
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996 .- 0148-0227 .- 2156-2202. ; 114
  • Journal article (peer-reviewed)abstract
    • We present, to our knowledge for the first time, results from subcanopy (forest trunk space) aerosol flux measurements with the eddy covariance (EC) technique. The measurements were performed at the SMEAR II measurement station located in a Scots pine forest in southern Finland during spring 2003 when the ground was snow covered and in situ biogenic particle formation took place almost every day. Spectral analysis showed that the EC method can be applied to estimate subcanopy aerosol fluxes and thereby ground deposition in a forest. By using the two-layer EC measurements we were able to investigate the fraction of the total aerosol deposition taking place in the canopy and on the forest floor. We found that the ratio of subcanopy to above- canopy flux is dependent on the strength of turbulence. When the friction velocity was low (u(*) < 0.25 m s(-1)), approximately 35% of the depositing particles penetrated the canopy and deposited on the ground. When u* increased, the fractional deposition on the forest floor decreased almost linearly, and in strongly turbulent conditions (u(*)> 1.0 m s(-1)) the ground deposition contributed only around 10% to the total deposition. Thus, it seems that increased turbulence enhances the importance of the canopy for aerosol deposition and the relative importance of the forest floor to the total deposition diminishes.
  •  
42.
  • Groot Zwaaftink, C. D., et al. (author)
  • Substantial contribution of northern high-latitude sources to mineral dust in the Arctic
  • 2016
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 121:22, s. 13678-13697
  • Journal article (peer-reviewed)abstract
    • In the Arctic, impurities in the atmosphere and cryosphere can strongly affect the atmospheric radiation and surface energy balance. While black carbon has hence received much attention, mineral dust has been in the background. Mineral dust is not only transported into the Arctic from remote regions but also, possibly increasingly, generated in the region itself. Here we study mineral dust in the Arctic based on global transport model simulations. For this, we have developed a dust mobilization scheme in combination with the Lagrangian particle dispersion model FLEXPART. A model evaluation, based on measurements of surface concentrations and annual deposition at a number of stations and aircraft vertical profiles, shows the suitability of this model to study global dust transport. Simulations indicate that about 3% of global dust emission originates from high-latitude dust sources in the Arctic. Due to limited convection and enhanced efficiency of removal, dust emitted in these source regions is mostly deposited closer to the source than dust from for instance Asia or Africa. This leads to dominant contributions of local dust sources to total surface dust concentrations (similar to 85%) and dust deposition (similar to 90%) in the Arctic region. Dust deposition from local sources peaks in autumn, while dust deposition from remote sources occurs mainly in spring in the Arctic. With increasing altitude, remote sources become more important for dust concentrations as well as deposition. Therefore, total atmospheric dust loads in the Arctic are strongly influenced by Asian (similar to 38%) and African (similar to 32%) dust, whereas local dust contributes only 27%. Dust loads are thus largest in spring when remote dust is efficiently transported into the Arctic. Overall, our study shows that contributions of local dust sources are more important in the Arctic than previously thought, particularly with respect to surface concentrations and dust deposition.
  •  
43.
  • Guo, Mingyang, et al. (author)
  • Validation and Sensitivity Analysis of a 1-D Lake Model Across Global Lakes
  • 2021
  • In: Journal of Geophysical Research - Atmospheres. - : American Geophysical Union (AGU). - 2169-897X .- 2169-8996. ; 126:4
  • Journal article (peer-reviewed)abstract
    • Lakes have important influence on weather and climate from local to global scales. However, their prediction using numerical models is notoriously difficult because lakes are highly heterogeneous across the globe, but observations are sparse. Here, we assessed the performance of a 1‐D lake model in simulating the thermal structures of 58 lakes with diverse morphometric and geographic characteristics by following the phase 2a local lake protocol of the Intersectoral Impact Model Intercomparison Project (ISIMIP2a). After calibration, the root‐mean‐square errors (RMSE) were below 2°C for 70% and 75% of the lakes for epilimnion and full‐profile temperature simulations, with an average of 1.71°C and 1.43°C, respectively. The model performance mainly depended on lake shape rather than location, supporting the possibility of grouping model parameters by lake shape for global applications. Furthermore, through machine‐learning based parameter sensitivity tests, we identified turbulent heat fluxes, wind‐driven mixing, and water transparency as the major processes controlling lake thermal and mixing regimes. Snow density was also important for modeling the ice phenology of high‐latitude lakes. The relative influence of the key processes and the corresponding parameters mainly depended on lake latitude and depth. Turbulent heat fluxes showed a decreasing importance in affecting epilimnion temperature with increasing latitude. Wind‐driven mixing was less influential to lake stratification for deeper lakes while the impact of light extinction, on the contrary, showed a positive correlation with lake depth. Our findings may guide improvements in 1‐D lake model parameterizations to achieve higher fidelity in simulating global lake thermal dynamics.
  •  
44.
  • Guseva, S., et al. (author)
  • Bulk Transfer Coefficients Estimated From Eddy-Covariance Measurements Over Lakes and Reservoirs
  • 2023
  • In: Journal of Geophysical Research - Atmospheres. - : American Geophysical Union (AGU). - 2169-897X .- 2169-8996. ; 128:2
  • Journal article (peer-reviewed)abstract
    • The drag coefficient, Stanton number and Dalton number are of particular importance for estimating the surface turbulent fluxes of momentum, heat and water vapor using bulk parameterization. Although these bulk transfer coefficients have been extensively studied over the past several decades in marine and large-lake environments, there are no studies analyzing their variability for smaller lakes. Here, we evaluated these coefficients through directly measured surface fluxes using the eddy-covariance technique over more than 30 lakes and reservoirs of different sizes and depths. Our analysis showed that the transfer coefficients (adjusted to neutral atmospheric stability) were generally within the range reported in previous studies for large lakes and oceans. All transfer coefficients exhibit a substantial increase at low wind speeds (<3 m s(-1)), which was found to be associated with the presence of gusts and capillary waves (except Dalton number). Stanton number was found to be on average a factor of 1.3 higher than Dalton number, likely affecting the Bowen ratio method. At high wind speeds, the transfer coefficients remained relatively constant at values of 1.6.10(-3), 1.4.10(-3), 1.0.10(-3), respectively. We found that the variability of the transfer coefficients among the lakes could be associated with lake surface area. In flux parameterizations at lake surfaces, it is recommended to consider variations in the drag coefficient and Stanton number due to wind gustiness and capillary wave roughness while Dalton number could be considered as constant at all wind speeds. Plain Language Summary In our study, we investigate the bulk transfer coefficients, which are of particular importance for estimation the turbulent fluxes of momentum, heat and water vapor in the atmospheric surface layer, above lakes and reservoirs. The incorrect representation of the surface fluxes above inland waters can potentially lead to errors in weather and climate prediction models. For the first time we made this synthesis using a compiled data set consisting of existing eddy-covariance flux measurements over 23 lakes and 8 reservoirs. Our results revealed substantial increase of the transfer coefficients at low wind speeds, which is often not taken into account in models. The observed increase in the drag coefficient (momentum transfer coefficient) and Stanton number (heat transfer coefficient) could be associated with the presence of wind gusts and capillary waves. In flux parameterizations at lake surface, it is recommended to consider them for accurate flux representation. Although the bulk transfer coefficients were relatively constant at high wind speeds, we found that the Stanton number systematically exceeds the Dalton number (water vapor transfer coefficient), despite the fact they are typically considered to be equal. This difference may affect the Bowen ratio method and result in biased estimates of lake evaporation.
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45.
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46.
  • Hare, B. M., et al. (author)
  • LOFAR Lightning Imaging : Mapping Lightning With Nanosecond Precision
  • 2018
  • In: Journal of Geophysical Research - Atmospheres. - : American Geophysical Union (AGU). - 2169-897X .- 2169-8996. ; 123:5, s. 2861-2876
  • Journal article (peer-reviewed)abstract
    • Lightning mapping technology has proven instrumental in understanding lightning. In this work we present a pipeline that can use lightning observed by the LOw-Frequency ARray (LOFAR) radio telescope to construct a 3-D map of the flash. We show that LOFAR has unparalleled precision, on the order of meters, even for lightning flashes that are over 20km outside the area enclosed by LOFAR antennas (approximate to 3,200km(2)), and can potentially locate over 10,000 sources per lightning flash. We also show that LOFAR is the first lightning mapping system that is sensitive to the spatial structure of the electrical current during individual lightning leader steps.
  •  
47.
  • He, Lixia, et al. (author)
  • An Analysis of Current and Electric Field Pulses Associated With Upward Negative Lightning Flashes Initiated from the Santis Tower
  • 2018
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 123:8, s. 4045-4059
  • Journal article (peer-reviewed)abstract
    • We present a study on the characteristics of current and electric field pulses associated with upward lightning flashes initiated from the instrumented Santis Tower in Switzerland. The electric field was measured 15km from the tower. Upward flashes always begin with the initial stage composed of the upward-leader phase and the initial-continuous-current (ICC) phase. Four types of current pulses are identified and analyzed in the paper: (1) return-stroke pulses, which occur after the extinction of the ICC and are preceded by essentially no-current time intervals; (2) mixed-mode ICC pulses, defined as fast pulses superimposed on the ICC, which have characteristics very similar to those of return strokes and are believed to be associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC-carrying channel at relatively small junction heights; (3) classical M-component pulses superimposed on the continuing current following some return strokes; and (4) M-component-type ICC pulses, presumably associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC-carrying channel at relatively large junction heights. We consider a data set consisting of 9 return-stroke pulses, 70 mixed-mode ICC pulses, 11 classical M-component pulses, and 19 M-component-type ICC pulses (a total of 109 pulses). The salient characteristics of the current and field waveforms are analyzed. A new criterion is proposed to distinguish between mixed-mode and M-component-type pulses, which is based on the current waveform features. The characteristics of M-component-type pulses during the initial stage are found to be similar to those of classical M-component pulses occurring during the continuing current after some return strokes. It is also found that about 41% of mixed-mode ICC pulses were preceded by microsecond-scale pulses occurring in electric field records some hundreds of microseconds prior to the onset of the current, very similar to microsecond-scale electric field pulses observed for M-component-type ICC pulses and which can be attributed to the junction of an in-cloud leader channel to the current-carrying channel to ground. Classical M-component pulses and M-component-type ICC pulses tend to have larger risetimes ranging from 6.3 to 430s. On the other hand, return-stroke pulses and mixed-mode ICC pulses have current risetimes ranging from 0.5 to 28s. Finally, our data suggest that the 8-s criterion for the current risetime proposed by Flache et al. is a reasonable tool to distinguish between return strokes and classical M-components. However, mixed-mode ICC pulses superimposed on the ICC can sometimes have considerably longer risetimes, up to about 28s, as observed in this study.
  •  
48.
  • He, Lixia, et al. (author)
  • Electromagnetic Fields Associated With the M-Component Mode of Charge Transfer
  • 2019
  • In: Journal of Geophysical Research - Atmospheres. - : AMER GEOPHYSICAL UNION. - 2169-897X .- 2169-8996. ; 124:13, s. 6791-6809
  • Journal article (peer-reviewed)abstract
    • In upward flashes, charge transfer to ground largely takes place during the initial continuous current (ICC) and its superimposed pulses (ICC pulses). ICC pulses can be associated with either M-component or leader/return-stroke-like modes of charge transfer to ground. In the latter case, the downward leader/return stroke process is believed to take place in a decayed branch or a newly created channel connected to the ICC-carrying channel at relatively short distance from the tower top, resulting in the so-called mixed mode of charge transfer to ground. In this paper, we study the electromagnetic fields associated with the M-component charge transfer mode using simultaneous records of electric fields and currents associated with upward flashes initiated from the Santis Tower. The effect of the mountainous terrain on the propagation of electromagnetic fields associated with the M-component charge transfer mode (including classical M-component pulses and M-component-type pulses superimposed on the initial continuous current) is analyzed and compared with its effect on the fields associated with the return stroke (occurring after the extinction of the ICC) and mixed charge transfer modes. For the analysis, we use a 2-Dimentional Finite-Difference Time Domain method, in which the M-component is modeled by the superposition of a downward current wave and an upward current wave resulting from the reflection at the bottom of the lightning channel (Rakov et al., 1995, model) and the return stroke and mixed mode are modeled adopting the MTLE (Modified Transmission Line with Exponential Current Decay with Height) model. The finite ground conductivity and the mountainous propagation terrain between the Santis Tower and the field sensor located 15 km away at Herisau are taken into account. The effects of the mountainous path on the electromagnetic fields are examined for classical M-component and M-component-type ICC pulses. Use is made of the propagation factors defined as the ratio of the electric or magnetic field peak evaluated along the mountainous terrain to the field peak evaluated for a flat terrain. The velocity of the M-component pulse is found to have a significant effect on the risetime of the electromagnetic fields. A faster traveling wave speed results in larger peaks for the magnetic field. However, the peak of the electric field appears to be insensitive to the M-component wave speed. This can be explained by the fact that at 15 km, the electric field is still dominated by the static component, which mainly depends on the overall transferred charge. The contribution of the radiation component to the M-component fields at 100 km accounts for about 77% of the peak electric field and 81% of the peak magnetic field, considerably lower compared to the contribution of the radiation component to the return stroke fields at the same distance. The simulation results show that neither the electric nor the magnetic field propagation factors are very sensitive to the risetimes of the current pulses. However, the results indicate a high variability of the propagation factors as a function of the branch-to-channel junction point height. For junction point heights of about 1 km, the propagation factors reach a value of about 1.6 for the E-field and 1.9 for the H-field. For a junction height greater than 6 km, the E-field factor becomes slightly lower than 1. The obtained results are consistent with the findings of Li, Azadifar, Rachidi, Rubinstein, Paolone, et al. (2016, ) in which an electric field propagation factor of 1. 8 was inferred for return strokes and mixed-mode pulses, considering that junction points lower than 1 km or so would result in a mixed mode of charge transfer, in which a downward leader/return-stroke-like process is believed to take place. It is also found that the field enhancement (propagation factor) for return stroke mode is higher for larger ground conductivities. Furthermore, the enhancement effect tends to decrease with increasing current risetime, except for very short risetimes (less than 2.5 mu s or so) for which the tendency reverses. Finally, model-predicted fields associated with different charge transfer modes, namely, return stroke, mixed-mode, classical M-component, and M-component-type ICC pulse are compared with experimental observations at the Santis Tower. It is found that the vertical electric field waveforms computed considering the mountainous terrain are in very good agreement with the observed data. The adopted parameters of the models that provide the best match with the measured field waveforms were consistent with observations. The values for the current decay height constant adopted in the return stroke and mixed-mode models (1.0 km for the return stroke and 0.8 km for the mixed-mode pulse) are lower than the value of 2.0 km typically used in the literature.
  •  
49.
  • Hepworth, Ehlke, et al. (author)
  • Association Between Extreme Atmospheric Anomalies Over Antarctic Sea Ice, Southern Ocean Polar Cyclones and Atmospheric Rivers
  • 2022
  • In: Journal of Geophysical Research - Atmospheres. - : American Geophysical Union (AGU). - 2169-897X .- 2169-8996. ; 127:7
  • Journal article (peer-reviewed)abstract
    • This study analyses the association of Southern Ocean extratropical cyclones and atmospheric rivers (ARs) with extreme temperature and/or moisture atmospheric anomalies over Antarctic sea ice. The hypothesis we test is whether the circulations associated with cyclones and ARs may routinely lead to the presence of unusually warm, moist air masses over ice-covered regions. The analysis is conducted over the extended Austral winter seasons (May-September) between May 1979 and September 2012, based on the European Centre for Medium-Range Weather Forecasts Interim reanalysis data. Approximately 27% of intense Southern Ocean cyclones and 20% of ARs occur in the vicinity of extreme temperature anomalies, while 12% of intense cyclones and 46% of ARs occur in the vicinity of extreme moisture anomalies. We summarize our results as follows: (a) extreme atmospheric anomalies over sea ice often occur in the absence of cyclones or ARs; (b) intense cyclones have a stronger association with extreme temperature anomalies than ARs; (c) approximately half of the ARs are in the vicinity of extreme moisture anomalies, while the latter's link with cyclones is weak; and (d) if an AR is in the vicinity of an extreme temperature anomaly, there will likely be a concurrent extreme moisture anomaly. This points to a strong association between ARs and moisture extremes, and a nuanced link between Southern Ocean polar cyclones and atmospheric anomalies over Antarctic sea ice.
  •  
50.
  • Hultgren, Kristoffer, et al. (author)
  • Tomographic and spectral views on the lifecycle of polar mesospheric clouds from Odin/OSIRIS
  • 2014
  • In: Journal of Geophysical Research - Atmospheres. - 2169-897X .- 2169-8996. ; 119:24, s. 14129-14143
  • Journal article (peer-reviewed)abstract
    • Vertical and horizontal structures of Polar Mesospheric Clouds (PMC) have been recovered by tomographic retrieval from the OSIRIS instrument aboard the Odin satellite. The tomographic algorithm has been used to return local scattering coefficients at seven wavelengths in the ultraviolet. This spectral information is used to retrieve PMC particle sizes, number density, and ice mass density. While substantial horizontal variations are found, local vertical structures are overall consistent with the idea of a growth-sedimentation process leading to a visible cloud. Large numbers of small particles are present near the top of the observed cloud layer. Toward lower altitudes, particle sizes increase while particle number densities decrease. A close relationship is found between the distribution of local PMC scattering coefficient and ice mass density. The bottom of the cloud often features large particles with mode radii exceeding 70 nm that rain out of the cloud before sublimating. The number density of these large particles is small, and they do not contribute significantly to the overall cloud brightness. As a consequence, the presence of these large particles can be difficult to identify for remote sensing techniques that integrate over the entire cloud column. When it comes to deriving absolute values of particle mode radius and number density, there is a strong sensitivity to assumptions on the mathematical form of the particle size distribution. We see a continued strong need to resolve this issue by co-analysis of various remote sensing techniques and observation geometries.
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