| 1. |
- Acosta Navarro, Juan Camilo, et al.
(författare)
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Global emissions of terpenoid VOCs from terrestrial vegetation in the last millennium
- 2014
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Ingår i: Journal of Geophysical Research - Atmospheres. - : Wiley-Blackwell. - 2169-897X .- 2169-8996. ; 119:11, s. 6867-6885
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Tidskriftsartikel (refereegranskat)abstract
- We investigated the millennial variability (1000 A.D.-2000 A.D.) of global biogenic volatile organic compound (BVOC) emissions by using two independent numerical models: The Model of Emissions of Gases and Aerosols from Nature (MEGAN), for isoprene, monoterpene, and sesquiterpene, and Lund-Potsdam-Jena-General Ecosystem Simulator (LPJ-GUESS), for isoprene and monoterpenes. We found the millennial trends of global isoprene emissions to be mostly affected by land cover and atmospheric carbon dioxide changes, whereas monoterpene and sesquiterpene emission trends were dominated by temperature change. Isoprene emissions declined substantially in regions with large and rapid land cover change. In addition, isoprene emission sensitivity to drought proved to have significant short-term global effects. By the end of the past millennium MEGAN isoprene emissions were 634 TgC yr-1 (13% and 19% less than during 1750-1850 and 1000-1200, respectively), and LPJ-GUESS emissions were 323 TgC yr-1(15% and 20% less than during 1750-1850 and 1000-1200, respectively). Monoterpene emissions were 89 TgC yr-1(10% and 6% higher than during 1750-1850 and 1000-1200, respectively) in MEGAN, and 24 TgC yr-1 (2% higher and 5% less than during 1750-1850 and 1000-1200, respectively) in LPJ-GUESS. MEGAN sesquiterpene emissions were 36 TgC yr-1(10% and 4% higher than during 1750-1850 and 1000-1200, respectively). Although both models capture similar emission trends, the magnitude of the emissions are different. This highlights the importance of building better constraints on VOC emissions from terrestrial vegetation.
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| 2. |
- Bender, Frida A. -M., et al.
(författare)
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Quantification of Monthly Mean Regional-Scale Albedo of Marine Stratiform Clouds in Satellite Observations and GCMs
- 2011
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Ingår i: Journal of Applied Meteorology and Climatology. - 1558-8424 .- 1558-8432. ; 50:10, s. 2139-2148
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Tidskriftsartikel (refereegranskat)abstract
- Planetary albedo the reflectivity for solar radiation is of singular importance in determining the amount of solar energy taken in by the Earth-atmosphere system. Modeling albedo, and specifically cloud albedo, correctly is crucial for realistic climate simulations. A method is presented herein by which regional cloud albedo can be quantified from the relation between total albedo and cloud fraction, which in observations is found to be approximately linear on a monthly mean scale. This analysis is based primarily on the combination of cloud fraction data from the Moderate Resolution Imaging Spectroradiometer (MODIS) and albedo data from the Clouds and the Earth's Radiant Energy System (CERES), but the results presented are also supported by the combination of cloud fraction and proxy albedo data from satelliteborne lidar [Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CA LIPSO)]. These data are measured and derived completely independently from the CERES-MODIS data. Applied to low-level marine stratiform clouds in three regions (off the coasts of South America, Africa, and North America), the analysis reveals regionally uniform monthly mean cloud albedos, indicating that the variation in cloud shortwave radiative properties is small on this scale. A coherent picture of low effective cloud albedo emerges, in the range from 0.35 to 0.42, on the basis of data from CERES and MODIS. In its simplicity, the method presented appears to be useful as a diagnostic tool and as a constraint on climate models. To demonstrate this, the same method is applied to cloud fraction and albedo output from several current-generation climate models [from the Coupled Model Intercomparison Project, phase 3 (CMIP3), archive]. Although the multimodel mean cloud albedo estimates agree to within 20% with the satellite-based estimates for the three focus regions, model-based estimates of cloud albedo are found to display much larger variability than do the observations, within individual models as well as between models.
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| 3. |
- Bender, Frida, et al.
(författare)
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Response to the eruption of Mount Pinatubo in relation to climate sensitivity in the CMIP3 models
- 2010
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Ingår i: Climate Dynamics. - : Springer. - 0930-7575 .- 1432-0894. ; 35:5, s. 875-886
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Tidskriftsartikel (refereegranskat)abstract
- The radiative flux perturbations and subsequent temperature responses in relation to the eruption of Mount Pinatubo in 1991 are studied in the ten general circulation models incorporated in the Coupled Model Intercomparison Project, phase 3 (CMIP3), that include a parameterization of volcanic aerosol. Models and observations show decreases in global mean temperature of up to 0.5 K, in response to radiative perturbations of up to 10 W m−2, averaged over the tropics. The time scale representing the delay between radiative perturbation and temperature response is determined by the slow ocean response, and is estimated to be centered around 4 months in the models. Although the magniude of the temperature response to a volcanic eruption has previously been used as an indicator of equilibrium climate sensitivity in models, we find these two quantities to be only weakly correlated. This may partly be due to the fact that the size of the volcano-induced radiative perturbation varies among the models. It is found that the magnitude of the modelled radiative perturbation increases with decreasing climate sensitivity, with the exception of one outlying model. Therefore, we scale the temperature perturbation by the radiative perturbation in each model, and use the ratio between the integrated temperature perturbation and the integrated radiative perturbation as a measure of sensitivity to volcanic forcing. This ratio is found to be well correlated with the model climate sensitivity, more sensitive models having a larger ratio. Further, if this correspondence between “volcanic sensitivity” and sensitivity to CO2 forcing is a feature not only among the models, but also of the real climate system, the alleged linear relation can be used to estimate the real climate sensitivity. The observational value of the ratio signifying volcanic sensitivity is hereby estimated to correspond to an equilibrium climate sensitivity, i.e. equilibrium temperature increase due to a doubling of the CO2 concentration, between 1.7 and 4.1 K. Several sources of uncertainty reside in the method applied, and it is pointed out that additional model output, related to ocean heat storage and radiative forcing, could refine the analysis, as could reduced uncertainty in the observational record, of temperature as well as forcing.
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| 4. |
- Decremer, Damien, et al.
(författare)
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Which significance test performs the best in climate simulations?
- 2014
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Ingår i: Tellus. Series A, Dynamic meteorology and oceanography. - : Stockholm University Press. - 0280-6495 .- 1600-0870. ; 66:1, s. 23139-
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Tidskriftsartikel (refereegranskat)abstract
- Climate change simulated with climate models needs a significance testing to establish the robustness of simulated climate change relative to model internal variability. Student's t-test has been the most popular significance testing technique despite more sophisticated techniques developed to address autocorrelation. We apply Student's t-test and four advanced techniques in establishing the significance of the average over 20 continuous-year simulations, and validate the performance of each technique using much longer (375-1000 yr) model simulations. We find that all the techniques tend to perform better in precipitation than in surface air temperature. A sizable performance gain using some of the advanced techniques is realised in the model Ts output portion with strong positive lag-1 yr autocorrelation (> +/- 0.6), but this gain disappears in precipitation. Furthermore, strong positive lag-1 yr autocorrelation is found to be very uncommon in climate model outputs. Thus, there is no reason to replace Student's t-test by the advanced techniques in most cases.
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| 5. |
- Ekman, Annica M. L.
(författare)
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Do sophisticated parameterizations of aerosol-cloud interactions in CMIP5 models improve the representation of recent observed temperature trends?
- 2014
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Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-897X. ; 119:2, s. 817-832
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Tidskriftsartikel (refereegranskat)abstract
- Model output from the Coupled Model Intercomparison Project phase 5 (CMIP5) archive was compared with the observed latitudinal distribution of surface temperature trends between the years 1965 and 2004. By comparing model simulations that only consider changes in greenhouse gas forcing (GHG) with simulations that also consider the time evolution of anthropogenic aerosol emissions (GHGAERO), the influence of aerosol forcing on modeled surface temperature trends, and the dependence of the forcing on the model representation of aerosols and aerosol indirect effects, was evaluated. One group of models include sophisticated parameterizations of aerosol activation into cloud droplets; viz., the cloud droplet number concentration (CDNC) is a function of the modeled supersaturation as well as the aerosol concentration. In these models, the temperature trend bias was reduced in GHGAERO compared to GHG in more regions than in the other models. The ratio between high- and low-latitude warming also improved compared to observations. In a second group of models, the CDNC is diagnosed using an empirical relationship between the CDNC and the aerosol concentration. In this group, the temperature trend bias was reduced in more regions than in the model group where no aerosol indirect effects are considered. No clear difference could be found between models that include an explicit aerosol module and the ones that utilize prescribed aerosol. There was also no clear difference between models that include aerosol effects on the precipitation formation rate and the ones that do not. The results indicate that the best representation of recent observed surface temperature trends is obtained if the modeled CDNC is a function of both the aerosol concentration and the supersaturation. Key Points CMIP5 GCMs disagree on late 20th century zonal average aerosol forcing Including aerosol indirect effects reduces the zonal mean temperature bias A more sophisticated parameterization of droplet activation is beneficial
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| 6. |
- Ekman, Annica M. L., et al.
(författare)
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Impact of Two-Way Aerosol-Cloud Interaction and Changes in Aerosol Size Distribution on Simulated Aerosol-Induced Deep Convective Cloud Sensitivity
- 2011
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Ingår i: Journal of the Atmospheric Sciences. - 0022-4928 .- 1520-0469. ; 68:4, s. 685-698
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Tidskriftsartikel (refereegranskat)abstract
- Recent cloud-resolving model studies of single (isolated) deep convective clouds have shown contradicting results regarding the response of the deep convection to changes in the aerosol concentration. In the present study, a cloud-resolving model including explicit aerosol physics and chemistry is used to examine how the complexity of the aerosol model, the size of the aerosols, and the aerosol activation parameterization influence the aerosol-induced deep convective cloud sensitivity. Six sensitivity series are conducted. A significant difference in the aerosol-induced deep convective cloud sensitivity is found when using different complexities of the aerosol model and different aerosol activation parameterizations. In particular, graupel impaction scavenging of aerosols appears to be a crucial process because it efficiently may limit the number of cloud condensation nuclei (CCN) at a critical stage of cloud development and thereby dampen the convection. For the simulated case, a 100% increase in aerosol concentration results in a difference in average updraft between the various sensitivity series that is as large as the average updraft increase itself. The change in graupel and rain formation also differs significantly. The sign of the change in precipitation is not always directly proportional to the change in updraft velocity and several of the sensitivity series display a decrease of the rain amount with increasing updraft velocity. This result illustrates the need to account for changes in evaporation processes and subsequent cooling when assessing aerosol effects on deep convective strength. The model simulations also show that an increased number of aerosols in the Aitken mode (here defined by 23 <= d <= 100.0 nm) results in a larger impact on the convective strength compared to an increased number of aerosols in the accumulation mode (here defined by 100 <= d <= 900.0 nm). When accumulation mode aerosols are activated and grow at the beginning of the cloud cycle, the supersaturation near the cloud base is lowered, which to some extent limits further aerosol activation. The simulations indicate a need to better understand and represent the two-way interaction between aerosols and clouds when studying aerosol-induced deep convective cloud sensitivity.
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| 7. |
- Ekman, Annica M. L., et al.
(författare)
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Sub-micrometer aerosol particles in the upper troposphere/lowermost stratosphere as measured by CARIBIC and modeled using the MIT-CAM3 global climate model
- 2012
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Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 117, s. D11202-
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Tidskriftsartikel (refereegranskat)abstract
- In this study, we compare modeled (MIT-CAM3) and observed (CARIBIC) sub-micrometer nucleation (N4-12, 4 <= d <= 12 nm) and Aitken mode (N-12, d > 12 nm) particle number concentrations in the upper troposphere and lowermost stratosphere (UT/LMS). Modeled and observed global median N4-12 and N-12 agree fairly well (within a factor of two) indicating that the relatively simplified binary H2SO4-H2O nucleation parameterization applied in the model produces reasonable results in the UT/LMS. However, a comparison of the spatiotemporal distribution of sub-micrometer particles displays a number of discrepancies between MIT-CAM3 and CARIBIC data: N4-12 is underestimated by the model in the tropics and overestimated in the extra-topics. N-12 is in general overestimated by the model, in particular in the tropics and during summer months. The modeled seasonal variability of N4-12 is in poor agreement with CARIBIC data whereas it agrees rather well for N-12. Modeled particle frequency distributions are in general narrower than the observed ones. The model biases indicate an insufficient diffusive mixing in MIT-CAM3 and a too large vertical transport of carbonaceous aerosols. The overestimated transport is most likely caused by the constant supersaturation threshold applied in the model for the activation of particles into cloud droplets. The annually constant SO2 emissions in the model may also partly explain the poor representation of the N4-12 seasonal cycle. Comparing the MIT-CAM3 with CARIBIC data, it is also clear that care has to be taken regarding the representativeness of the measurement data and the time frequency of the model output.
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| 8. |
- Engström, Anders, 1982-
(författare)
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Aerosol-cloud interaction from an observational and modeling perspective
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Clouds may respond strongly to changes in the atmospheric aerosol population, and the response of clouds to an increased global aerosol burden could to some extent mask the warming caused by enhanced greenhouse gas concentrations. However, estimates of the impact of aerosols on cloud properties are associated with large uncertainties, both because of difficulties representing the aerosol-cloud interaction within models, and because of problems of unequivocally isolating the effect of aerosols on cloud properties in observational data. This thesis focuses in part on underlying meteorological factors that significantly correlate with both aerosol and cloud properties, and on how sensitive clouds are to small variations in meteorological conditions. It was found that meteorological covariations must be taken into account when estimating the strength of the relationship between aerosols and cloud properties. By studying the response of shallow convective clouds to perturbations in meteorological conditions and aerosol concentration, it was further concluded that variations in meteorological conditions can enhance or mask the relationship between aerosols and cloud properties, making it difficult to isolate the aerosol signature from small meteorological differences. Additionally, the impact of deep convective clouds on the redistribution of aerosols within a cloud life cycle is examined. It was found that mid-tropospheric aerosols can have a substantial source in evaporating cloud droplets within deep convection. Lastly, this thesis focuses on the implications of meteorological analysis uncertainties, in part related to the difficulties of constraining meteorological variability in observational data of clouds and aerosols, but mainly the impact of analysis errors on atmospheric trajectory calculations. A method is presented to consistently estimate the uncertainty in trajectory calculations. It was concluded that the spatial and temporal trajectory error can be substantially underestimated if the analysis error is not taken into account.
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| 9. |
- Engström, Anders, et al.
(författare)
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Impact of meteorological factors on the correlation between aerosol optical depth and cloud fraction
- 2010
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 37, s. L18814-
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Tidskriftsartikel (refereegranskat)abstract
- The aerosol optical depth has in several recent studies been found to correlate with cloud fraction. This study examines the global distribution of the total correlation between aerosol optical depth, cloud fraction and meteorological conditions using satellite observations together with atmospheric re-analysis data from the ECMWF. The results show large regional differences in the correlation between aerosol optical depth and cloud fraction, where a higher correlation is found over remote ocean. The one meteorological variable that correlates significantly with both aerosol optical depth and cloud fraction is the 10-meter wind speed. Constructing the partial correlation between aerosol optical depth and cloud fraction, with the impact from 10-meter wind speed removed, yields a significant difference compared to the total correlation. In several regions the remaining partial correlation is reduced from 0.4 to below 0.1. The results highlight the need to investigate all possible correlations between meteorological variables, cloud properties and aerosols. Citation: Engstrom, A., and A. M. L. Ekman (2010), Impact of meteorological factors on the correlation between aerosol optical depth and cloud fraction
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| 10. |
- Kim, Dongchul, et al.
(författare)
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The responses of cloudiness to the direct radiative effect of sulfate and carbonaceous aerosols
- 2014
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Ingår i: Journal of Geophysical Research: Atmospheres. - 2169-897X. ; 119:3, s. 1172-1185
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Tidskriftsartikel (refereegranskat)abstract
- This study investigates the responses of the direct radiative effect of light absorbing and scattering carbonaceous and sulfate aerosols on cloudiness and associated radiative fluxes using an interactive aerosol-climate model coupled with a slab ocean model. We find that without including the impact of aerosols on cloud microphysics in the model (indirect effect), the direct radiative effect of aerosols alone can cause a change in cloud coverage and thus in cloud flux change which is consistent with several previous studies. More notably, our result indicates that the direct radiative effect of absorbing aerosols can cause changes in both low-level and high-level clouds with opposite signs. As a result, the global mean cloud radiation response to absorbing aerosols has a rather small value. The change of cloud solar radiative response (all-sky effect minus clear-sky effect) at the top of the atmosphere due to the existence of direct radiative effect of scattering, absorbing, and both types of aerosols is 0.72, 0.08, and 0.81Wm(-2), respectively, all are comparable in quantity to the current estimation of aerosol direct radiative forcing. The cloud response due to the longwave radiative effect is 0.09, 0.18, and 0.27Wm(-2), respectively. The global means of the radiative flux and cloud radiative responses appear to be linearly additive; however, this is definitely not the case for the zonal mean or at the regional scale. Key Points The effect of absorbing and scattering aerosols with an aerosol-climate model Cloud responses on the direct radiative are examined Nonlinearity from absorbing and scattering aerosols exists
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