scholarly journals A new method for evaluating the impact of vertical distribution on aerosol radiative forcing in general circulation models

2013 ◽  
Vol 13 (7) ◽  
pp. 18809-18853
Author(s):  
M. R. Vuolo ◽  
M. Schulz ◽  
Y. Balkanski ◽  
T. Takemura
Keyword(s):  
Vertical Distribution ◽  
Radiative Forcing ◽  
General Circulation ◽  
Cloud Amount ◽  
General Circulation Models ◽  
Individual Species ◽  
Clear Sky ◽  
Atmospheric Column ◽  
Aerosol Forcing ◽  
The Impact

Abstract. The quantification and understanding of direct aerosol forcing is essential in the study of climate. One of the main issues that makes its quantification difficult is the lack of a complete comprehension of the role of the aerosol and clouds vertical distribution. This work aims at reducing the incertitude of aerosol forcing due to the vertical superposition of several short-lived atmospheric components, in particular different aerosols species and clouds. We propose a method to quantify the contribution of different parts of the atmospheric column to the forcing, and to evaluate model differences by isolating the effect of radiative interactions only. Any microphysical or thermo-dynamical interactions between aerosols and clouds are deactivated in the model, to isolate the effects of radiative flux coupling. We investigate the contribution of aerosol above, below and in clouds, by using added diagnostics in the aerosol-climate model LMDz. We also compute the difference between the forcing of the ensemble of the aerosols and the sum of the forcings from individual species, in clear-sky. This difference is found to be moderate on global average (14%) but can reach high values regionally (up to 100%). The non-additivity of forcing already for clear-sky conditions shows, that in addition to represent well the amount of individual aerosol species, it is critical to capture the vertical distribution of all aerosols. Nonlinear effects are even more important when superposing aerosols and clouds. Four forcing computations are performed, one where the full aerosol 3-D distribution is used, and then three where aerosols are confined to regions above, inside and below clouds respectively. We find that the forcing of aerosols depends crucially on the presence of clouds and on their position relative to that of the aerosol, in particular for black carbon (BC). We observe a strong enhancement of the forcing of BC above clouds, attenuation for BC below clouds, and a moderate enhancement when BC is found within clouds. BC forcing efficiency amounts to 44, 171, 333 and 178 W m-2 per unit optical depth for BC below, within, above clouds and for the 3-D BC distribution, respectively. The different behaviour of forcing nonlinearities for these three components of the atmospheric column suggests that, an important reason for differences between cloudy-sky aerosol forcings from different models may come from different aerosol and clouds vertical distributions. Our method allows to evaluate the contribution to model differences due to aerosol and clouds radiative interactions only, by reading 3-D aerosol and cloud fields from different GCMs, into the same model. This method avoids differences in calculating optical aerosol properties and forcing to enter into the discussion of inter-model differences. It appears that the above and in-cloud amount of BC is larger for SPRINTARS (190 compared to 179), increasing its cloudy-sky forcing efficiency with respect to LMDz, being thus potentially an important factor for inter-model differences.

scholarly journals A new method for evaluating the impact of vertical distribution on aerosol radiative forcing in general circulation models

2014 ◽  
Vol 14 (2) ◽  
pp. 877-897 ◽  
Author(s):  
M. R. Vuolo ◽  
M. Schulz ◽  
Y. Balkanski ◽  
T. Takemura
Keyword(s):  
Vertical Distribution ◽  
Radiative Forcing ◽  
General Circulation ◽  
Climate Model ◽  
Nonlinear Effects ◽  
General Circulation Models ◽  
Individual Species ◽  
Aerosol Radiative Forcing ◽  
Atmospheric Column ◽  
The Impact

Abstract. The quantification and understanding of direct aerosol forcing is essential in the study of climate. One of the main issues that makes its quantification difficult is the lack of a complete understanding of the role of the vertical distribution of aerosols and clouds. This work aims at reducing the uncertainty of aerosol top-of-the-atmosphere (TOA) forcing due to the vertical superposition of several short-lived atmospheric components, in particular different aerosol species and clouds. We propose a method to quantify the contribution of different parts of the atmospheric column to the TOA forcing as well as to evaluate the contribution to model differences that is exclusively due to different spatial distributions of aerosols and clouds. We investigate the contribution of aerosol above, below and in clouds by using added diagnostics in the aerosol–climate model LMDz. We also compute the difference between the TOA forcing of the ensemble of the aerosols and the sum of the forcings from individual species in clear sky. This difference is found to be moderate for the global average (14%) but can reach high values regionally (up to 100%). Nonlinear effects are even more important when superposing aerosols and clouds. Four forcing computations are performed: one where the full aerosol 3-D distribution is used, and then three where aerosols are confined to regions above, inside and below clouds, respectively. We find that the TOA forcing of aerosols depends crucially on the presence of clouds and on their position relative to that of the aerosol, in particular for black carbon (BC). We observe a strong enhancement of the TOA forcing of BC above clouds, attenuation for BC below clouds, and a moderate enhancement when BC is found within clouds. BC above clouds accounts for only about 30% of the total BC optical depth but for 55% of the forcing, while forcing efficiency increases by a factor of 7.5 when passing from below to above clouds. The different behaviour of forcing nonlinearities for these three components of the atmospheric column encouraged us to develop the method for application to inter-model variability studies by reading 3-D aerosol and cloud fields from different general circulation models (GCMs) into the same model. We apply the method to the comparison of forcing due to the aerosols and clouds distributions of the general circulation models LMDz and SPRINTARS. The different amount of BC above but also within clouds is revealed to play a major role on the differences of cloudy-sky forcings between the two models, which can exceed 100% regionally.

scholarly journals Large methane releases lead to strong aerosol forcing and reduced cloudiness

2011 ◽  
Vol 11 (3) ◽  
pp. 9057-9081
Author(s):  
T. Kurtén ◽  
L. Zhou ◽  
R. Makkonen ◽  
J. Merikanto ◽  
P. Räisänen ◽  
...  
Keyword(s):  
Radiative Forcing ◽  
General Circulation ◽  
Cloud Droplet ◽  
Fold Increase ◽  
General Circulation Models ◽  
Large Temperature ◽  
Atmospheric Lifetime ◽  
Temperature Changes ◽  
Aerosol Forcing ◽  
Rapid Amplification

Abstract. The release of vast quantities of methane into the atmosphere as a result of clathrate destabilization is a potential mechanism for rapid amplification of global warming. Previous studies have calculated the enhanced warming based mainly on the radiative effect of the methane itself, with smaller contributions from the associated carbon dioxide or ozone increases. Here, we study the effect of strongly elevated methane (CH4) levels on oxidant and aerosol particle concentrations using a combination of chemistry-transport and general circulation models. A 10-fold increase in methane concentrations is predicted to significantly decrease hydroxyl radical (OH) concentrations, while moderately increasing ozone (O3). These changes lead to a 70% increase in the atmospheric lifetime of methane, and an 18% decrease in global mean cloud droplet number concentrations (CDNC). The CDNC change causes a radiative forcing that is comparable in magnitude to the longwave radiative forcing ("enhanced greenhouse effect") of the added methane. Together, the indirect CH4-O3 and CH4-OH-aerosol forcings could more than double the warming effect of large methane increases. Our findings may help explain the anomalously large temperature changes associated with historic methane releases.

Diagnosing Cloud Feedbacks in General Circulation Models

2007 ◽  
Vol 20 (11) ◽  
pp. 2602-2622 ◽  
Author(s):  
Ping Zhu ◽  
James J. Hack ◽  
Jeffrey T. Kiehl
Keyword(s):  
Radiative Forcing ◽  
General Circulation ◽  
Cloud Amount ◽  
General Circulation Models ◽  
Cloud Feedback ◽  
Radiative Fluxes ◽  
Cloud Feedbacks ◽  
Flux Change ◽  
Gfdl Model ◽  
The Individual

Abstract In this study, it is shown that the NCAR and GFDL GCMs exhibit a marked difference in climate sensitivity of clouds and radiative fluxes in response to doubled CO2 and ±2-K SST perturbations. The GFDL model predicted a substantial decrease in cloud amount and an increase in cloud condensate in the warmer climate, but produced a much weaker change in net cloud radiative forcing (CRF) than the NCAR model. Using a multiple linear regression (MLR) method, the full-sky radiative flux change at the top of the atmosphere was successfully decomposed into individual components associated with the clear sky and different types of clouds. The authors specifically examined the cloud feedbacks due to the cloud amount and cloud condensate changes involving low, mid-, and high clouds between 60°S and 60°N. It was found that the NCAR and GFDL models predicted the same sign of individual longwave and shortwave feedbacks resulting from the change in cloud amount and cloud condensate for all three types of clouds (low, mid, and high) despite the different cloud and radiation schemes used in the models. However, since the individual longwave and shortwave feedbacks resulting from the change in cloud amount and cloud condensate generally have the opposite signs, the net cloud feedback is a subtle residual of all. Strong cancellations between individual cloud feedbacks may result in a weak net cloud feedback. This result is consistent with the findings of the previous studies, which used different approaches to diagnose cloud feedbacks. This study indicates that the proposed MLR approach provides an easy way to efficiently expose the similarity and discrepancy of individual cloud feedback processes between GCMs, which are hidden in the total cloud feedback measured by CRF. Most importantly, this method has the potential to be applied to satellite measurements. Thus, it may serve as a reliable and efficient method to investigate cloud feedback mechanisms on short-term scales by comparing simulations with available observations, which may provide a useful way to identify the cause for the wide spread of cloud feedbacks in GCMs.

scholarly journals Relative Humidity in the Troposphere with AIRS

2014 ◽  
Vol 71 (7) ◽  
pp. 2516-2533 ◽  
Author(s):  
Alexander Ruzmaikin ◽  
Hartmut H. Aumann ◽  
Evan M. Manning
Keyword(s):  
Relative Humidity ◽  
Surface Temperature ◽  
Radiative Forcing ◽  
General Circulation ◽  
Large Scale ◽  
Statistical Significance ◽  
General Circulation Models ◽  
Positive Trend ◽  
Drying Time ◽  
Clear Sky

Abstract New global satellite data from the Atmospheric Infrared Sounder (AIRS) are applied to study the tropospheric relative humidity (RH) distribution and its influence on outgoing longwave radiation (OLR) for January and July in 2003, 2007, and 2011. RH has the largest maxima over 90% in the equatorial tropopause layer in January. Maxima in July do not arise above 60%. Seasonal variations of about 20% in zonally averaged RH are observed in the equatorial region of the low troposphere, in the equatorial tropopause layer, and in the polar regions. The seasonal variability in the recent decade has increased by about 5% relative to that in 1973–88, indicating a positive trend. The observed RH profiles indicate a moist bias in the tropical and subtropical regions typically produced by the general circulation models. The new data and method of evaluating the statistical significance of bimodality confirm bimodal probability distributions of RH at large tropospheric scales, notably in the ascending branch of the Hadley circulation. Bimodality is also seen at 500–300 hPa in mid- and high latitudes. Since the drying time of the air is short compared with the mixing time of moist and dry air, the bimodality reflects the large-scale distribution of sources of moisture and the atmospheric circulation. Analysis of OLR dependence on surface temperature shows a 0.2 W m−2 K−1 difference in sensitivities between clear-sky and all-sky OLR, indicating a positive longwave cloud radiative forcing. Diagrams of the clear-sky OLR as functions of percentiles of surface temperature and relative humidity in the tropics are designed to provide a new measure of the supergreenhouse effect.

scholarly journals A global satellite view of aerosol cloud interactions

2004 ◽  
Vol 4 (5) ◽  
pp. 6823-6836 ◽  
Author(s):  
C. Luo
Keyword(s):  
North Africa ◽  
North Atlantic ◽  
Radiative Forcing ◽  
General Circulation ◽  
Large Scale ◽  
Regional Scale ◽  
Cloud Amount ◽  
General Circulation Models ◽  
Aerosol Cloud ◽  
Aerosol Cloud Interactions

Abstract. Long-term and large-scale correlations between Advanced Very High-Resolution Radiometer (AVHRR) aerosol optical depth and International Satellite Cloud Climatology Project (ISCCP) monthly cloud amount data show significant regional scale relationships between cloud amount and aerosols, consistent with aerosol-cloud interactions. Positive correlations between aerosols and cloud amount are associated with North American and Asian aerosols in the North Atlantic and Pacific storm tracks, and mineral aerosols in the tropical North Atlantic. Negative correlations are seen near biomass burning regions of North Africa and Indonesia, as well as south of the main mineral aerosol source of North Africa. These results suggest that there are relationships between aerosols and clouds in the observations that can be used by general circulation models to verify the correct forcing mechanisms for both direct and indirect radiative forcing by clouds.

scholarly journals Global and regional radiative forcing from 20 % reductions in BC, OC and SO<sub>4</sub> – an HTAP2 multi-model study

2016 ◽  
Author(s):  
Camilla Weum Stjern ◽  
Bjørn Hallvard Samset ◽  
Gunnar Myhre ◽  
Huisheng Bian ◽  
Mian Chin ◽  
...  
Keyword(s):  
East Asia ◽  
Radiative Forcing ◽  
General Circulation ◽  
Anthropogenic Aerosols ◽  
Aerosol Forcing ◽  
Source Regions ◽  
Model Study ◽  
Range Transport ◽  
Emission Changes ◽  
The Impact

Abstract. In the Hemispheric Transport of Air Pollution Phase 2 (HTAP) exercise, a range of global atmospheric general circulation and chemical transport models performed coordinated perturbation experiments with 20 % reductions in emissions of anthropogenic aerosols, or aerosol precursors, in a number of source regions. Here, we compare the resulting changes in the atmospheric load and vertically resolved profiles of black carbon (BC), organic aerosols (OA) and sulfate (SO4) from 10 models that include treatment of aerosols. We use a set of temporally, horizontally and vertically resolved profiles of aerosol forcing efficiency (AFE) to estimate the impact of emission changes in six major source regions on global radiative forcing (RF) pertaining to the direct aerosol effect. Results show that mitigations in South and East Asia have substantial impacts on the radiative budget in all investigated receptor regions, especially for BC. In Russia and the Middle East, more than 80 % of the forcing for BC and OA is due to extra-regional emission reductions. Similarly, for North America, BC emissions control in East Asia is found to be more important than domestic mitigations, which is consistent with previous findings. Comparing fully resolved RF calculations to RF estimates based on vertically averaged AFE profiles allows us to quantify the importance of vertical resolution to RF estimates. We find that locally in the source regions, a 20 % emission reduction strengthens the radiative forcing associated with SO4 by 25 % when including the vertical dimension, as the AFE for SO4 is strongest near the surface. Conversely, the local RF from BC weakens by 37 % since BC AFE is low close to the ground. The influence of inter-continental transport on BC forcing, however, is enhanced by one third when accounting for the vertical aspect, because long-range transport leads primarily to aerosol changes at high altitudes, where the BC AFE is strong.

scholarly journals Global and regional radiative forcing from 20 % reductions in BC, OC and SO<sub>4</sub> – an HTAP2 multi-model study

2016 ◽  
Vol 16 (21) ◽  
pp. 13579-13599 ◽  
Author(s):  
Camilla Weum Stjern ◽  
Bjørn Hallvard Samset ◽  
Gunnar Myhre ◽  
Huisheng Bian ◽  
Mian Chin ◽  
...  
Keyword(s):  
East Asia ◽  
Radiative Forcing ◽  
General Circulation ◽  
Temperature Response ◽  
Anthropogenic Aerosols ◽  
Aerosol Forcing ◽  
Source Regions ◽  
Intercontinental Transport ◽  
Emission Changes ◽  
The Impact

Abstract. In the Hemispheric Transport of Air Pollution Phase 2 (HTAP2) exercise, a range of global atmospheric general circulation and chemical transport models performed coordinated perturbation experiments with 20 % reductions in emissions of anthropogenic aerosols, or aerosol precursors, in a number of source regions. Here, we compare the resulting changes in the atmospheric load and vertically resolved profiles of black carbon (BC), organic aerosols (OA) and sulfate (SO4) from 10 models that include treatment of aerosols. We use a set of temporally, horizontally and vertically resolved profiles of aerosol forcing efficiency (AFE) to estimate the impact of emission changes in six major source regions on global radiative forcing (RF) pertaining to the direct aerosol effect, finding values between. 51.9 and 210.8 mW m−2 Tg−1 for BC, between −2.4 and −17.9 mW m−2 Tg−1 for OA and between −3.6 and −10.3 W m−2 Tg−1 for SO4. In most cases, the local influence dominates, but results show that mitigations in south and east Asia have substantial impacts on the radiative budget in all investigated receptor regions, especially for BC. In Russia and the Middle East, more than 80 % of the forcing for BC and OA is due to extra-regional emission reductions. Similarly, for North America, BC emissions control in east Asia is found to be more important than domestic mitigations, which is consistent with previous findings. Comparing fully resolved RF calculations to RF estimates based on vertically averaged AFE profiles allows us to quantify the importance of vertical resolution to RF estimates. We find that locally in the source regions, a 20 % emission reduction strengthens the radiative forcing associated with SO4 by 25 % when including the vertical dimension, as the AFE for SO4 is strongest near the surface. Conversely, the local RF from BC weakens by 37 % since BC AFE is low close to the ground. The fraction of BC direct effect forcing attributable to intercontinental transport, on the other hand, is enhanced by one-third when accounting for the vertical aspect, because long-range transport primarily leads to aerosol changes at high altitudes, where the BC AFE is strong. While the surface temperature response may vary with the altitude of aerosol change, the analysis in the present study is not extended to estimates of temperature or precipitation changes.

scholarly journals The Precipitation Response to an Idealized Subtropical Continent

2016 ◽  
Vol 29 (12) ◽  
pp. 4543-4564 ◽  
Author(s):  
Elizabeth A. Maroon ◽  
Dargan M. W. Frierson ◽  
Sarah M. Kang ◽  
Jacob Scheff
Keyword(s):  
General Circulation ◽  
Hadley Circulation ◽  
General Circulation Models ◽  
Atmospheric Model ◽  
Opposite Hemisphere ◽  
Clear Sky ◽  
Tropical Precipitation ◽  
Precipitation Response ◽  
Atmospheric General Circulation Models ◽  
The Impact

Abstract A subtropical continent is added to two aquaplanet atmospheric general circulation models (AGCMs) to better understand the influence of land on tropical circulation and precipitation. The first model, the gray-radiation moist (GRaM) AGCM, has simplified physics, while the second model, the GFDL Atmospheric Model version 2.1 (AM2.1), is a fully comprehensive AGCM. Both models have a continent that is 60° wide in longitude from 10° to 30°N, in an otherwise slab-ocean-covered world. The precipitation response varies with cloudy- and clear-sky feedbacks and depends on continental albedo. In GRaM simulations with a continent, precipitation in the Northern Hemisphere decreases mostly as a result of decreased evaporation. In AM2.1 simulations, precipitation also shifts southward via Hadley circulation changes due to increasing albedo, but the radiative impact of clouds and moisture creates a more complex response. Results are similar when a seasonal cycle of insolation is included in AM2.1 simulations. The impact of a large, bright subtropical continent is to shift precipitation to the opposite hemisphere. In these simulations, the hemisphere of greater tropical precipitation is better predicted by the hemisphere with greater atmospheric energy input, as has been shown in previous literature, rather than the hemisphere that has higher surface temperature.

scholarly journals Large methane releases lead to strong aerosol forcing and reduced cloudiness

2011 ◽  
Vol 11 (14) ◽  
pp. 6961-6969 ◽  
Author(s):  
T. Kurtén ◽  
L. Zhou ◽  
R. Makkonen ◽  
J. Merikanto ◽  
P. Räisänen ◽  
...  
Keyword(s):  
Radiative Forcing ◽  
General Circulation ◽  
Cloud Droplet ◽  
Fold Increase ◽  
General Circulation Models ◽  
Large Temperature ◽  
Atmospheric Lifetime ◽  
Temperature Changes ◽  
Aerosol Forcing ◽  
Rapid Amplification

Abstract. The release of vast quantities of methane into the atmosphere as a result of clathrate destabilization is a potential mechanism for rapid amplification of global warming. Previous studies have calculated the enhanced warming based mainly on the radiative effect of the methane itself, with smaller contributions from the associated carbon dioxide or ozone increases. Here, we study the effect of strongly elevated methane (CH4) levels on oxidant and aerosol particle concentrations using a combination of chemistry-transport and general circulation models. A 10-fold increase in methane concentrations is predicted to significantly decrease hydroxyl radical (OH) concentrations, while moderately increasing ozone (O3). These changes lead to a 70 % increase in the atmospheric lifetime of methane, and an 18 % decrease in global mean cloud droplet number concentrations (CDNC). The CDNC change causes a radiative forcing that is comparable in magnitude to the longwave radiative forcing ("enhanced greenhouse effect") of the added methane. Together, the indirect CH4-O3 and CH4-OH-aerosol forcings could more than double the warming effect of large methane increases. Our findings may help explain the anomalously large temperature changes associated with historic methane releases.

scholarly journals Mitigation of Non-CO2 Aviation’s Climate Impact by Changing Cruise Altitudes

Aerospace ◽  
2021 ◽  
Vol 8 (2) ◽  
pp. 36
Author(s):  
Sigrun Matthes ◽  
Ling Lim ◽  
Ulrike Burkhardt ◽  
Katrin Dahlmann ◽  
Simone Dietmüller ◽  
...  
Keyword(s):  
Co2 Emissions ◽  
Temperature Change ◽  
Radiative Forcing ◽  
General Circulation ◽  
General Circulation Models ◽  
Sensitivity Study ◽  
Climate Impact ◽  
Flight Altitude ◽  
Co2 Effects ◽  
The Impact

Aviation is seeking for ways to reduce its climate impact caused by CO2 emissions and non-CO2 effects. Operational measures which change overall flight altitude have the potential to reduce climate impact of individual effects, comprising CO2 but in particular non-CO2 effects. We study the impact of changes of flight altitude, specifically aircraft flying 2000 feet higher and lower, with a set of global models comprising chemistry-transport, chemistry-climate and general circulation models integrating distinct aviation emission inventories representing such alternative flight altitudes, estimating changes in climate impact of aviation by quantifying radiative forcing and induced temperature change. We find in our sensitivity study that flying lower leads to a reduction of radiative forcing of non-CO2 effects together with slightly increased CO2 emissions and impacts, when cruise speed is not modified. Flying higher increases radiative forcing of non-CO2 effects by about 10%, together with a slight decrease of CO2 emissions and impacts. Overall, flying lower decreases aviation-induced temperature change by about 20%, as a decrease of non-CO2 impacts by about 30% dominates over slightly increasing CO2 impacts assuming a sustained emissions scenario. Those estimates are connected with a large but unquantified uncertainty. To improve the understanding of mechanisms controlling the aviation climate impact, we study the geographical distributions of aviation-induced modifications in the atmosphere, together with changes in global radiative forcing and suggest further efforts in order to reduce long standing uncertainties.

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