Radiative forcing by aerosol modification of deep convective cloud cover: An evaluation with cloud-resolving simulations and satellite observations

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Wednesday, 5 February 2014: 11:15 AM
Room C207 (The Georgia World Congress Center )
Eric M. Wilcox, DRI, Reno, NV; and D. J. Posselt and T. Yuan

It has been suggested that the modification of deep convective clouds by aerosols can impact the spatial coverage of anvil and cirrus clouds detraining in the upper-troposphere. This has the potential to significantly modify radiative forcing in the tropics owing to the large optical thickness and strong greenhouse effect of tropical clouds. Our integrated analysis of multiple NASA satellite and reanalysis data sets reveals strong systematic relationships among organized deep convective cloud system size and structure, and the convective available potential energy (CAPE) and vertical shear of horizontal wind in the convective environment. These relationships are established based on a database of hundreds of thousands of clouds over the Indian Ocean and South Asia during the winter and summer monsoons. Systematic differences in cloud system spatial cover are found between cloud system developing in clean and polluted environments. The radiative forcing at the top of the atmosphere owing to these differences is evaluated using NASA CERES radiative flux observations. However, cloud system spatial coverage is far more sensitive to CAPE and shear than it is to variations in aerosol optical thickness, therefore our methodology carefully controls for this sensitivity when evaluating the aerosol impact. The observed sensitivity of cloud system size and structure to variations in aerosol load are compared with a similar database of thousands of clouds generated with season-long, large-domain integrations with the Goddard Cumulus Ensemble cloud-resolving model where the concentration of cloud condensation nuclei are systematically varied. The Goddard Satellite Data Simulation Unit is applied to the model output to simulate the MODIS and AMSR-E brightness temperatures used to evaluate cloud system size and structure. Using this approach we apply the exact same analysis methodology to the model as is applied to satellite data. This presentation will evaluate whether numerical cloud models and observations exhibit any consistency in the sensitivity of deep convective cloud coverage and radiative forcing to variations in aerosols in the tropics.