An Observation and CRM Based Analysis of Large Scale Aerosol-Convection Interaction in the Tropics

Aerosol effects on cloud albedo and lifetime in tropical cumulus, stratocumulus, and isolated deep convective clouds have been extensively studied in recent years. However, the majority of tropical precipitation and cloud radiative forcing is produced by organized clusters of convective systems. Deconvolving the effect of aerosol on tropical MCSs is challenging because these cloud systems interact strongly with their dynamic and thermodynamic environment. Recent idealized experiments indicate aerosols exert a non-trivial influence on both the properties of organized deep convection and their interaction with the surrounding environment.

In this presentation, we describe results of combined observational- and cloud-resolving model-based studies of the aerosol influence on deep convective cloud systems. An integrated analysis of hundreds of thousands of clouds over the Indian Ocean and South Asia during the winter and summer monsoons is performed using multiple NASA satellite and reanalysis data sets. The results indicate 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. Systematic differences in cloud system spatial cover are found between cloud systems developing in clean and polluted environments.

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 of the Goddard Cumulus Ensemble cloud-resolving model. In these simulations, the concentration of cloud condensation nuclei are varied, and the output converted to simulated MODIS and AMSR-E brightness temperatures using the Goddard Satellite Data Simulation Unit. Simulated satellite observations are processed in an identical manner as the real-world observations and the results are used to (1) determine whether numerical cloud models and observations exhibit consistent aerosol sensitivity and (2) explore the mechanisms behind the observed differences between clean and polluted environments.