Impacts of aerosols on cloud system-resolving model simulations of tropical deep convection during TWP-ICE

Description

This presentation will summarize results from several-hundred-member ensemble simulations of aerosol indirect effects on tropical deep convection and associated outflow cirrus clouds. A two-dimensional cloud system-resolving model with a sophisticated two-moment liquid and ice microphysics scheme is used. Simulations are performed with aerosol loadings representing either pristine or polluted conditions and applying forcing data from a 16-day period of monsoon conditions during the 2006 Tropical Warm Pool-International Cloud Experiment (TWP-ICE). Aerosols are shown to have little impact on the atmospheric storage of water and static energy tropospheric as well as tropospheric destabilization, and therefore, surface precipitation is practically insensitive to aerosols given the prescribed large-scale forcing and SST. The spread of the TOA radiative fluxes among different ensemble members for the same aerosol loading is surprisingly large, exceeding 30 W/m2 even when averaged over the entire 16-day period. The ensemble approach demonstrates that there is a statistically significant small weakening of convection in the polluted simulations compared to pristine, in contrast to the hypothesis of convective invigoration as a result of aerosol loading. Despite this weakening, the cloud-top heights and ice water paths are higher in polluted conditions because of smaller ice particle sizes and thus smaller fall velocities. Such a conclusion offers a different interpretation of recent satellite observations of tropical deep convection in pristine and polluted environments. A more extensive analysis of satellite data (CALIPSO, CLOUDSAT, and MODIS) shows that aerosol impacts are significantly smaller than suggested by previous studies.