Examining aerosol indirect effects on tropical deep convection

Aerosols are known to have both direct and indirect effects on clouds through their role as cloud condensation nuclei (ccn). These effects are currently not fully understood, but are known to depend on cloud type and environment. Especially challenging is the problem of understanding the full impact of aerosols on deep convective clouds. These clouds prove more complex in their response due to the presence of both liquid water and ice. Aerosol indirect theory proposes that increasing the number of ccn can lead to a reduction in precipitation due to a less efficient warm rain process. However, past studies of deep convective clouds suggest that the interaction with the ice phase may promote convective invigoration, which in some cases can lead to an increase, rather than a decrease, in precipitation. This study seeks to examine the differing effects of aerosols on the liquid and ice phase of deep convective clouds, in order to better understand the effect of aerosols on precipitation

A series of model simulations have been conducted under a radiative-convective equilibrium framework, which has been utilized in the past to successfully simulate the tropics. These model simulations are large-scale, three-dimensional channel runs (approximately 5,000 km x 250 km) using a cloud resolving model with 1 km spacing in the horizontal and variable vertical grid spacing. The model being used is the Regional Atmospheric Modeling System (RAMS), which allows for the prognosis of ccn number concentration based on background aerosol concentration and various environmental conditions. The set of simulations differ only in the background aerosol concentration, so as to simulate the response of tropical convection to changes in the amount of aerosols that are available to act as ccn. The typical tri-modal cloud distribution that has been observed in the tropics is well represented in these model simulations, but given the focus of this study, only deep convective clouds are considered.

Preliminary findings have shown significant differences resulting from increases in aerosols available to act as ccn. Deep convective clouds forming in polluted simulations have increased cloud water content, likely due to a decrease in warm rain efficiency. The larger available cloud water content allows an increase in riming efficiency, and so more graupel can be found within the clouds. Some evidence exists for convective invigoration, or enhanced updrafts due to an increase in latent heat release, but the full response is not yet fully understood. These findings will be investigated in more detail by analyzing aspects such as the latent heating profiles through these deep convective systems and examining specific microphysical processes related to precipitation formation. The results of these analyses will be presented.