Simulations of cumulus clouds using a spectral microphysics cloud‐resolving model

Abstract

We have investigated the effects of aerosols on the development of cumulus clouds using a two‐dimensional spectral‐bin cloud‐resolving model. A convective cloud event occurring on 24 August 2000 in Houston, Texas, was simulated and the model results were compared with available radar and rain gauge measurements. Simulations assuming different aerosol chemical compositions were conducted to examine the impacts on cumulus development. The cloud microphysical and macrophysical properties changed considerably with the aerosol chemical properties. With varying the aerosol composition from only (NH4)2SO4, (NH4)2SO4 with soluble organics, to (NH4)2SO4 with slightly soluble organics, the number of activated aerosols in cloud decreased accordingly, leading to a decrease in the cloud droplet number concentration and an increase in the droplet size. Increasing activated aerosols resulted in the increase of ice crystal formation by homogeneous freezing, more extensive riming, lower supersaturation (Sw and Sice), less efficient growth of graupel, and more melting precipitation. Ice microphysical processes were more sensitive to the changes of aerosol chemical properties than the warm rain processes. The changes in macrophysical properties were more evident: The increase of activated aerosols resulted in longer cell lifetime, larger cell size, stronger secondary convective cell, and more accumulated precipitation. The simulation with the aerosol composition of (NH4)2SO4 with slightly soluble organics and an activation scheme of a reformulation of the Köhler theory to include the effect of slightly soluble organics and soluble HNO3 agreed well with the observations. The simulation captured the major convective cell observed from the field measurements. The predicted convective cell intensity, cell size, cell lifetime, and accumulated rain were in agreement with the observations.