Multicellular Deep Convection Sensitivity to Aerosols and Midlevel Dry Layers

The importance of environmental parameters, such as humidity, vertical wind shear, and instability, in modulating the sensitivity of deep convective precipitation to aerosol loading has been demonstrated in several recent studies. Favorable conditions for midlatitude deep convection, such as may occur near the dryline or in the high plains of the United States, are often characterized by midlevel dry layers and enhanced aerosol concentrations. The goal of this research is therefore to investigate the sensitivity of multicellular deep convection both to midlevel dry layers and to aerosol loading. This goal is addressed through the use of idealized cloud-resolving model (CRM) simulations conducted with the Regional Atmospheric Modeling System (RAMS), a CRM containing a sophisticated double moment bin-emulating bulk microphysical scheme and budget tracking of microphysical process rates.

A common storm splitting situation is simulated wherein the left-moving storm evolves into a multicellular cluster and the right-moving storm becomes a supercell. The left-mover is the focus of this study, as it is more sensitive than the right-mover to both midlevel dryness and aerosol perturbations. The results demonstrate that, while midlevel dryness has a larger impact on the multicellular precipitation than do aerosols, the precipitation may either increase or decrease with increased aerosol loading, depending on the height of the dry layer. This is found to result from changes in both the cloud and rain microphysical properties, which impacts the latent cooling rates and cold pool strength. Enhanced aerosol concentrations result in (1) more numerous, smaller cloud droplets that evaporate more efficiently; and (2) fewer, larger raindrops that evaporate less efficiently. Such microphysical changes have competing effects on the latent cooling rates, and the net effect is found to depend on the altitude of the dry layer. The resultant feedbacks to the strength of the downdrafts, cold pool, associated cold pool dynamical forcing, and subsequent secondary convection, are quantified and will be presented.