Aerosol Impacts on Deep Convective Processes using Bulk and Bin Microphysical Schemes

The updrafts, anvils and cold pools of deep convective storms are all potentially impacted by variations in aerosol concentrations. Suppression of warm rain processes under environments with higher aerosol conditions reduces the initial loss of liquid water through precipitation, and thereby enhances the vertical advection of greater amounts of liquid water above the freezing level. This strengthens the updraft through the buoyancy response to the increased latent heat released by the freezing of the larger lofted liquid water mass. The additional liquid water and the size distribution of the liquid water species in the mid and upper levels of the storm will influence the mass and types of ice species in the detraining convective anvil, and hence the anvil cloud radiative forcing. Finally, aerosol-induced impacts on the raindrop size distributions influences evaporation rates, and hence the strength of the evaporatively-generated cold pool.

In order to represent aerosol impacts on deep convection, accurate parameterizations of such processes are needed within cloud-resolving models (CRMs). Microphysical schemes currently available in CRMs vary from more simple but computationally efficient bulk microphysical schemes in which the shape of the hydrometeor size distribution is typically constant throughout the simulation, to more complex but computationally intensive spectral bin microphysical schemes in which the hydrometeor size distribution is able to vary in time. Given the influence of aerosols on hydrometeor size distributions, the utilization of a bulk or bin scheme might significantly influence the representation of aerosol influences on deep convective storm processes and subsequent feedbacks. The goals of this research are therefore two-fold and include: (1) to enhance our understanding of aerosol impacts on deep convective updrafts, anvils and cold pools; and (2) to assess the use of simple through more complex microphysical schemes in representing these effects. These goals will be achieved through the use of the Regional Atmospheric Modeling System (RAMS), which includes single- and two-moment bulk schemes, as well as the recently implemented HUCM spectral bin model. Idealized supercell simulations will be performed for clean and polluted conditions, using the single and two-moment bulk schemes and the bin scheme. The simulations will be initialized using a typical environmental sounding supportive of supercells, and will utilize a horizontal grid spacing of 500m, variable grid spacing in the vertical, and a grid domain of 200 by 200 km. While aerosol concentrations are prognosed when using the two-moment bulk and spectral bin schemes, this capability does not exist for single-moment schemes. Therefore, variations in cloud droplet number concentrations will be prescribed in the single-moment scheme in order to represent aerosol effects. The sensitivity of the deep convective updraft, anvil characteristics, cold pool intensity and surface precipitation to the scheme complexity and the aerosol loading will be presented.