Effects of aerosols on the dynamics and microphysics of squall lines simulated by spectral bin and bulk parameterization schemes

Abstract

A new spectral bin microphysical scheme (SBM) was implemented into the Weather Research and Forecasting model referred to as Fast‐SBM, which uses a smaller number of size distribution functions than the original version of the scheme referred to as Exact‐SBM. It was shown that both schemes produced similar dynamical and microphysical structure of a squall line simulated. An excellent agreement in the simulated precipitation amounts between the schemes was found within a range of cloud condensation nuclei concentrations from 100 to 3000 cm−3. The Fast‐SBM requires about 40% of the computing power of the Exact‐SBM, allowing it to be used for “real‐time” simulations over limited domains. The results obtained using the SBM simulations have been compared with those using a modified version of the Thompson bulk parameterization scheme. The main extension of the bulk scheme was the implementation of the process of drop nucleation, so that drop concentration is no longer prescribed a priori but rather calculated using the prescribed aerosol concentration. This scheme is referred to as the Drop scheme. A large set of sensitivity studies have been performed, in which microphysical parameters and precipitation, droplet nucleation above cloud base, etc., have been compared with those obtained from SBM. The SBM scheme produces more realistic dynamical and microphysical structure of the squall line. The Drop scheme did relatively little to change the cloud structures simulated by the bulk scheme. Unlike the SBM simulations that show different precipitation sensitivities to aerosol concentrations in relatively dry and humid environments, the Drop scheme indicates monotonic decrease in precipitation with increasing aerosol concentrations.