PNNL

Abstract

A new spectral bin microphysical scheme (SBM) was implemented into the WRF (referred to as Fast-SBM), which uses a smaller number of size distribution functions than the previous (standard) version of SBM (referred to as Exact-SBM). It was shown that both the Fast SBM and Exact SBM reproduced the typical structure of an idealized squall line quite realistically. The schemes simulated similar dynamical and microphysical structures, and there was excellent agreement in the simulated precipitation amounts between the schemes under a very wide range of aerosol conditions in which initial condensation nuclei concentration varied from 100 cm-3 to 3000 cm-3. Moreover, the Fast-SBM uses about 40% of the computing power of the exact-SBM, allowing it to be used for “real-time” simulations over limited domains. The results of SBM simulations have been compared with a modified version of the Thompson bulk parameterization scheme within the same dynamical framework (2D WRF). 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 depending on the prescribed aerosol concentration. This scheme is referred to as DROP scheme. A large set of sensitivity studies have been performed, in which the sensitivity of results (microphysical parameters and precipitation) to aerosol concentration, droplet nucleation above cloud base, etc. has been compared with the results from the SBM. It is shown that the SBM scheme produces more realistic dynamical and microphysical structure of the squall line. The addition of the DROP scheme did relatively little to change the underlying results of the bulk scheme, and unlike the SBM simulations, that show different precipitation sensitivities to aerosol concentrations in relatively clean and polluted environments, the DROP scheme simulates large monotonic decrease in precipitation with increasing aerosol concentrations.