Coupling Spectral-bin Cloud Microphysics with the MOSAIC Aerosol Model in WRF-Chem: Methodology and Preliminary Results

Abstract Aerosol particles acting as cloud condensation nuclei (CCN) and ice nuclei (IN) may strongly affect cloud properties and precipitation through modifying cloud microphysical processes and the feedback to dynamics. Cloud-revolving models typically employ either bulk or bin treatments of cloud microphysics. The bulk treatments are computationally much less expensive but have deficiencies arising from their simplified representation of hydrometeor size distributions. Due to the high computational costs, spectral-bin cloud microphysics are often run with very simple aerosol treatments (e.g., prescribed CCN spectrum and composition), which results in large uncertainties in simulations involving complicated aerosol environments or multi-day duration. The goal of this study is to better represent cloud-aerosol interaction processes in the Chemistry version of Weather Research and Forecast model (WRF-Chem) by coupling the spectral-bin microphysics (SBM) with the MOSAIC sectional aerosol module. A flexible interface is built to exchange cloud and aerosol information between them. The treatments of CCN spectrum and droplet nucleation in the original SBM are modified with a bin aerosol activation based on the realistic aerosol description from MOSAIC. Correspondingly, the tendencies of aerosol resuspension and in-cloud wet removal processes in the original MOSAIC are replaced with those calculated in the SBM. The initial test of the coupled modeling system is conducted with a maritime stratocumulus clouds case during the Variability of the American Monsoons Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx). Prescribed aerosol composition and size distribution are used to test the coupling system against the WRF-SBM simulation in which the aerosol is from the mean of MOSAIC. Comparison of first minute of the two simulations shows that aerosol activation in the new modeling system is generally reasonable. An evaluation of the WRF-SBM-MOSAIC simulation over a longer time (24-h) suggests that the aerosol resuspension through cloud evaporation plays an important role in replenishing aerosols in maritime boundary layer. This is consistent with the observational evidence that most of the drizzle evaporated before reaching surface during the VOCALS. The new coupled model is expected to allow much more realistic cloud and precipitation simulations for the regions with complex aerosol distributions.

Keywords: bin microphysics, sectional aerosol, WRF-Chem, coupling, stratocumulus cloud

Author: Wenhua Gao State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, No. 46, Zhongguancun South Street, Haidian District, Beijing, 100081, China. E-mail: gaowh@cma.gov.cn