Comparison of two double-moment microphysics schemes in aspects of warm-rain droplet spectra and raindrop budget

摘要

Simulation results of the WDM6 scheme and the Thompson scheme, both of which are commonly-used double-moment bulk microphysics schemes, are compared within the Weather Research and Forecasting model. The purpose of the comparison is to study the difference in the aspects of the warm-rain hydrometeor number concentrations, the droplet size distributions, and the budgets of the rain mixing ratio and number concentration. It is found that the WDM6 scheme overestimates the ratio and the amount of large precipitation, and underestimates those of small precipitation, compared to the Thompson scheme. The cloud number concentration (CNC) predicted in the WDM6 scheme is one to three orders of magnitude smaller than that of the Thompson scheme, which is set to the specific value of CNC. The cloud droplet spectra of the WDM6 scheme are broader. The WDM6 scheme produces a larger rain number concentration and smaller mass mean diameter of raindrops under the influence of both warm and cold rain processes— specifically, autoconversion and melting of snow and graupel. The WDM6 scheme produces a larger autoconversion rate and smaller total melting rate of snow and graupel than the Thompson scheme in the rain mixing ratio budget. The sign of the difference in the rain-cloud collection varies with region, and the rain-cloud collection process together with evaporation of rain have a major influence on the sign of the surface precipitation difference between the two schemes.