Thompson Microphysics Updates in the Unified Forecast System

Abstract

The Thompson microphysics scheme was evaluated in the Unified Forecast System (UFS) for medium-range weather application in both atmosphere-only and fully coupled atmosphere-ocean-ice-wave system configurations. Initial tests based on the Global Forecast System (GFS) version 16 configuration showed that the Thompson microphysics scheme became unstable with a typical GFS time step. An inner-loop time-splitting approach and a new semi-Lagrangian sedimentation algorithm for rain and graupel were implemented in the scheme to alleviate this numerical instability problem. To reduce biases of radiative fluxes  at the surface and at the top of the atmosphere, the conversions from cloud ice to snow and from snow to graupel in the scheme were modified along with the falling velocity of cloud ice. A few other parameters related to the cloud ice formation process were also adjusted to help improve the accuracy of radiative fluxes. Convective cloud condensate was included in the calculations of the total cloud cover and radiative transfer. Both atmosphere-only and air-sea coupled experiments were conducted to examine the impacts of these changes on global and regional forecast skills at different temporal and spatial scales.