Improving numerics of bulk microphysics schemes in WRF model: time integration scheme impact on warm rain processes and precipitation

Description

Bulk microphysics (BLK) schemes that use Eulerian forward-in-time integration are conditionally stable and not positively definite. Implemented in the Weather Research and Forecasting (WRF) model, these schemes might show better performance for finer spatial resolutions when time steps that are used to advance microphysical prognostic equations have an order of magnitude from seconds to tenths of seconds. For coarser spatial resolutions, time steps are usually increased from hundredths up to thousands of seconds, but it might lead to the degradation of WRF BLK schemes’ performance because of corrections such as “mass adjustment” (for single-moment schemes) and additional “concentration adjustment” (for double-moment schemes). Based on analytic stability and positive definiteness criteria, we analyze the impact of Eulerian forward-in-time (EFTI), adaptive substepping (ADSS), semi-implicit (SITI), and fully implicit (FITI) time integration techniques on warm rain processes in different BLK schemes implemented in the WRF model. We also analyze spatial and temporal distribution of accumulated precipitation and time series of maximal surface precipitation rates obtained in idealized large-scale WRF simulations. Differences between the control run (Morrison-Curry-Khvorostyanov BLK scheme with EFTI) and runs with ADSS, SITI, and FITI will be presented and discussed.