Improving numerics of bulk microphysics schemes in WRF model: warm rain processes

Poster PDF

Description

We are developing an adaptive sub-stepping (ADSS) technique based on analytic stability and positive definiteness criteria to analyze numerics of warm rain processes in different bulk microphysics (BLK) schemes implemented in the Weather Research and Forecasting (WRF) model. We implement description of warm rain processes based on Eulerian explicit (EE) time integration in WRF BLK schemes into our microphysics package as a standalone program. We use results of sensitivity runs with EE and ADSS schemes for a wide range of cloud and rain water contents and droplet concentration to calculate auto-conversion and accretion growth rates as well as maximal time step permitted to keep the positivity of the solution. We highlight that all WRF BLK schemes are not positive definite and might show better performance for finer spatial resolutions when time steps 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 thousandths of seconds, but it might lead to 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). We 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 standard EE time integration) and runs with ADSS time integration will be presented during the meeting.