Examining a Multi-Scale Version of the Kain-Fritsch Convective Parameterization in a Regional Climate Downscaling Application

The U.S. Environmental Protection Agency is modifying portions of the Weather Research and Forecasting (WRF) model to improve dynamical downscaling of global climate information. One such modification involves adaptation of the Kain-Fritsch convective parameterization scheme to allow model grid resolutions in the meso-gamma scale (2-20 km). In this work, we use the Advanced Research WRF (WRF-ARW) with a new Multi-scale Kain-Fritsch (MSKF) scheme and 108/36/12-km grid nesting to downscale global climate model information for the 2025 to 2035 period.

The MSKF scheme includes modifications related to the following issues: subgrid-scale convective cloud-radiation interactions, convective adjustment time scale, horizontal entrainment of environmental air, linear mixing of updraft vertical velocity with grid-scale vertical velocity, stabilizing capacity, and dealing with coincident resolved and sub-grid convective precipitation. It was previously tested by downscaling meteorological reanalysis products over seasonal periods and evaluating the results against observed data and observational analyses, but this is the first multi-year application and the first application based on future global climate information. The global climate model data come from the NASA/GISS ModelE2 version AR5 simulating Representative Concentration Pathway 6.0. These data were previously downscaled from their 2° x 2.5° grid spacing using WRF-ARW with 108/36-km nesting with the 36-km grid covering most of North America. In this work, the 12-km grid covers the same area over North America so that comparisons can be made between previous results from the standard WRF-ARW model and results from this work with the new MSKF scheme and finer horizontal resolution.

Preliminary evaluations indicate that a positive bias in precipitation seen in 36-km downscaling is significantly reduced with the MSKF scheme applied with 12-km grid spacing. This work will investigate the relative intensity of extremes in temperature, wind and precipitation between the previous 36-km downscaling and this 12-km experiment. It will also examine extraordinary meteorological episodes within the 2025-2035 time frame to show how finer horizontal resolution in our dynamical downscaling with MSKF can provide more relevant and reliable future climate information for climate-change adaptation efforts.