Improving US extreme precipitation simulation: dependence on cumulus parameterization and underlying mechanism

Abstract

Regional Climate-Weather Research and Forecasting model (CWRF) simulations driven by the ECMWF-Interim reanalysis (ERI) showed that cumulus parameterization significantly impacts daily 95th percentile precipitation (P95) over the US Gulf States (GS) and Central-Midwest States (CM). This study compared interannual variations across ERI and five CWRF cumulus parameterization members based on CM and GS regional mean composites during P95 events. A structural equation model framework was used to build regressions of these variations among optimally selected fields to identify the underlying processes affecting P95. We discovered five distinct physical mechanisms, each involving unique interplays among water and energy supplies and surface and cloud forcings, with varying degrees of relative importance (%). In CM summer and CM and GS autumn, water supply (~ 60%), energy supply (~ 20%), and cloud forcing (~ − 20%) jointly determined P95. In GS spring and winter, surface forcing was predominant (84–87%), while energy and water supplies evenly accounted for the remaining impact. In CM spring, surface forcing was also predominant (85%), but was accompanied by energy supply alone. In GS summer, cloud forcing was predominant (− 84%), while water supply had the opposite impact (− 8%) to energy supply (6%). In CM winter, water supply (− 62%) also counteracted energy supply (31%) while cloud forcing played a positive role (7%). The seasonal reversal in the roles of water supply and cloud forcing occurred because the prevailing precipitation system changed from convective to stratiform processes. The choice of cumulus parameterization affected how water and energy supplies acted through surface and cloud forcings, thus determined CWRF’s ability to simulate extreme precipitation.