Using interior grid nudging to imrpove the large-scale circulation and extreme precipitation events in WRF as a regional climate model

One philosophical paradigm for global climate downscaling is that the regional climate model (RCM) should add mesoscale features to the simulated climate while retaining synoptic features that are prescribed by the global model. Following that philosophy, we use WRF as a RCM and investigate the effects of interior grid nudging to keep the RCM consistent with the global model. Three simulations driven by NCEP-DOE AMIP-II Reanalysis have been completed to model the continental United States at 36-km resolution for the period 1988-2007. These simulations use (1) grid point (analysis) nudging; (2) spectral nudging; and (3) no interior nudging. Initial statistical evaluation of near-surface fields, including 2-m temperature and precipitation, has revealed that, relative to not using any nudging, both types of interior nudging improve the mean climate and variability on time scales ranging from interseasonal to interannual. This presentation will show why nudging improves the representation of the climate. In particular, the Midwest drought of 1988 and flood of 1993 are used to diagnose the impacts of interior nudging on the large-scale circulation in WRF.