Contribution of the North Atlantic Subtropical High to Regional Climate Model (RCM) Skill in Simulating Southeastern United States Summer Precipitation

Abstract:

This study assesses the skill of advanced regional climate models (RCMs) in simulating Southeastern United States (SE US) summer precipitation and explores the mechanisms responsible for the simulation skill at a process level. Analysis of the RCM output for the North American Regional Climate Change Assessment Program (NARCCAP) indicates that the RCM simulations of summer precipitation show large biases and a remarkable spread over the SE US. The causes of such a spread are investigated by performing simulations using the Weather Research and Forecasting (WRF) model, a next-generation RCM developed by the US National Center for Atmospheric Research. The results show that the simulated biases in SE US summer precipitation are due mainly to the misrepresentation of the modeled North Atlantic Subtropical High (NASH) western ridge. In WRF simulations, the NASH western ridge shifts 7-deg northwestward compared to that in the reanalysis ensemble, leading to a dry bias in the simulated precipitation according to the "NASH western ridge - Southeast summer precipitation" relationship. Experiments utilizing the Four Dimensional Data Assimilation technique further suggest that the improved representation of the circulation patterns associated with the NASH western ridge substantially reduces the bias in simulating SE US summer precipitation. Our analysis of circulation dynamics indicates that the NASH western ridge in the WRF simulations is significantly influenced by planetary boundary layer (PBL) processes over the Gulf of Mexico. Specifically, a decrease (increase) in the simulated PBL height tends to stabilize (destabilize) the lower troposphere over the Gulf of Mexico, and thus inhibits (favors) the onset and/or development of convection. The changes in tropical convecton induce a tropical-extratropical teleconnection pattern, which modulates the circulation along the NASH western ridge in the WRF simulations and contributes to the precipitation biases over the SE US. In conclusion, our study demonstrates that the NASH western ridge is an important factor responsible for RCM skill in simulating SE US summer precipitation. Furthermore, improvements in the PBL parameterizations for the Gulf of Mexico might help advance RCM skill in representing the NASH western ridge and SE US summer precipitation.