Assimilating precipitation to improve simulations of the North American summer circulation

We are evaluating the impact of precipitation assimilation on regional simulations of the US summertime climate. The merged precipitation analyses used for the precipitation assimilation is a combination of the Higgins' precipitation analyses over the U.S., and the National Centers for Environmental Prediction/Department of Energy Reanalysis-II rain rates over the rest of the regional model domain, which includes Mexico, the Caribbean Sea, and portions of the North Atlantic and Pacific Oceans. We are comparing these regional simulations to the North American Regional Reanalysis, which used similar precipitation observations as part of the reanalysis. In particular, a continuous precipitation assimilation was performed over North America, starting on July 1st, 1986 and ending December 31st, 1996. This regional simulation was compared to a control simulation that did not include precipitation assimilation and to the bounding NCEP/DOE Reanalysis II, needed for the regional simulations. By using the merged precipitation analyses, we were better able to simulate the summertime monsoon circulation. During two distinct extreme summers, 1988 and 1993, which can be characterized as dry and wet summers, the assimilation of the merged precipitation analyses improved the regional simulation of atmospheric specific humidity, temperature and winds, as well as precipitation extremes.