Modelling the effects of land surface forcing on the North American Monsoon System

A high resolution climate model (RAMS) is used to examine the sensitivity of warm season rainfall across western North America to the presence of prescribed springtime soil moisture anomalies. The study is motivated by empirical results suggesting that spring snowpack in the southern U.S. Rocky Mountains is inversely correlated with the subsequent summer rainfall across the American Southwest, and follows several modeling studies demonstrating that coarse-resolution global GCMs have difficulty simulating the observed distribution of summer precipitation in this region of very complex terrain. We use RAMS to address the following questions: (1) Does this high resolution model produce a more realistic control simulation than coarse-resolution GCMs? (2) Do prescribed springtime land surface anomalies persist long enough to provide significant forcing for the summer monsoon circulation? (3) Does enhanced springtime soil moisture produce excess summer precipitation (which would be the case if local evaporation were the leading term in the local moisture budget) or deficient summer precipitation (as suggested by empirical studies)?