Dynamical Effect of Land Surface Processes on Summer Precipitation over the Southwestern United States

The physical mechanism of summertime precipitation over Arizona and New Mexico (AZNM) is examined using regional model experiments. Two sets of regional model simulations with different physics packages produce very different precipitation (P) over the Southwest including AZNM. The better simulation that produces a wet monsoon similar to the observations has larger evaporation (E) over AZNM and stronger moisture flux from the Gulf of California into AZNM. Diagnostics of the simulations suggested that the increase in precipitation is not due to the increase in evaporation locally but rather to the change in moisture flux. Regional model experiments were then designed to isolate the impact of local E and the large-scale flow. Both regional model experiments and diagnostics support the following physical mechanism: There is an increase in E in the realistic simulation due to the change in land surface physics. This increase in E is compensated by the decrease in sensible heat, which leads to the colder land surface. Associated with this cooling, the surface pressure raises and the Southwest heat low weakens due to the increase in the surface pressure. This alters the large-scale low-level circulation and increases the occurrence of the low-level moisture surge events from the Gulf of California into AZNM, and accordingly, increases P. The mechanism is also found in observations of day-to-day variation of precipitation over AZNM.