WRF and MM5 Modeling of the 1999 North American Monsoon Onset and the Las Vegas Flood

The Arizona/Great Basin onset of the 1999 North American monsoon (NAM) was simulated using the WRF and MM5 models. This onset also produced a major flooding event in Las Vegas, Nevada. Our working hypothesis states that sea surface temperatures (SST) in the northern Gulf of California (GC) may play a critical role in the timing and amount of summer rainfall over the U.S. southwest. In particular, the onset of relatively heavy rainfall occurs after these SSTs exceeded 29°C. This study explores the impact of GC SSTs on factors affecting deep convective precipitation: the regional atmospheric circulation, water vapor mixing ratio, convective available potential energy (CAPE) and convective inhibition (CIN). The impact of GC SSTs on rainfall is also addressed. The results generally reproduce the observations under “local scale mechanism”, with the removal of the Gulf of California marine boundary layer inversion as SSTs in the northern GC approach 30°C. This increased the water vapor mixing ratio at lower levels over the GC and over the southwestern United States, as low-level winds over the northern GC were from the southeast. This in turn increased thunderstorm activity and rainfall amounts over the Southwest. The northern GC SST-rainfall relationship shown under “local scale mechanism” was reproduced in this modeling study. This study indicates that in order to capture the physics that is mechanistically related to NAM rainfall, high vertical resolution of the boundary layer and good boundary layer physics is a must. Also critical is the use of reliable SST data.