MM5 Simulations of the Las Vegas flash flood of 8 July 1999: The role of SSTs

Recent work (http://www.dri.edu/Projects/Monsoon) indicates heavy and more frequent monsoon rainfall occurs over the U.S. desert southwest after sea surface temperatures (SST) in the northern Gulf of California (GOC) exceed about 28.5 deg.C, based on satellite data from six monsoon seasons. Last year, the Las Vegas flash flood of 8 July occurred 2 days after this threshold SST was exceeded. Last years monsoon will be evaluated for the U.S. southwest via satellite fields of SST, rainfall, upper-tropospheric water vapor, precipitable water, etc. These results support the hypothesis that northern GOC SSTs play a critical role in the timing and amounts of monsoon rainfall over the U.S. southwest.

To test this hypothesis, we selected the Las Vegas flash flood of 8 July 1999 for our numerical experiments. The non-hydrostatic NCAR/Penn State Mesoscale Model Version 5 (MM5) is used in the simulation of the monsoon onset and the Las Vegas flash flood of July 08, 1999. The MM5 model accommodates four-dimensional data assimilation, multi-nested domains and other physical parametrizations, which allows us to simulate the general features of the monsoon. Our work focuses on simulations with larger coarse domain, including the Gulf of California (GOC), especially the Northern Gulf, exploring the idea that sea surface temperatures (SSTs) in the Northern GOC exceeding about 28.5 deg.C could support widespread convection and precipitation in the U.S. desert southwest. Although normally fixed, a 'do loop' for incrementing SSTs will be implemented into one of the MM5 decks to simulate observed increases in the SSTs in the GOC. Domain boundaries will be initialized with water vapor mixing ratios characteristic of those over the Pacific off the Baja Peninsula. The experiment-driven processes and simulation results will be discussed at the workshop.