Numerical Simulation of Heavy Precipitation in Northern Baja California and Southern California

Abstract

The fifth-generation Pennsylvania State University?NCAR Mesoscale Model (MM5) was used to simulate the heavy-precipitation events of 6?21 January 1993 during a moderate El Niño that produced severe flooding, landslides, and the loss of many lives near the border of California and Baja California, Mexico. The mean synoptic-scale condition consisted of a strong upper-level subtropical westerly jet over the U.S.?Mexico western border and the passage of three surface fronts, along with intense low-level advection of tropical moisture and convective activity over the region. The MM5 reproduced the mean observed circulation patterns of the study period but with less intensity. During the first extreme event on 6 January, the model sounding profile in San Diego, California, was more unstable and saturated than observed, the horizontal winds were weaker in the lower troposphere, and the simulated precipitation was slightly underestimated. The model precipitation was verified with hourly and daily precipitation records in California and Baja California, respectively, and regional errors were obtained for the 16-day period. The simulation showed an improvement when the resolution increased from 90 to 30 km, but there was not a significant improvement from 30 to 10 km; however, extreme rainfall events (>10 mm day?1) were better resolved by the 10-km grid, possibly due to the finer-scale topography.