Simulations of the North American Monsoon System. Part I: Model Analysis of the 1993 Monsoon Season

Abstract

The North American monsoon system is known to produce significant summertime precipitation on the west coast of Mexico and the southwestern United States, with some areas receiving greater than 50% of their yearly rainfall between the months of July and September. The onset of the monsoon is attributed to a shift in the large-scale upper-level anticyclonic flow over the central United States, and the associated increases in moisture flux and resulting precipitation are tied to the low-level jets from the Gulf of California and the Gulf of Mexico. Individual monsoon surge events vary in intensity, as does the magnitude of the diurnal cycle of the low-level jets and precipitation. Numerical modeling and forecasting of these interacting large-and mesoscale monsoon features is often difficult in terms of accurately recreating the varying flow regimes aloft and near the surface and over both the flat and steep terrain that are encompassed within the monsoon region of influence. The Regional Atmospheric Modeling System (RAMS) at Colorado State University has been utilized to investigate seasonal monsoon simulations for the 1988 (United States drought), 1993 (Midwest flood), and 1997 (El Niño year) monsoon seasons. In Part I of this paper the credibility of RAMS, as far as its ability to reproduce observed features of the North American monsoon system, is evaluated. Part II provides interseasonal comparisons of model-simulated monsoon features from the three simulated extreme seasons and results of sensitivity studies to SSTs and soil moisture variability. Part III presents the development of potential vorticity anomalies associated with convection over Mexico and their downstream influence over the central United States.