SURGES OVER THE GULF OF CALIFORNIA DURING THE MEXICAN MONSOON

Northward surges of relatively cool, moist, maritime air from the trop ical Pacific into the southwestern United States occur via the Gulf of California every summer during the Mexican monsoon season. These surg es advect large amounts of moisture northward and promote increased co nvective activity in Arizona. Mesoscale model output from 32 successiv e 24-h simulations are used to examine both the large-scale and mesosc ale features associated with surge events. Model data are compared to high-resolution aircraft measurements on two days, showing clearly tha t the model is capable of reproducing many of the detailed structures found in the observations. Results from a large-scale analysis indicat e that strong surges are produced in the model when a midlatitude trou gh is in the proper phase relationship with a tropical easterly wave. weak surges occur when this relationship is missing.