A scientific experiment; to investigate the atmospheric circulations, the mesoscale convective systems and the precipitation associated with the monsoon over northwestern Mexico and southwestern United States was carried out during the northern summer of 1990. A number of institutions and universities participated in the experiment. Special observations were made over the Mexican States of Baja California, Baja California Sur, Sonora, Sinaloa, Durango, Coahuila, Chihuahua and in the United States, Arizona. A wide variety of meteorological observations were made, including special pilot balloon and radiosonde soundings, research aircraft flights (including use of airborne Doppler radar), special surface observations, as well as more routine satellite, surface and upper-air observations over the region. The observations have been processed and compiled by the National Severe Storms Laboratory (NSSL) and the Center for Scientific Study and Higher Education of Ensenada (CICESE), in a data bank with the cooperation of National Water Commission (CNA) and the Sonora Institute of Technology (ITSON). Different atmospheric phenomena are being investigated, including the horizontal transport of water vapor and the formation and characteristics of cumulus convective systems and associated precipitation processes. Preliminary studies to date using the 1990 data identify the 'Sierra Madre Occidental', particularly important to the formation of mesoscale convective systems and the generation of summer precipitation, as a region where moist air masses from the tropical Americas converge. The Mexican watersheds from the Pacific and the Gulf of California show higher precipitation over the Sierra Madre, as compared with the watersheds from Chihuahua and the Gulf of Mexico, suggesting the Pacific ocean as the main source of moisture for the monsoon circulation over northwestern Mexico.
