Moving and Stationary Mesoscale Convective Systems over Northwest Mexico during the Southwest Area Monsoon Project

Abstract

Some characteristics of the life cycle and motion of Mesoscale Convective Systems (MCSs), which formed over the northern Sierra Madre Occidental during the Southwest Area Monsoon Project (SWAMP), are examined. The motion characteristics of MCSs and presence of residual circulations (mesoscale vortices) are determined using Geostationary Operational Environmental Satellite infrared and visible satellite imagery. Composite wind fields associated with moving and stationary systems were constructed using soundings from the operational upper-air network and from special pilot balloon stations that were operated in Mexico as part of the SWAMP experiment. The MCSs were classified as moving or stationary based on the observed displacement during a 6-h period. Moving and stationary systems occurred with about the same frequency. The moving systems were distributed uniformly along the length of the Sierra Madre, while the stationary systems were primarily confined to the southern Gulf of California. The stationary systems also tended to have mesoscale vortices associated with them after the convection had dissipated. The midlevel composite winds show the presence of stronger easterlies present prior to and during the formation of moving MCSs, while the stationary systems had weak westerlies over the southern Gulf of California. These composite winds are consistent with the large displacement of the moving systems. The weak cyclonic shear associated with the stationary systems may represent a source of vorticity for the mesoscale vortices.