This talk will describe special meteorological observations made as part of a field program over northwestern Mexico during the summer of 1993. One objective of these observations was to help explain the observed maximum of convective cloudiness over the foothills of the Sierra Madre Occidental during the months of July and August. This region has one of the highest frequencies of convective cloudiness anywhere in North America during July and August.
Frequent pilot balloon observations were made at a coastal site and at a station about 100 km from the coast, the latter site being close to the foot of the mountains. These observations have been be used to describe the intensity and timing of the sea-land and valley-mountain breeze circulation respectively. The depth of the sea breeze circulation at the coastal site is about 1 km, whereas the valley breeze at the interior site is much deeper, as suggested by the depth of significant diurnal variation in the winds. Radiosonde and frequent tethered balloon soundings at the interior site have been used to describe the mean diurnal evolution of the boundary layer, and the morning profiles will be compared with the radiosonde observations at the coastal site.
The observations have been stratified into wet and dry days, based on rainfall observations over the region. The difference in the evolution of the diurnal cycle and in the mean profiles of temperature and moisture will be shown.
Finally, some ideas regarding the source of moisture for the region of convective cloudiness will be presented.