Diurnal Variation of the Lower-Tropospheric Flow over the Arizona Low Desert from SWAMP-1993 Observations

Abstract

This paper describes the tropospheric circulation over the lower deserts of Arizona, California, and northwestern Mexico using observations from a special rawinsonde network operated during July and August as part of the 1993 Southwest Area Monsoon Project. The observations show that the flow over the low desert is diffluent, divergent, and upslope during the afternoon and confluent, weakly convergent, and downslope or slope parallel in the early morning hours. This diurnal cycle may help to explain the observed tendency for summer thunderstorms to occur most frequently over the low desert during the nighttime. Despite the relatively short observation interval (18 days), the authors show that this period is similar to multiyear mean conditions during July and August. A heat budget indicates that radiative heating in the lower troposphere over the desert is approximately balanced by the influx of cooler air from over the Gulf of California but uncertainties in the estimates of vertical motion and horizontal temperature advection make the budget uncertain in the middle troposphere.