Intraseasonal Variability of Summer Storms over Central Arizona during 1997 and 1999

Abstract

Although previous climatologies over central Arizona show a summer diurnal precipitation cycle, on any given day precipitation may differ dramatically from this climatology. The purpose of this study is to investigate the intraseasonal variability of diurnal storm development over Arizona and explore the relationship to the synoptic-scale flow and Phoenix soundings during the 1997 and 1999 North American monsoons. Radar reflectivity mosaics constructed from Phoenix and Flagstaff Weather Surveillance Radar-1988 Doppler reflectivity data reveal six repeated storm development patterns or regimes. The diurnal evolution of each regime is illustrated by computing frequency maps of 25 dBZ and greater reflectivity during 3-h periods. These regimes are named to reflect their regional and temporal characteristics: dry regime, eastern mountain regime, central-eastern mountain regime, central-eastern mountain and Sonoran-isolated regime, central-eastern mountain and Sonoran regime, and nondiurnal regime. Composites constructed from the NCEP?NCAR 40-Year Reanalysis Project data show that regime occurrence is related to the north?south location of the 500-hPa geopotential height ridge axis of the Bermuda high and the east?west location of the 500-hPa monsoon boundary, a boundary between dry air to the west and moist air to the east. Consequently, precipitable water from the 1200 UTC Phoenix soundings is the best parameter for discriminating the six regimes.