A Moisture Budget Analysis of the Protracted Heat Wave in the Southern Plains during the Summer of 1980

Abstract

Extreme heat and drought plagued much of the south central United States during the summer of 1980 in response to a quasi-stationary persistent anticyclone aloft. Numerous weather reporting stations in the Southern Plains experienced a dewpoint temperature decrease of about 1°C per week from late June to late July in response to the intense heat. In this paper evaporation is computed as a residual from the large-scale moisture budget based upon the observed twice daily radiosonde data. From 1 June to 31 August 1980 the average daily evaporation amounted to 8 mm for a 4 × 105 km2 region centered over Oklahoma. On individual days the evaporation was as high as 10–15 mm. Strong diurnal variations were computed in the water vapor flux, vertical velocity and horizontal divergence fields. These variations, when superimposed over persistent large-scale subsidence, yielded enhanced subsidence, low-level divergence and anticyclonic vorticity production by day and weakened subsidence by night. The maximum diurnal variation in vertical motion was located near 700 mb and averaged 2–3 cm s−1. Comparison with 1979 results, a slightly cooler and wetter than normal summer, showed that the diurnal signal was quite amplified in 1980. The more normal Great Plains pattern of nighttime convergence, weak surface cyclonic vorticity and ascent was seldom seen in 1980 and then only in conjunction with the passage of rare synoptic-scale disturbances. The vertical profiles of equivalent potential temperature across the Southern Plains were comparable in 1979 and 1980, ranging from θe∼340–345 K at the surface to ∼330 K at 600 mb. In contrast, the surface saturation equivalent potential temperature was much warmer in 1980 than 1979 (∼385 K vs 360 K). Consequently, the level of free convection was much higher in 1980 (∼600 mb) and organized convective systems could not be triggered in the absence of synoptic-scale forcing. Evaporation in the absence of precipitation resulted in the presumed depletion of soil moisture and even higher surface temperatures as almost all of the available solar insulation went into sensible heat. Daytime subsidence reinforced the heating and drying of the atmosphere in a positive feedback process that likely contributed to the maintenance of the hot, dry conditions under the favorable synoptic-scale anticyclone regime.