INFLUENCE OF EVAPOTRANSPIRATION RATES ON THE DEVELOPMENT OF THERMALS

Abstract

Hourly amounts of heat required for evapotranspiration (E) for seven days of the Great Plains Turbulence Field Program are determined by subtracting sensible heat conducted into the soil (G) and transferred to the air (Q) from net radiation (R). Q was calculated from wind and temperature profile data with a relationship derived by Webb. The daily sums of E ranged from 228 cal/sq cm on the day with the greatest soil moisture (S.M. = 9%) to 79 cal/sq cm on the day with the least, (2.5%). Corresponding values of Q ranged from 122 for the former to 218 for the latter, although (R-G) decreased from 350 to 258 cal/sq cm for the same two days. Bowen ratios determined from daytime sums were greater than unity (1.5, 2.8 and 1.7) for 3 days with S.M. <6% and less than unity (0.54, 0.52 and 0.45) for 3 days with S.M. > 7%. Average Q was at a maximum an hour later for the former than for the latter. Average cumulative curves of Q/(R-G) for both were nearly linear, but divergent until midday. After noon the "dry-day" curye was concave upward and the "wet-day" curve concave downward. It is concluded that relatively small changes in available soil moisture, by changing the amount of evapotranspiration and hence the amount of Q, can make relatively large differences in convective layer growth and therefore in thermal strengths. Moreover, the different cumulative patterns for "wet" and "dry" days may account for correspondingly different times of maximum thermal development, independent of cloudiness.