THE MUTUAL VARIATION OF WIND, SHEAR, AND BAROCLINICITY IN THE CUMULUS CONVECTIVE ATMOSPHERE OF THE HURRICANE

Abstract

The enhanced cumulus convection in tropical disturbances acts in two opposing ways. In one sense the condensation heat from the cumuli acts to warm the inner portions of the disturbance and induce vertical shear of the horizontal wind through the thermal wind relationship. In the opposite sense the cumuli also act to suppress vertical wind shear by transfer of horizontal momentum within their up- and downdrafts. The operation of this dual or ?paradox? role of the cumulus cloud is of fundamental importance for understanding the steady-state dynamics of the hurricane's inner core region and is also hypothesized to be of basic importance in the development of tropical storms and generation of easterly waves. Observational information and calculations on the radial distribution of baroclinicity and vertical shear collected on 102 individual flight legs (on 19 flight levels) into hurricanes flown by the Research Flight Facility of the U.S. Weather Bureau are presented. The measured baroclinicity in the lower half of the troposphere at the inner 50?70-km. radii is two to three times larger than cylindrical thermal wind balance would require. This baroclinicity may be 50 to 100 times larger in the inner areas of the hurricane than in the easterly wave, yet vertical wind shear in the hurricane may be but one to three times as large. An extensive discussion is presented on the characteristics of the vertical motion within the hurricane. A steady-state model of mean flow conditions in the inner 80-km. radius based on flight observations and best known surface stress conditions is assumed. Upon this mean vortex motion five sizes of cumulus up- and downdrafts are superimposed with their characteristic eddies and resulting stress. A discussion of the terms in the steady-state equations of motion reveals that sizable amounts of cumulus-produced radial and tangential frictional acceleration is required to satisfy the broadscale mean flow conditions. These cumulus-produced horizontal accelerations account for the imbalance in the thermal wind relationship. Tropical storm development is not viewed as being possible unless the cumulus-induced vertical momentum transfers act in a dominant way to oppose the thermal wind requirement and inhibit increase of vertical wind shear as baroclinicity increases. Vortex development thus requires a continual imbalance of pressure over wind acceleration.