On the Formation and Development of Tropical Cyclones

Abstract

Research in tropical meteorology has revealed that the average easterly winds found in tropical regions are perturbed by large numbers of synoptic-scale disturbances. The mechanisms responsible for the initiation of such disturbances and the intensification of some of them into hurricanes may be considered as a problem of the instability of the basic easterly current and its perturbations. This study presents results of investigations into this problem for the region of the tropical North Atlantic Ocean, Caribbean Sea and Gulf of Mexico. Initial wave formation and subsequent intensification to the vortex stage are emphasized. Empirical relationships between the vertical (geostrophic) wind shear and wave or vortex intensification are consistent with the predictions of the Bjerknes-Homboe theory modified for easterly flow. The 1000?500-mb shear (thickness) patterns were used as a measure of the depth of the easterlies. A net easterly shear in this layer was found to be favorable for intensification and a net westerly shear unfavorable. Empirical relationships between sequenccs of events in the westerlies and associated variations in the structure and position of sub-tropical anticyclones are found to predict the time and place of formation of one class of tropical disturbances. Methods of forecasting initial development and changes in intensification of tropical disturbances based on the combined empirical and theoretical work were developed and tested on independent data. The results were quite favorable, on the whole, although the verification system is not completely free of subjectivity.