Observational Analysis of Tropical Cyclogenesis in the Western North Pacific. Part II: Budget Analysis

Abstract

This is the second of a two-part study which uses a combination of composite rawinsonde and individual FGGE case analyses to identify the important physical processes associated with tropical cyclone formation. Part I examined the structural evolution and circulation patterns for genesis and nongenesis cloud clusters in the western North Pacific. This paper analyzes moisture, energy and angular (tangential) momentum budgets for both types of cloud clusters. Results show that both the genesis and nongenesis cloud clusters have similar cumulus activity and similar moisture and energy budgets. An enhanced surface energy flux is found only after the formation of a tropical cyclone with a well-defined center. The results of Part I showed that genesis cloud clusters have comparatively strong 2°?8° radius middle- to low-level cyclonic circulations in comparison with nondeveloping cloud clusters. Transports by the mean transverse circulation are not efficient to produce the observed buildup of the tangential wind preceding the formation of cyclones. It is hypothesized that sizable eddy fluxes contribute to the cyclonic circulation buildup. The magnitude of the eddy fluxes is estimated as a residual of the tropospheric tangential momentum budget. The buildup of strong cyclonic circulation before cyclogenesis appears to be due in part to strong surrounding environment forcing by wind surge-type processes. The wind surges produce large inward eddy vorticity fluxes. Such eddy fluxes allow an increase in tangential wind without the need to increase the mean transverse circulation.