Observational Analysis of Tropical Cyclone Formation. Part III: Budget Analysis

Abstract

Vertically integrated budgets of moisture, heat, angular momentum and kinetic energy are calculated from the composite data sets of Part I (McBride, 1981). The transition from cloud cluster to typhoon/hurricane is characterized by a warming of the troposphere and increase of tangential wind. Observations are presented to show that these effects are not restricted to the system's inner core region, but rather take place over a volume extending out to at least 6° latitude radius from the system center. Accordingly, in this paper cyclogenesis is investigated by analyzing budgets over this large scale. The heat budget calculations show that the observed warming of the troposphere is an order of magnitude smaller than the other terms in the budget equation. Most of the released latent heat LP0 is exported laterally through the boundaries of the region through conversion to the term ?·Vs. The portion of LP0 which is released within the volume acts to counter the net radiative cooling QR. All the composite weather systems export moist static energy h through their transverse circulation. This means that intensification cannot be brought about simply in response to increased cumulus heating due to increased mass circulation. To bring about an increase in h, any change must be such that the quantity (E0 ? ?·Vh) is increased, where E0 is the surface evaporation. All the composite weather systems export kinetic energy. The export takes place completely in the upper tropospheric outflow layer. The kinetic energy budgets show a residual requirement for a generation of kinetic energy by subgrid-scale processes. This eddy generation appears to be of the same magnitude as the generation by the mean radial flow, ?V?·???. Compared to non-developing systems, developing cloud clusters have twice to three times as much import of relative angular momentum through their lateral boundaries. This is related to the developing system having greater outer radius low-level positive and upper level negative surrounding tangential wind fields.