Environmental forcing of supertyphoon Paka's (1997) latent heat structure

The distribution and intensity of total (i.e., combined stratified and conv ective processes) rain rate/latent heat release (LHR) were derived for Trop ical Cyclone Paka during the period 9-21 December 1997 from the F-10, F-11, F-13, and F-14 Defense Meteorological Satellite Special Sensor Microwave I mager and the Tropical Rainfall Measuring Mission Microwave Imager observat ions. These observations were frequent enough to capture three episodes of inner-core convective bursts and a convective rainband cycle that preceded periods of rapid intensification. During these periods of convective bursts , satellite sensors revealed that the rain rates/LHR 1) increased within th e inner-core region, 2) were mainly convectively generated (nearly a 65% co ntribution), 3) propagated inward, 4) extended upward within the mid- and u pper troposphere, and 5) became electrically charged. These factors may hav e increased the areal mean ascending motion in the mid- and upper-troposphe re eyewall region, creating greater cyclonic angular momentum, and, thereby , warming the center and intensifying the system. Radiosonde measurements from Kwajalein Atoll and Guam, sea surface temperat ure observations, and the European Centre for Medium-Range Forecasts analys es were used to examine the necessary and sufficient conditions for initiat ing and maintaining these inner-core convective bursts. For example, the ne cessary conditions such as the atmospheric thermodynamics [i.e., cold tropo pause temperatures, moist troposphere, and warm SSTs (>26 degreesC)] fulfil l the necessary conditions and suggested that the atmosphere was ideally su ited for Paka's maximum potential intensity to approach supertyphoon streng th. Further, Paka encountered moderate vertical wind shear (<15 m s(-1)) be fore interacting with the westerlies on 21 December. The sufficient conditi ons that include horizontal moisture and the upper-tropospheric eddy relati ve angular momentum fluxes, on the other hand, appeared to have some influe nce on Paka's convective burst. However, the horizontal moisture flux conve rgence values in the outer core were weaker than some of the previously exa mined tropical cyclones. Also, the upper-tropospheric outflow generation of eddy relative angular momentum flux convergence was much less than that fo und during moderate tropical cyclone-trough interaction. These results indi cated how important the external necessary condition and the internal forci ng (i.e., convective rainband cycle) were in generating Paka's convective b ursts as compared with the external sufficient forcing mechanisms found in higher-latitude tropical cyclones. Later, as Paka began to interact with th e westerlies, both the necessary (i.e., strong vertical wind shear and cold er SSTs) and sufficient (i.e., dry air intrusion) external forcing mechanis ms helped to decrease Paka's rain rate.