THE ENVIRONMENTAL INFLUENCE ON TROPICAL CYCLONE PRECIPITATION

The intensity, spatial, and temporal changes in precipitation were exa mined in three North Atlantic hurricanes during 1989 (Dean, Gabrielle, and Hugo) using precipitation estimates made from Special Sensor Micr owave/Imager (SSM/I) measurements. In addition, analyses from a barotr opic hurricane forecast model and the European Centre for Medium-Range Weather Forecast model were used to examine the relationship between the evolution of the precipitation in these tropical cyclones and exte rnal forcing. The external forcing parameters examined were 1) mean cl imatological sea surface temperatures, 2) vertical wind shear, 3) envi ronmental tropospheric water vapor flux, and 4) upper-tropospheric edd y relative angular momentum flux convergence. The analyses revealed th at 1) the SSM/I precipitation estimates were able to delineate and mon itor convective ring cycles similar to those observed with land-based and aircraft radar and in situ measurements; 2) tropical cyclone inten sification was observed to occur when these convective rings propagate d into the inner core of these systems (within 111 km of the center) a nd when the precipitation rates increased; 3) tropical cyclone weakeni ng was observed to occur when these inner-core convective rings dissip ated; 4) the inward propagation of the outer convective rings coincide d with the dissipation of the inner convective rings when they came wi thin 55 km of each other; 5) in regions with the combined warm sea sur face temperatures (above 26-degrees-C) and low vertical wind shear (le ss than 5 m s-1), convective rings outside the region of strong lower- tropospheric inertial stability could be initiated by strong surges of tropospheric moisture, while convective rings inside the region of st rong lower-tropospheric inertial stability could be enhanced by upper- tropospheric eddy relative angular momentum flux convergence.