Satellite-Derived Latent Heating Distribution and Environmental Influences in Hurricane Opal (1995)

Abstract

The total (i.e., convective and stratiform) latent heat release (LHR) cycle in the eyewall region of Hurricane Opal (October 1995) has been estimated using observations from the F-10, F-11, and F-13 Defense Meteorological Satellite Program Special Sensor Microwave/Imagers (SSM/Is). This LHR cycle occurred during the hurricane?s rapid intensification and decay stages (3?5 October 1995). The satellite observations revealed that there were at least two major episodes in which a period of elevated total LHR (i.e., convective burst) occurred in the eyewall region. During these convective bursts, Opal?s minimum pressure decreased by 50 mb and the LHR generated by convective processes increased, as greater amounts of latent heating occurred at middle and upper levels. It is hypothesized that the abundant release of latent heat in Opal?s middle- and upper-tropospheric region during these convective burst episodes allowed Opal?s eyewall to become more buoyant, enhanced the generation of kinetic energy and, thereby, rapidly intensified the system. The observations also suggest that Opal?s intensity became more responsive to the convective burst episodes (i.e., shorter time lag between LHR and intensity and greater maximum wind increase) as Opal became more intense. Analyses of SSM/I-retrieved parameters, sea surface temperature observations, and the European Centre for Medium-Range Weather Forecasts (ECMWF) data reveal that the convective rainband (CRB) cycles and sea surface and tropopause temperatures, in addition to large-scale environmental forcing, had a profound influence on Opal?s episodes of convective burst and its subsequent intensity. High sea surface (29.7°C) and low tropopause (192 K) temperatures apparently created a greater potential for Opal?s maximum intensity. Strong horizontal moisture flux convergence within Opal?s outer-core regions (i.e., outside 333-km radius from the center) appeared to help initiate and maintain Opal?s CRBs. These CRBs, in turn, propagated inward to help generate and dissipate the eyewall convective bursts. The first CRB that propagated into Opal?s eyewall region appeared to initiate the first eyewall convective burst. The second CRB propagated to within 111 km of Opal?s center and appeared to dissipate the first CRB, subjecting it to subsidence and the loss of water vapor flux. The ECMWF upper-tropospheric height and wind analyses suggest that Opal interacted with a diffluent trough that initated an outflow channel, and generated high values of upper-tropospheric eddy relative angular momentum flux convergence. The gradient wind adjustment processes associated with Opal?s outflow channel, in turn, may have helped to initiate and maintain the eyewall convective bursts. The ECMWF analyses also suggest that a dry air intrusion within the southwestern quadrant of Opal?s outer-core region, together with strong vertical wind shear, subsequently terminated Opal?s CRB cycle and caused Opal to weaken prior to landfall.