On the Rapid Intensification of Hurricane Opal (1995) over the Gulf of Mexico

Hurricane Opal, which occurred on 3-5 October 1995 over the Gulf of Mexico, intensified rapidly (from category 1 to Category 4) within a short period of 24 hours (i.e., between 1200 UTC on 3 October and 1200 UTC 4 October). Much rapid deepening was confined, however, within a short period of 2 hours (0900-1100 UTC 4 October) as the central pressure dropped from 933 to 916 hPa as the eye contracted from a radius of 30 km to 15 km (Marks et al. 1998). It is well known that tropical cyclone intensification largely depends on three physical processes: 1) internal storm dynamics, 2) large-scale upper-tropospheric influences, and 3) air-sea interactions. In the case of Opal, previous studies have shown that its rapid intensification can be explained by influences of upper-tropospheric-trough interactions (e.g., Rodgers et al. 1998; Bosart et al. 1999) and air-sea interactions (Shay et al. 1998; Hong et al. 1999). Although a greater understanding of Opal's rapid intensification has been achieved from these observational and numerical modeling studies, the role of internal storm-scale dynamics during Opal's short but strong rapid intensification phase (0900-1100 UTC) is not well understood. Furthermore, it is not clear what environmental factors were responsible for the abrupt weakening of Opal after 1200 UTC 4 October prior to landfall. Therefore, the purpose of this study is to investigate not only the role of eyewall convective processes in the rapid intensification of phase of Opal but also the effect of dry air intrusion on the rapid weakening of Opal using both observational and numerical simulation results. Preliminary results from both the NLDN lightning and ECMWF gridded data analysis revealed the occurrence of increased frequency of cloud-to-ground lightning strokes associated with vigorous cumulonimbus convection close to the eye center between 0800 and 1400 UTC on 4 October, which coincided well with the inner eyewall convective burst episode indicated by large convective latent heat release derived from the SSM/I data. It is hypothesized that Opal's rapid intensification can be explained by the sudden increase in convective latent heat release (LHR), which must have contributed to strong updraft velocities as evident from an increase in lightning flashes, and hence induced compensating subsidence within the eye to cause a rapid decrease in hydrostatic pressure through adiabatic warming. This short-term convective burst appears to have been caused by an increase in CAPE resulting from enhanced surface moisture fluxes during Opal's passage over the WOE as well as by the convective destabilization caused by the upper-tropospheric trough. These results including the MM5 model simulations in capturing the intensification and weakening phases of Opal will be presented at the conference.