Abstract
Hurricane Bret underwent a rapid intensification (RI) and subsequent weakening between 1200 UTC August 21 and 1200 UTC August 22, 1999, before it made landfall as a category 3 hurricane on the Texas coast 12 h later. Its minimum sea-level pressure dropped 35 hPa from 979 to 944 hPa within 24 h. During this period, aircraft of the National Oceanographic and Atmospheric Administration (NOAA) flew several research missions that sampled the environment and inner core of the storm. These data sets combined with gridded data from the National Centers for Environmental Prediction's (NCEP) Global Model and the National Center for Atmospheric Research (NCAR) reanalyses are used to document the atmospheric and oceanic environments of the tropical cyclone (TC) as well as their relation to the observed structural and intensity changes. Bret's RI was linked to movement over a warm ocean eddy and high sea surface temperatures (SSTs) in the Gulf of Mexico coupled with a simultaneous decrease in vertical wind shear. SSTs at the beginning of the storm?s RI were approximately 29 degrees Celcius and steadily increased to 30 degrees Celcius as it moved northward. The vertical wind shear relaxed to less than 10 kt during this time. Mean values of oceanic heat content (OHC) beneath the storm were about 20 % higher at the beginning of the RI period than 6 h before. Cooling of near-coastal shelf waters (to between 25 and 26 degrees Celcius) by pre-storm mixing combined with an increase in vertical wind shear were responsible for the weakening of the storm. The available observations suggested that intrusion of dry air into the circulation core did not contribute to the intensity evolution. In order to quantitatively describe the influence of environmental conditions on the intensity forecast, sensitivity studies with the Statistical Hurricane Intensity Prediction Scheme (SHIPS) model were conducted. Four different cases with modified vertical wind shear and/or SSTs were studied. Differences between all cases were relatively small due to the model design, but much cooler prescribed SSTs resulted in the greatest intensity changes. Model runs with idealized environmental conditions demonstrated the model?s general lack of capability to forecast RIs and also stressed the need of more accurate SST observations in the coastal shelf regions when predicting the intensity of landfalling TCs.