Numerical Prediction of Hurricane Opal

Abstract

The main theme of this paper is on the intensity forecast of a hurricane (Opal) and interpretation of factors contributing toward it. The paper illustrates the results of assimilation and prediction for Hurricane Opal of 1995 from a very high-resolution global model. The assimilation makes use of a detailed physical initialization that vastly improves the nowcasting skill of rainfall and the model-based outgoing longwave radiation. Some of the interesting aspects of Hurricane Opal?s history occurred between 1200 UTC 1 October 1995 and 1200 UTC 5 October 1995. During this period the storm made landfall over the Florida panhandle. The storm reached maximum wind speed of over 130 kt on 4 October 1995. The intensity issue of Opal has drawn much attention. Issues such as the potential vorticity impact from a middle-latitude trough, the angular momentum of the lower-tropospheric inflow layer, the warm ocean temperature anomalies of the northern Gulf of Mexico, and the possible role of mesoconvective concentric eyewall are discussed in this paper. The main finding of this study is that a reduction of the gradient of angular momentum occurs above the regions of maximum convective heating. This contributes toward stronger cyclonic spinup of parcels that enter the storm environment from the middle latitudes. Another major contributor is the import of angular momentum along the lower-tropospheric inflow channels of the storm. These channels were found to be open, that is, uncontaminated with a plethora of deep convection and heavy rain. This permitted the high angular momentum to advance toward the storm?s interior thus contributing to its intensification.