A Numerical Study of the Genesis of Tropical Storms Observed during the FGGE Year

Abstract

This study utilizes the First GARP Global Experiment's (FGGE) analyzed dataset and a relatively fine scale regional model in combination to investigate the feasibility of numerically simulating tropical disturbances during the FGGE year, 1979. Four different cases were investigated including a cyclone. TC-17, in the Indian Ocean, a developing hurricane, David, and a nondeveloping wave in the Atlantic, and a multi-storm case, Tip and Roger, in the Pacific. The results were promising when using ECMWF FGGE data in that simulations of genesis or nongenesis were achieved in the three developing cases and in the one nondeveloping case. The accuracy of the intensification rates varied from case to case. For example, in the simulation of TC-17, the maximum low level winds were simulated to be ?45 m s?1 while observations indicated winds of only 22 m s?1. However, in the case of David, the maximum winds increased at a slower rate than observed, while in the case of Tip the slow intensification rate was correctly simulated. An interesting result was the high correlation between model precipitation patterns in the simulations and observed satellite cloud photos. These results indicate that the environment in which an incipient disturbance is embedded plays a major role in the genesis process. An additional striking result was the wide variability of storm development and structure from case to case. Tropical storm David was simulated to be a relatively small scale storm whereas Tip was simulated to be a storm with an enormous area of gale force winds. The model simulations also produced different distributions of the low level wind maximum relative to the moving storm with banding of a number of meteorological fields, including precipitation and vorticity. The formation of storms was related to the presence of an incipient disturbance possessing cyclonic low level vorticity, and ample high relative humidity together with a strong coupling between the disturbance phase speed and the upper level flow field. Most cases including the nondeveloping wave contained upper level anticyclonic conditions. All cases included a weak warm, upper level anomaly including the nondeveloping wave case. Also, it was found that environmental upward motion is not always an accurate indicator of genesis.