Abstract
Abstract. Special forecasts from the Global Forecast System (GFS) model were used in this study to evaluate how the intensification process in a tropical cyclone is represented in this model. Several tropical cyclones that developed in 2005 were analyzed in terms of the storm-scale circulation rather than more traditional measures such as maximum wind or minimum central pressure. The primary balance governing the circulation in the planetary boundary layer is between the convergence of environmental vorticity, which tends to spin up the storm, and surface friction, which tends to spin it down. In addition, we employ recently developed ideas about the relationship between precipitation and the saturation fraction of the environment to understand the factors controlling mass, and hence vorticity convergence. The budget of moist entropy is central to this analysis. Two well-known governing factors for cyclone intensification emerge from this study; surface moist entropy fluxes, dependent in the model on sea surface temperature and cyclone-generated surface winds, and ventilation of the system by dry environmental air. Quantitative expressions for the role of these factors in cyclone intensification are presented in this paper.
Highlights
The ability of numerical models to correctly simulate tropical cyclones has increased over time as a result of improved data assimilation techniques and observational platforms, a better representation of the physical processes and an increase in computational resolution
Results from the Global Forecast System (GFS) model have been used by the National Hurricane Center (NHC) for several years to predict the track of tropical cyclones that develop over the Atlantic and the East Pacific basins; in addition, the model is showing some ability to predict tropical cyclogenesis and intensity change
We believe that it is valuable to determine the factors which control genesis and intensity in the GFS model; this exercise should be useful in formulating hypotheses about how tropical cyclogenesis occurs in nature, suggesting what should be sought in observational programs
Summary
The influence of the ocean on formation and intensification has been extensively studied for many years. Palmen (1948) found that tropical cyclones only originate when sea surface temperatures (SST) are larger than 26◦ C. Marın et al.: Intensification of tropical cyclones in the GFS model studies (Gray, 1968; Knaff and Weaver, 2000; Emanuel et al, 2004) have mentioned this mechanism as a strong inhibitor of cyclone development. Raymond et al (1998) showed that the circulation tendency in the planetary boundary layer (PBL) around a tropical cyclone depends primarily on the imbalance between the convergence of absolute vorticity into the system and the Reynolds stress due to surface friction, the former causing spinup and the latter spindown This imbalance governs cyclone spinup and spindown in the real world, and presumably in the GFS model.
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