An Examination of a Model's Components during Tropical Cyclone Recurvature

Abstract

The main goal of this study is to investigate the relative contributions from the components of dynamics and physics of a forecast model, toward the understanding of the recurvature dynamics of hurricanes. A number of experiments were conducted using the Florida State University Global Spectral Model (FSU GSM), run at a global resolution of 126 waves. The method of physical initialization was used to ?spin up? the model, 24 h prior to the 5-day forecast period to better define the initial water vapor, sensible heat fluxes, and rainfall rates. The usage of the FSU GSM employed a partitioning of the dynamics and physics into separate components, that assumes a residue-free budget of the models' components. The model dynamics were broken down into a nonlinear advective component and also a linear dynamics (rest of the dynamics) partition. The model physics were partitioned into four components: deep convective heating, large-scale precipitation (nonconvective stable rain), total radiation, and shallow convection and surface fluxes. A total of four cases were examined, two each for Hurricanes Cindy and Dennis?1200 UTC 26 and 27 August, and 1200 UTC 28 and 29 August, occurring during the 1999 Atlantic hurricane season. The series of model runs were formulated to examine the tropical cyclone forecast tracks, suppressing one or more of the partitions for each time step, through day 5 of a forecast. Initial experiments coupling both the nonlinear advective and the linear dynamics (summed to equal the ?total dynamics?) found that the total dynamics component resulted in a weakly recurving track for each of the storm cases. The addition of the physics components incrementally sharpened the recurving track through time. While the full model dynamics was used as a baseline, the results of this study indicated that the deep convective heating (also referred to as deep convection) and total dynamics combined to produce a recurving track for both storms, for 50% of the four examined cases. The remaining cases required that the shallow convection and surface fluxes partition be included along with the deep convection and total dynamics. It was found that incremental improvements occurred with both the deep convective heating and shallow convection and surface fluxes partitions, however, the additions of the large-scale precipitation and radiation partitions did not significantly improve the forecast track relative to the full model, and their resulting magnitudes were significantly smaller than the rest.