Comparison of observed gale radius statistics

Abstract

Forecasts of tropical cyclone track and intensity have long been used to characterize the evolution and expected threat from a tropical storm. However, in recent years, recognition of the contributions of subtropical cyclogenesis to tropical storm formation and the process of extratropical transition to latter stages of the once-tropical storm’s lifecycle have raised awareness about the importance of storm structure. Indeed, the structure of a cyclone determines the distribution and intensity of the significant weather associated with that storm. In this study, storm structure is characterized in terms of significant wind radii. The radii of tropical storm, damaging, and hurricane-force winds, as well as the radius of maximum winds are all analyzed. These wind radii are objectively derived from the H*Wind surface wind analysis system. Initially, six years of these data are examined for consistency with previous studies. Having ascertained that the H*Wind radii are realistic, detailed comparisons are performed between the H*Wind and NHC Best Track wind radii for two years (2004 and 2005) of North Atlantic tropical storm and hurricane cases. This intercomparison reveals an unexpected bias: the H*Wind radii are consistently larger than the NHC Best Track for all but the smallest and least intense storms. Further examination of the objectively-determined H*Wind tropical storm force wind radius data compared to subjectively-determined radii for the same storm times demonstrates that the objective wind radii are underestimating the extent of the tropical storm force wind area. Since the objective H*Wind radii are large compared to the NHC Best Track – and yet underestimate the area of tropical storm force winds – this argues for further examination of the methods used to ascertain these significant wind radii.