Evaluation of Bogus Vortex Techniques with Four-Dimensional Variational Data Assimilation

Abstract

The effectiveness of a four-dimensional variational data assimilation (4DVAR) technique for creating ?bogus? vortices in numerical simulations of hurricanes is evaluated in this study. A series of numerical experiments is conducted to generate initial vortices for Hurricane Georges and Bonnie (1998) in the Atlantic Ocean by assimilating bogus sea level pressure and wind information into a mesoscale numerical model (MM5). Several different strategies are tested for investigating the sensitivity of the initial vortex representation to the type of bogus information. While some of the results in this study confirm conclusions made in previous studies, some significant differences are obtained regarding the role of bogus wind data in creating a realistic bogus vortex. In contrast with previous studies in which the bogus wind data had only a marginal impact on creating a realistic hurricane, this study concludes that the wind information is very important because 1) with assimilation of only bogus sea level pressure information, the response in wind field is contained largely within the divergent component, with strong low-level convergence leading to strong upward motion near the center; and 2) with assimilation of bogus wind data only, an expected dominance of the rotational component of the wind field is generated. In this latter case, the minimum pressure is also adjusted significantly, although the adjusted sea level pressure does not always match the actual hurricane minimum pressure. The generated vortex offers a smooth start to the forecast and leads to a significant improvement in the forecast. Only when both the bogus sea level pressure and wind information are assimilated together does the model produce a vortex that represents the actual intensity of the hurricane and results in significant improvements to forecasts of both hurricane intensity and track. As the 4DVAR experiments are performed with relatively coarse horizontal grid resolution in this study, the impact of vortex size on the structure of the initial vortex is also evaluated. The authors find that when the scale of the specified bogus vortex is smaller than that which can be resolved by the model, the assimilation method may result in structures that do not completely resemble observed structures in hurricanes. In contrast, when the vortex is sufficiently large for it to be resolved on the horizontal grid, but not so large as to be unrealistic, more reasonable hurricane structures are obtained.