Mechanisms for Rapid Intensity Changes in Hurricanes

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9714253 Frank The U.S. Weather Research Program (USWRP) is an interagency activity designed to perform and implement the research necessary to improve the delivery of weather services to the nation. Under this Program, the National Science Foundation, the National Oceanic and Atmospheric Administration (NOAA), the National Aeronautics and Space Administration and the Office of Naval Research are jointly evaluating and supporting research of high priority to the USWRP. The intense winds and rainfall! associated with landfalling hurricanes are organized on smaller scales than those that can be forecast using currently available analyses and models. Although much of the damage caused by hurricanes results from small-scale and rapidly changing features of the core, the physical mechanisms governing changes in the winds, convection and related phenomena are poorly understood. There is, however, growing evidence that the major changes and asymmetries observed in the cores of hurricanes are strongly modulated by larger- scale features that can be predicted. A promising approach to improving forecasts of the structure and intensity of hurricanes near landfall is to improve our quantitative understanding of these causal relationships. This research program will study the mechanisms by which hurricane structure and intensity vary near and after landfall using a series of numerical simulations of hurricanes. A state-of-the-art non-hydrostatic model will be used to simulate rapid, small-scale variations in the hurricane core as it responds to imposed variations in wind speed, vertical wind shear, large-scale gradients and anomalies of potential vorticity, and changes in boundary layer processes. A primary focus of the study will be on the manner in which the imposed variations in external circulations and surface properties affect the hurricane's secondary circulation, and hence modulate the magnitude and distribution of convection. The re search will also analyze the manner in which asymmetries in the storm convection, including convection in the spiral rainbands, affect the storm's intensity and core structure. The results should provide significantly improved understanding of the processes governing rapid changes in hurricane structure, and they should be of immediate use in improving forecasts of the winds, storm surge and rainfall in hurricanes. ***