Collaborative Research: Dynamic and Thermodynamic Control of Tropical Cyclone Intensity in Sheared Environments

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In this collaborative research effort, the Principal Investigators will undertake a comprehensive investigation of the physics of the interaction between tropical cyclones and sheared flow in which they are embedded, focusing on the mutual operation of dynamic and thermodynamic processes. They hypothesize that tropical cyclones are weakened by the injection of low-entropy, middle level air into the vortex core by vortex Rossby waves excited by the interaction between the vortex and environmental shear flow. The research emphasis (mutual importance of dynamical and thermodynamical processes) departs from most previous investigations, which have focused almost exclusively on direct dynamical effects. The research will have a direct path to improved prediction of tropical cyclones through the improvement of an operational forecast model.The starting point of the research is two key theoretical developments that have matured over the previous decade: the quantitative theory of thermodynamic control of hurricane intensity, and the theory governing the generation, behavior and wave/mean flow interaction of vortex Rossby waves. From these points, the Principal Investigators will develop an extended theory for the generation and maintenance of vortex Rossby waves as a consequence of the interaction between vortices and ambient shear flow, addressing as well the rate at which these waves flux passive tracers in and out of the vortex core region. This theory will be used as guidance in modifying the thermodynamic cycle to account for dry air injection at middle levels by vortex Rossby waves. Having developed this theoretical framework, the Principal Investigators will test it using a suite of models, focusing on fully three dimensional simulations at high resolution using a nonhydrostatic model. It is expected that the theory will be modified based on the results of these tests. Finally, the researchers will use this theory to develop a parameterization of Rossby wave-induced fluxes of low entropy air, for use in axisymmetric models, including the aforementioned operational forecast model.The intellectual merit of the project: The nature of the interactions between tropical cyclones and the ambient wind field has remained enigmatic for many years. Previous efforts to understand and quantify this interaction have focused almost exclusively on the dynamics. In this project, the researchers will marry thermodynamic and dynamic aspects of the interaction to create a general and complete description of the process.The broader impacts of the project: The skill of hurricane intensity forecasts remains poor in spite of substantial advances in the skill of storm track forecasts. Research on tropical cyclone intensity is a high priority under the U.S. Weather Research Program. This research may well lead to an advanced understanding of the effect of environmental shear on hurricane intensity; such advanced understanding will be directly incorporated into an existing hurricane intensity prediction model, thereby leading directly, and within the time frame of this award, to improved hurricane intensity forecasts. In the process of carrying out this research, the Principal Investigators will train advanced graduate students in the science of hurricanes and hurricane prediction, thereby helping to foster a new generation of researchers and forecasters.