Study of Processes Leading to Tropical Cyclone Intensity Change

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One of the primary research areas under the U.S. Weather Research Program (USWRP) is to perform the research necessary to improve track and intensity forecasts of tropical cyclones (TC). The major goal of this research is to understand the processes that lead to intensity changes in tropical cyclones. While there are many processes responsible for tropical cyclone intensity change, the PI will focus on those occurring in the inner core region (within about 150 km from the cyclone center), including interactions between inner and outer spiral rainbands, eyewall convection; vortex Rossby waves, and eyewall breakdown process. The working hypothesis is that the effect of any external forcing on TC intensity occurs either directly or indirectly through mesoscale processes in the inner core region. The following scientific questions will be addressed: a. How does the internally generated asymmetric structure in the inner core region interact with the symmetric vortex and affect the TC intensity even without any external forcing from environmental flow? b. How do the outer spiral rainbands form, why they most frequently form between 80 km and 150 km from the TC center, and how do they propagate and interact with the inner spiral rainbands and eyewall convection, affecting the TC intensity? c. How does the eyewall respond to external forcing, such as that from vertical shear in the environmental flow, and what is the consequence of this response? To answer these questions, the PI will perform idealized numerical simulations using a fully compressible, nonhydrostatic, three-dimensional multiply nested, movable mesh, high-resolution model (TCM4) developed by the Principal Investigator. Comprehensive diagnostics of the numerical results will be performed to elucidate the physical mechanisms. The broader impacts of the research include the improved understanding of the physical processes that affect the tropical cyclone structure and intensity changes, leading to improved prediction of tropical cyclone structure and intensity change, especially by numerical weather prediction models. In particular, results from this research will address the possible role of inner core asymmetric dynamics and their response to external forcing in causing the intensity change of tropical cyclones. Educational benefits of the work include the training of graduate students. This research is of high priority within the USWRP.