Collaborative Research: Environmental Control of Tropical Cyclone Activity

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This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).Recent research has established that much and perhaps most of the interannual to interdecadal variation of tropical cyclone activity integrated over ocean basins is controlled by the large-scale atmospheric and oceanic environment in which the storms develop. Statistical and model-based hindcasts of Atlantic tropical cyclone activity capture as much as 65% of the interannual variance of tropical cyclone frequency, despite taking little or no account of potential initiating disturbances, such as African easterly waves. These same techniques, however, produce greatly divergent results when applied to global warming scenarios. This highlights the need for a better physical understanding of the link between tropical cyclones and climate. This project will seek such an understanding using several different, but complementary, approaches. The first is an investigation, using a cloud-scale model, of the formation, or failure to form, of tropical cyclones from seed cyclonic disturbances in large-scale environments of radiative-convective equilibrium with shear. Previous results with this method will be extended to a wider range of wind profiles that include speed and directional shear, allowing for a careful study of cyclogenesis (or failure) in realistic environments that still can be systematically adjusted and compared. A second approach builds on the success of the Genesis Potential Index (GPI), which, when applied to historical data, has been shown to explain much of the interannual variance and long-term trend in tropical cyclone activity. Here it will be used to understand the physical factors responsible for predicted variations in the GPI in climate models simulating the response to global warming. The index will be improved using results from the cloud-resolving model simulations. Finally, one or two cases of genesis in the cloud-resolving model will be analyzed in some depth, in an attempt to test the hypothesis that tropical cyclones can develop only in mesoscale patches that are nearly saturated through most of the troposphere.Broader impacts are in advancing understanding of the relationships between tropical climate and tropical cyclone activity. This could lead to better and more useful assessments of how tropical cyclone activity may or may not change with a changing climate. This work may also yield a more fundamental understanding of the process of tropical cyclone formation, ultimately leading to better short-term forecasts of the formation of tropical depressions and storms. The project will support the training of a young scientist or post-doc and a graduate student and will contribute to further development of a flexible idealized hurricane modeling system based on the widely-used Weather Research and Forecasting (WRF) model.