Thermodynamics of Tropical Cyclone Overland Maintenance and Intensification

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The energy that fuels a tropical storm's or a hurricane's winds is typically drawn from warm ocean waters. However, for some tropical cyclones, the maximum surface winds have been observed to be maintained or even increase over land. A particularly noteworthy case is given by Tropical Storm Erin over Oklahoma, which dramatically and unexpectedly intensified into a strong tropical storm as it approached Oklahoma City from the west, resulting in millions of dollars of wind-related damage. The few prior investigations into this phenomenon generally agree that the required energy is drawn from the underlying land surface. However, they disagree on the extent to which the energy is associated with land warmth versus wetness, as well as on the physical processes that allow for a sufficient amount of energy to be concentrated near the surface so that it can be transferred to the tropical cyclones. Furthermore, the ideas advanced by these previous studies have generally been developed using highly simplified numerical weather prediction model simulations, with few instances in which these ideas have been tested for actual tropical cyclones. Consequently, this project seeks to significantly advance basic understanding of the energetics supporting tropical cyclone maintenance and strengthening over land while rigorously testing this understanding for a large sample of observed events. A partnership with the Ronald E. McNair Post-Baccalaureate Achievement Program at University of Wisconsin-Milwaukee will support the professional development and academic persistence of an undergraduate scholar from a traditionally underrepresented background, helping to increase representation of minority individuals within the workforce pipeline in the atmospheric and related sciences.This research tests three guiding hypotheses: remote surface energy exchange is the primary but not exclusive control on tropical cyclone intensity change over land; non-desert soils cannot be sufficiently warmed to result in sufficient upward enthalpy flux for tropical cyclone maintenance or intensification over land; and intensity change over land is equally sensitive to surface energy exchange and initial finite-amplitude atmospheric variability. Factor separation applied to idealized numerical simulations is used to test the first two hypotheses, whereas ensemble-initialized real-data simulations are used to test the applicability of the idealized simulation results to the real atmosphere. An expanded climatology of atmospheric and substrate properties associated with overland tropical cyclone maintenance and intensification is to support the numerical model simulations and advance knowledge of the environments in which these events occur. Through these activities, this research will reconcile competing theories regarding the thermodynamic processes necessary to support non- or weakly baroclinic tropical cyclone maintenance and intensification over land through quantifying the respective contributions of local and non-local land-surface energy exchange to overland tropical cyclone intensity change. Given the ongoing scientific debate regarding the surface latent heat flux magnitudes needed to permit tropical cyclone intensification over water, findings from the research will also advance knowledge of the energetics of the traditional overwater tropical cyclone intensification process.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.