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Dissertation/Thesis Abstract

Understanding the Evolution of Tropical Cyclones through the ψ – χ Framework
by Das, Sweta, Ph.D., The Florida State University, 2020, 85; 27543543
Abstract (Summary)

The understanding of the evolution of a Tropical Cyclone (TC) has been a topic of research for several years. During the hurricane season not all thunderstorm events embedded in the African easterly waves or otherwise evolve into organized convection with a closed low pressure system, manifesting into TCs. In this work we suggest that one of the ways to objectively analyze the evolution of the TC is to understand the evolution of the conversion of the available potential energy into kinetic energy on the scale of the disturbance. This study explores the energetics of the interaction between Streamfunction (Psi: ψ )-Velocity Potential (Chi: χ) in the numerical simulations of the TCs.

Using the output of separate 48-hour WRF simulations of three Atlantic TCs: Cindy and Irma of 2017, and Michael 2018, we analyze the time history of the conversion of their kinetic energy from the irrotational to the non-divergent components of the winds. All of these TCs had varied intensities with Cindy being the weakest and Irma being the strongest over the simulation period, which WRF simulated with reasonable fidelity in the evolution of their peak intensities. We show that at 850 hPa, the fractional conversion of the kinetic energy from the irrotational to the non-divergent component of the wind increases as the TC intensifies and is higher for the stronger TCs than weaker TCs. Contrastingly, in the outflow level of the TC this transfer of kinetic energy is weaker for stronger TCs than the weaker TCs. Our analysis reveals that when the gradients of the streamfunction and velocity potential are large and oriented parallel to each other both in the large-scale TC environment and in the region of the primary circulation of the TC, then the TC is favored to intensify with robust conversion of the kinetic energy of the irrotational flow(Kχ) to kinetic energy of non-divergent flow (Kψ) at 850 hPa. In contrast, however in the outflow layer, we require a slower conversion of Kχ to Kψ for a TC to intensify otherwise it leads to increased inertial instability and weakening of the TC. We arrive at similar conclusions when we contrast the evolution of the tropical cyclones from its genesis to intensifying stages. Likewise, when we examine the sensitivity of the simulations of the tropical cyclones to the choice of microphysics, we find that parameterizations that engenders strong conversion of Kχ to Kψ at 850 hPa and weak conversion of Kχ to Kψ at the outflow level leads to the simulation of stronger TCs. Therefore, analyzing this conversion rate of kinetic energy of the flow field helps in understanding the evolution of the intensity of TCs.

Indexing (document details)
Advisor: Misra, Vasubandhu, Liu, Guosheng
Commitee: Chicken, Eric, Bourassa, Mark, Chagnon, Jeffrey
School: The Florida State University
Department: Earth, Ocean & Atmospheric Science
School Location: United States -- Florida
Source: DAI-B 82/1(E), Dissertation Abstracts International
Subjects: Meteorology
Keywords: Tropical cyclones
Publication Number: 27543543
ISBN: 9798662459786
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