A Convectively-Coupled Framework for Understanding Hurricane Azimuthal Asymmetries and Secondary Eyewall Formation
CitationGUO, JIAHUA. 2020. A Convectively-Coupled Framework for Understanding Hurricane Azimuthal Asymmetries and Secondary Eyewall Formation. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
AbstractTropical cyclones (TCs) or hurricanes are among the most devastating natural disasters. Improvement of intensity forecasts of hurricanes is not yet satisfactory. Among all the factors that can influence the intensity of a hurricane, the structure is crucial for accurate intensity and track forecasts. Important structural features include asymmetric features such as spiral rainbands and axisymmetric features such as secondary eyewall formation. Throughout this dissertation, we are going to investigate the potential mechanisms associated with these features from a convectively coupled perspective.
In Chapter 2, we applied space-time spectral analysis to study the asymmetric perturbations in a hurricane and found two distinct power peaks in most of the variables. We obtained the structure of each mode by regressing each field onto a window-filtered index. We found that the structure of the fast-propagating wave is similar to that of the unstable mixed vortex-Rossby-inertia-gravity wave. The slow-propagating wave has a retrograde intrinsic propagating speed and a vertical structure that resembles that of convectively coupled waves.
Inspired by the findings from Chapter 2, we then developed a convectively coupled eigenfrequency model to investigate the linear instability of TC-like vortices in Chapter 3. We confirmed that the instability of the mixed vortex-Rossby-inertia-gravity wave is consistent with the fast-propagating signal observed in the WRF simulations. With convectively coupled dynamics, a branch of the nearly stationary wave becomes unstable.
To more accurately constrain the convective parameterization used in Chapter 3, we adopted the methodology developed by Kuang  in Chapter 4. We first constructed several sets of linear response functions along the radius of the hurricane. We then derived the parameters in the convective parameterization from these linear response functions through model order reductions. The framework helps to estimate the parameters with more quantitative accuracy.
In Chapter 5, we introduced an axisymmetric framework that can potentially be applied to study the mechanisms behind secondary eyewall formation (SEF). The axisymmetric framework consists of a dry axisymmetric dynamic model that simulates hurricane-scale circulation and a cloud-resolving model that resolves local convection. Potentially, this framework allows more flexibility to investigate various hypotheses of SEF.
Citable link to this pagehttps://nrs.harvard.edu/URN-3:HUL.INSTREPOS:37365905
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