Collaborative Research: Understanding Tropical Cyclone Energetics and Intensification in Environmental Vertical Wind Shear

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Environmental vertical wind shear has long been recognized as a major inhibiting factor for tropical cyclone (TC) intensification since it acts to tilt the vortex. The wind shear also facilitates the dry-air intrusion into the TC core region to reduce the buoyancy of convective updrafts. Yet, TCs do develop or even undergo rapid intensification (RI) in light to moderate shear conditions. An accurate prediction of both timing and rate of TC intensification in such conditions, in particular RI, poses a great challenge in numerical forecasts of TCs. Understanding the physical processes that overcome the shear induced negative effects on TC intensification is, thus, important for the potential improvement of TC intensity forecasts. Research to date showed that several processes, such as the reduction in ventilation and boundary-layer recovery due to the enhancement of surface enthalpy fluxes, could overcome the negative impacts imposed by the shear, leading to the intensification of a TC vortex. However, details in TC energetics and dynamical route to TC intensification associated with these processes remain poorly understood. The overall goal of this project is to advance the understanding of mechanisms underlying the TC intensity change after the genesis stage in a sheared environment. The identified key physical processes that differentiate the TC intensification rates in different shear and thermodynamic environments will provide useful guidance for operational forecast of TC intensity change including both RI and slow intensification.To achieve the research objectives, this project aims to provide new insights into the key thermodynamic changes and their dynamic responses that govern TC intensification in a sheared environment using idealized Hurricane Weather Research and Forecast (HWRF) sensitivity numerical experiments and the data generated by the HWRF Ensemble Data Assimilation System (HEDAS) that possesses the ability to assimilate storm-relative observations with different time and spatial resolutions in the numerical system. Comprehensive analyses on the HEDAS dataset and HWRF idealized simulations are carried out. Research activities include (a) analyses of TC energetics during post-genesis stages within the moist static energy (MSE) framework; (b) Exploration of the linkage between TC energetics and vortex spin-up dynamics using a novel diagnostic tool that can yield an improved understanding of the TC intensification driven by various dynamic and thermodynamic forcing in an unbalance framework; and (c) Investigation of key issues regarding the role of the reduction of mid- and low-level ventilation and boundary-layer recovery in TC intensification in a sheared environment using parcel trajectory analyses.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.