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Georgina Paull, Konstantinos Menelaou, and M. K. Yau

Abstract

This study examines the influences of an axisymmetric heat source on the tangential wind structure of a tropical cyclone (TC). Specifically, the response of a TC due to the effect of convection located in varying inertial stability profiles was calculated. Using an idealized heat source, the thermodynamic efficiency hypothesis and the dynamic hypothesis for lower-level tangential wind acceleration are studied with the use of a balanced 2D model. These two frameworks for calculating the lower-level tangential wind acceleration are then compared to an idealized but thermally forced version of a nonlinear 3D model (WRF). It is found that using either of the 2D balanced model approaches to calculate the tangential wind acceleration results in an underestimation when compared to the full nonlinear simulation. In addition, the thermodynamic efficiency approach also shows a radial shift in the location of the maximum lower-level tangential wind acceleration. Sensitivity experiments in the context of the WRF Model in varying background inertial instabilities were investigated. It is shown that as the eyewall-like heating is shifted to larger values of inertial stability, there is a decrease in the induced secondary circulation in tandem with a spinup of the lower-level tangential winds. This intensification appears to be modulated by the low-level radial advection of absolute vorticity.

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Georgina Paull, Konstantinos Menelaou, and M. K. Yau

Abstract

Latent heat release from condensational heating has been recognized as one of the dominating energy sources of a tropical cyclone. Here we argue that other microphysical processes may also play an important role. From an analysis of a real-case simulation of Hurricane Katrina (2005), it was found that cooling from evaporation and melting of some frozen hydrometeors radially outside the eyewall region can have similar magnitudes as condensational heating. Based on this finding, idealized thermally forced experiments were performed. The specified heating and cooling functions mimic those found in the Hurricane Katrina run. The results indicated that the addition of cooling enhances the lower-level inward radial winds, which in turn increases the acceleration of the lower-level tangential winds through an enhanced transport of absolute vorticity. Sensitivity experiments on varying the structure of the cooling functions and the background state of the vortex demonstrate that the lower-level tangential wind acceleration is more sensitive to changes in the vertical structure and location of the cooling than the radial characteristics. In addition, the lower-level acceleration is sensitive to variations in the inertial and static stabilities rather than the vertical tangential wind shear of the initial vortex and its environment.

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