A thermodynamic pathway leading to rapid intensification of tropical cyclones under shear

Abstract:

Rapid intensification (RI) forecast of tropical cyclones under moderate vertical wind shear remains challenging in the operational forecast center. Based on a successful ARW-WRF simulation of Typhoon Vicente (2012) (Chen et al., 2017, 2018, 2019), this study analyzes the thermodynamic processes that help saturate the TC inner core before RI onset using a column-integrated moist static energy (MSE) framework. Results indicate that the nearly saturated inner core in the lower-middle troposphere is achieved by an increase in the column-integrated MSE, as column water vapor accumulates while the mean column temperature cools.

The sign of the column-integrated MSE tendency depends on the competition between surface enthalpy fluxes, radiation, and vertical wind shear-induced ventilation effect. The reduction of ventilation above the boundary layer due to vertical alignment is crucial to accumulate the energy within the inner core region. The vertical alignment of the vortex is achieved via downshear reformation, with the aid of vigorous deep convection in the left-of-shear quadrants A comparison of the RI simulation with a null simulation further highlights the impact of vortex structure on the thermodynamic state adjustment and TC intensification.