Thermodynamic Pathways of Vertical Wind Shear Impacting Tropical Cyclone Intensity Change: Energetics and Lagrangian Analysis

Abstract

Abstract This study analyzes the variations in the thermodynamic cycle and energy of a tropical cyclone (TC) under the influence of vertical wind shear (VWS), exploring the possible thermodynamic pathways through which VWS affects TC intensity. The maximum energy harnessed by the TC diminishes alongside a decrease in storm intensity in the presence of VWS. In the sheared TC, the ascending branch of the thermodynamic cycles of TC shifts toward lower entropy, which is related to the reduction in entropy in the eyewall and/or the increase in entropy and enhanced upward motion outside the eyewall. Moreover, the descending leg shifts toward higher entropy due to the increase in entropy and weakening of downward motion in both the ambient environment and the upper troposphere. These changes in the ascending and descending branches could reduce the work done by the heat engine cycle, with the former playing a primary role in the presence of VWS. Given that the ascending branch is influenced by the eyewall and the rainbands outside the eyewall under VWS, the thermodynamic pathways could be categorized into inner ventilation and outer ventilation based on the location of their roles. The pathways associated with inner ventilation primarily reduce the entropy in the eyewall. In addition to the conventional low- and midlevel ventilation, the inner ventilation also encompasses new pathways entering the midlevel eyewall after descending from the upper level and ascending from the boundary layer. Conversely, the pathways of outer ventilation are related to the increase in the entropy outside the eyewall. These include the ascent of high-entropy air to the middle and upper troposphere related to the inner and outer rainbands, the outward advection of high-entropy air from the eyewall in the midlevel and upper level, and air warming by the descending draft from the upper- to midlevel troposphere. These insights contribute to a nuanced understanding of the sophisticated interactions within TCs and their response to VWS. Significance Statement Vertical wind shear (VWS) is known to modify the thermodynamic structure of tropical cyclones (TCs), thereby affecting their development. The incorporation of multiple thermodynamic mechanisms amplifies the complexity of related studies and predictions. Within the context of a unified heat engine framework, this study aims to paint a relatively comprehensive picture of the key thermodynamic pathways through which VWS impacts the intensity of TCs. Contrary to previous studies, we emphasize the increase in entropy and the intensified upward motion outside the eyewall, which play pivotal roles in diminishing the intensity of TCs in the presence of VWS. Gaining an understanding of these critical mechanisms and structural changes offers insights and guidance that can enhance the prediction of sheared TCs.