The Synergistic Effect of Midlevel Dry Air and Vertical Wind Shear on the Development of the Tropical Cyclone Secondary Circulation

Cloud Model 1 (CM1) is used to investigate the synergistic effect of midlevel dry air and environmental vertical wind shear (VWS) on the development of the tropical cyclone (TC) secondary circulation, focusing on the period between model initialization to minimal hurricane intensity. The experiments vary in the initial relative humidity (RH) profile above the subcloud layer and the deep-layer vertical wind shear (VWS). The RH is initialized at a constant value in the free troposphere, ranging from 20% to 80% in increments of 20%. The VWS magnitude ranges from 0 to 12.5 m s^-1 in increments of 2.5 m s^-1. The VWS profile is westerly and increases linearly from 850 to 200 hPa. The combinations of RH and VWS give a total of 24 experiments in the RH–VWS bivariate parameter space.

A moist entropy budget is used to quantify the effect of mid- and low-level ventilation on the development of the TC secondary circulation for each experiment. Budgets terms related to mid- and low-level ventilation are compared in shear-relative quadrants to better understand the spatial structure of the ventilation pathways. Additionally, the magnitudes of the mid- and low-level ventilation terms are compared across experiments to determine the relative importance of these ventilation pathways in the RH–VWS bivariate parameter space. Results show that low-level ventilation is most sensitive to the initial RH profile, with greater low-level ventilation for an initially drier free troposphere. In contrast, mid-level ventilation is similar across the RH–VWS bivariate parameter space.