Effects of Low-Level Flow Orientation and Vertical Shear on the Structure and Intensity of Tropical Cyclones

This article explores the simultaneous effect of vertical wind shear (VWS) and low-level mean flow (LMF) on tropical cyclone (TC) structure evolution. The structural evolution of 180 western North Pacific TCs from 2002 to 2014 was measured by a new parameter, the RV-ratio. This parameter is defined as the ratio of a TC’s radius of 35-kt wind (R_Vmax) to its maximum wind speed (V_max) at peak intensity. Whereas TCs with an RV-ratio in the lowest quartile of all 180 samples favored intensification over expansion and 76% of these TCs experienced rapid intensification, TCs with an RV-ratio in the top-most quartile favored size-expansion and only 16% of these experienced rapid intensification. Results also showed that the RV-ratios of vertically-sheared TCs were found to significantly correlate with the relative orientation of LMF with respect to the deep-layer VWS vector. Specifically, when a TC interacted with an LMF with a left-of-shear (right-of-shear) orientation, the TC was more likely to favor intensification (expansion) over expansion (intensification). Furthermore, under the effect of weak VWS magnitudes and left-of-shear LMF orientations, the TCs analyzed in our data exhibited an 80% probability of forming a more intense but smaller storm.

To understand the interaction between TC, VWS, and LMF, this study further analyzed the asymmetric surface flow and asymmetric convection of TCs using QuikSCAT winds and CMORPH satellite rainrates. Processes associated with these asymmetric features induced by VWS and LMF were hypothesized to affect the structural development of vertically-sheared TCs.