Climatological Analysis of Tropical Cyclone Intensity Changes under Moderate Vertical Wind Shear

Even though vertical wind shear (VWS) is generally an inhibiting factor for tropical cyclone (TC) intensification, there are numerous examples of TCs in nature that intensify in moderately sheared environments. Potential explanations have been proposed to explain the ability of a TC to withstand VWS (e.g., vortex re-alignment, downshear reformation); however, most of the literature focuses on either individual TCs or idealized numerical simulations. This study presents a global climatological analysis of TC intensity changes under moderate VWS with the aim of identifying factors that favor or inhibit intensification. Vortex-scale factors (e.g., intensity, size, latitude) were obtained from best track data, and large-scale factors [e.g., sea surface temperature (SST), environmental moisture] were obtained from the 0-hr Statistical Hurricane Intensity Prediction System (SHIPS) analyses. Moderate VWS was defined as 6.25-12.5 m s^-1 based on percentiles of the global distribution of 200-850 hPa shear magnitude. Two groups were compared: intensifying (24-h intensity change of at least 10 m s^-1) and weakening (24-h intensity change of at least -10 m s^-1) TCs characterized by moderate VWS. Preliminary results show that intensification is likely to occur for TCs with warm SSTs and high tropospheric moisture. Moreover, intensifying TCs are weaker, smaller, and further equatorward than weakening TCs. This result contradicts theoretical work that predicts large, strong, high-latitude TCs to be more resilient to VWS than small, weak, low-latitude TCs. While further investigation is needed, the discrepancy between results hints that TC intensity changes under moderate VWS depend on both environmental and internal TC characteristics.