A Multi-Case Perspective on Tropical Cyclone Intensification 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 that have intensified in moderately sheared environments. Previous studies have proposed mechanisms for how a TC can withstand VWS (e.g., vortex re-alignment, downshear reformation); however, most of the literature focuses on either individual TCs or idealized numerical simulations. These limited approaches prevent a complete understanding of TC intensity changes in sheared environments. To overcome these limitations, this study presents a global climatological analysis (1982–2014) of factors that differentiate intensifying versus steady-state TCs that experience moderate VWS. Here, moderate VWS is defined as 4.5−11.0 m s^–1, which represents the 25th and 75th percentiles of the global distribution of 200−850 hPa VWS magnitude.

The comparison of intensifying and steady-state TCs, which is based on best tracks and environmental diagnoses, shows significant differences between the groups. Intensifying TCs are stronger, larger, less tilted, and closer to the equator than steady-state TCs. Furthermore, intensifying TCs move over warmer sea surface temperature and greater tropospheric water vapor—especially in the upshear half—than steady-state TCs. Contrary to previous modeling studies, the comparison shows no systematic differences between the wind profiles of intensifying and steady-state TCs. This lack of systematic differences is attributed to the timescale of VWS variability, which is not accounted for in idealized model simulations. Based on these results, intensification in spite of moderate VWS seems more likely under conditions that favor more axisymmetric and deeper convection.