The Implication of Outflow Structure for the Rapid Intensification of Tropical Cyclones under Vertical Wind Shear

Abstract

The implication of outflow structure for tropical cyclone (TC) rapid intensification (RI) is investigated via a climatological study using the best track, reanalysis, and infrared brightness temperature data during 1980–2019. Composite analyses are performed in a shear-relative framework for the RI events under different strengths of environmental shear. Results show that for the RI events under moderate (4.5–11 m s21) or strong (.11 m s21) environmental shear the RI onset follows a significant increase of upper-level outflow upshear of the storm, which is intimately linked with the increasing active convection upshear. The intensified outflow blocks the upper-level environmental flow and thus decreases the local shear, building an environment favorable for RI. In contrast, the RI under weak environmental shear (,4.5 m s21) is found to be less attributed to this outflow-blocking mechanism. Comparison between the RI and non-RI cases under moderate or strong environmental shear reveals that the RI cases tend to have stronger outflow and convection in the upshear flank than the non-RI cases, confirming the importance of outflow blocking on the occurrence of RI. Statistical analysis further indicates that the 24-h future intensity change under moderate or strong shear is more negatively correlated with the local shear than with the environmental shear, implicating the potential of local shear and upshear outflow as predictors to improve the forecasting of TC intensity change and especially RI. Further analysis suggests that the environmental thermodynamic conditions may play an important role in modulating the upshear convection and thus outflow blocking.