Midlevel Warm‐Core Development Associated With the Rapid Intensification of a Convectively Asymmetric Tropical Cyclone Under Strong Vertical Wind Shear

Abstract

Under strong environmental vertical wind shear (VWS), tropical cyclones (TCs) typically exhibit pronounced vertical tilt and precipitation asymmetry that are generally considered unfavorable for rapid intensification (RI). Nonetheless, some of these TCs still experience RI, presenting challenges for intensity forecasts. To better understand such events, this study investigates the characteristics and mechanisms of warm‐core evolution during the RI of the asymmetric Typhoon Krosa (2019). Unlike the double warm cores typically found in more symmetric RI TCs, both observations and a high‐resolution simulation show that the majority of Krosa's RI process features the development of a single midlevel warm core, which dominates the surface pressure drop. A potential temperature budget reveals that this midlevel warm core is driven by the intrusion of warm air from outer‐core region, contrary to the typical view that midlevel air intrusion weakens TCs. Backward trajectory analysis shows that the warm air contributing to the midlevel warming mainly originates left of shear and undergoes near‐adiabatic subsidence before entering the eye. This subsidence is associated with a branch of descending inflow between 5–9‐km heights, which is part of the secondary circulation induced by stratiform precipitation downwind of downshear convection, and is driven by negative buoyancy. This descending inflow does not organize until the shear‐induced asymmetric precipitation shield extends left of the shear, indicating the critical role of shear‐relative precipitation structure changes and associated circulation asymmetries in asymmetric RI. This work highlights the distinct thermodynamic features and evolving mechanisms of asymmetric RI TCs compared to more symmetric ones.