Inner-Core Humidification and Prelandfall Rapid Intensification of Typhoon Mekkhala (2020) in Strong Vertical Wind Shear

Abstract A numerical simulation was conducted in this study to investigate the physical processes governing the unusual rapid intensification (RI) of Typhoon Mekkhala (2020) under strong vertical wind shear. The vortex tilt exhibited a gradual decrease from approximately 70 to below 10 km over a period of 6 h prior to and following the onset of the RI. Nevertheless, due to substantial midlevel ventilation, the storm intensity exhibited only slight variations in the early phase of the vortex alignment, 6 h prior to the RI onset. The RI occurred subsequent to the precipitation within the inner core becoming increasingly axisymmetric as the stratiform clouds developed cyclonically. The precipitation axisymmetrization was found to be closely associated with inner-core humidification in the left-of-shear and upshear quadrants, occurring 3 h before the RI onset. As previously documented, horizontal vapor advection greatly influenced the moistening process. A new finding of this study was that the “showerhead effect,” namely, moistening due to sublimation from stratiform clouds and evaporation from stratiform rain, could be predominantly conducive to the humidification of the inner core compared with horizontal vapor advection. Another noteworthy finding in the tangential wind budget was that the eddy radial vorticity flux contributed negatively and positively during the pre-RI and RI of Mekkhala, respectively, in conjunction with the dominant positive contribution resulting from the mean radial vorticity flux. Significance Statement Rapid intensification has been a long-standing part of the challenge of tropical cyclone forecasting. This study aims to understand the dynamics of the rapid intensification of a typhoon in the presence of strong vertical wind shear, which is generally known to be unfavorable for rapid intensification. The results suggest that evaporation of stratiform precipitation could contribute more substantially to the humidification in the inner core than horizontal advection of moisture. In this circumstance, inner-core precipitation axisymmetrization and rapid intensification occurred. These results will hopefully shed more light on how the tropical cyclone community perceives the effects of humidification near the tropical cyclone core on tropical cyclone rapid intensification.