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 km to below 10 km over a period of six hours 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, six hours 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 three hours 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 to 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.
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