Mesoscale processes for super heavy rainfall of Typhoon Morakot (2009) over Southern Taiwan

Analyses and simulations show that under favorable large-scale situations, this unprecedented precipitation was caused first by the convergence of the southerly component of the pre-existing strong southwesterly monsoonal flow and the northerly component of the typhoon circulation.

Then the westerly component of southwesterly flow pushed the highly moist air (mean specific humidity > 16 g/kg between 950 and 700 hPa from NCEP GFS data set) eastward against the Central Mountain Range, and forced it to lift in the preferred area. From the fine-scale numerical simulation, not only did the convergence itself provide the source of the heavy rainfall when it interacted with the topography, but also convective cells existed within the typhoon's main rainband.