A numerical investigation of the convective systems in the vicinity of southern Taiwan associated with Typhoon Fanapi (2010): Formation mechanism of double rainfall peaks

The westbound Typhoon Fanapi (2010) brought 872 mm daily rainfall over the coastal plain in southern Taiwan, which is comparable to the extreme rainfall of 1080 mm over the southern Central Mountain Range (CMR). In this study, the Weather Research and Forecast model was used to investigate the formation mechanism of the two rainfall peaks and the associated dynamics of the convective systems in the vicinity of southern Taiwan. As the vortex that deflects somewhat southward near landfall moves over the CMR, the vortex slows down significantly during westward departure and induces strong near‐coastal convergence at lower levels with the westerly to southwesterly outer flow. The associated intense latent heating of the near‐coastal rainfall greatly contributes to positive potential vorticity (PV) with enhanced vorticity stretching near the convex eyewall when the vortex becomes more asymmetric and longitudinally elongated. The positive PV tendency near the coast is induced not only by vertical differential diabatic heating (DDH) but also by the longitudinal DDH mainly associated with positive vorticity tilting. The longitudinal DDH indeed dominates all other processes in the PV generation inside the eyewall further offshore, which is vital to the maintenance of the eyewall with the strong banded convergence and is special to typical intense cyclones over the open ocean. The formation of the double intense rainfall peaks cannot occur without a precise departing track near the southwest coast in association with the slowdown of the vortex core when the CMR height is significantly reduced or latent heating is deactivated after landfall. Key Points The latent heating of the near‐coastal rainfall greatly contributes to potential vorticity with enhanced stretching near the convex eyewall The longitudinal differential heating dominates other processes in the PV generation inside the eyewall further offshore The formation of double rainfall peaks cannot occur without a precise departing track associated with the slowdown of the vortex core