A numerical study of near-equatorial genesis of Typhoon Vamei

The processes that lead to the formation and intensification of the near-equatorial Typhoon Vamei (December, 2001) were examined based on the simulation of a high-resolution model. The simulated Typhoon formed in the South China Sea, south of 2N, had a short lifetime, and reached only category one intensity, similar to the observed.

The formation involves the interactions among large scale of winter monsoonal circulation, the scale of the regional cyclonic circulation (the Borneo Vortex of order ~ 100 km that occurs in the lee of Borneo) and the scale of the numerous mesoscale convective vortices (MCVs of order ~ 10 km) within them. MCVs form in the presence of horizontal shear vorticity in the lower troposphere along a convergence line on the eastern edge of an exceptionally strong monsoonal northerly wind surge. Areas of high potential vorticity (PV) in the lower troposphere develop within MCVs, (consistent with previous studies of “hot towers”) and can persist well after the convection has ceased. The Borneo Vortex acts to circulate the MCVs or associated remnant PV (or vorticity) anomalies, towards the center. The formation of a Typhoon is precluded by the development of a dominant region of elevated PV associated with convection near the center of the regional cyclonic circulation. Development then proceeds through the merging of other MCVs with the central MCV.

The intensification period from tropical depression to Typhoon is marked by two distinct ~ 3 hourly rapid intensification periods. The two RI periods are both associated with the merging with the storm core of a MCV as it rotates around to the northwest side into the storm's path. The intensification occurs as the vorticity associated with the MCV is absorbed by the storm core and as enhanced convergence triggers strong convection in the storm core stretching the vortex column in the vertical and lowering the central pressure.

Evidence is presented that corroborates the theory that increases in PV at upper levels are associated with the evaporation of upper level precipitation. The consequent large gradient in equivalent potential temperature at upper levels leads to very high PV as the tropical cyclone develops and cyclonic vorticity is advected upward. Thus it appears that during tropical cyclogenesis, sources of PV at both upper and lower levels are crucial for the build up of high PV Throughout the troposphere.

Analysis of the vorticity budget reveals that the convergence of planetary vorticity played an insignificant role in the storm development. The largely topographically driven Borneo Vortex provides the necessary background vorticity. Although the tilting term and the vertical vorticity advection term play a significant role in the vorticity budget during the storm development, the horizontal vorticity advection and divergence terms provide the largest sources for the low-level vorticity growth.