Simulation of formation of a near-equatorial typhoon Vamei (2001)

A community mesoscale model is used to simulate and understand processes that led to the formation and intensification of the near-equatorial typhoon Vamei that formed in the South China Sea in December, 2001. The simulated typhoon resembles the observed in that it had a short lifetime and a small size, formed near the equator (south of 2 degrees N), and reached category-one intensity. The formation involved the interactions between the scales of the background cyclonic circulation (the Borneo Vortex of order similar to 100 km) and of mesoscale convective vortices (MCVs, in the order similar to 10 km). Before tropical cyclone formation MCVs formed along a convergent, horizontal shear vorticity line on the eastern edge of an exceptionally strong monsoonal northerly wind surge. The typhoon genesis is marked by three rapid intensification periods, which are associated with the rapid growth of potential vorticity (PV). A vorticity budget analysis reveals that the increases in low-level vorticity during the rapid intensification periods are attributed to enhanced horizontal vorticity fluxes into the storm core. The increase of the horizontal vorticity flux is associated with the merging of areas of high PV associated with MCVs into the storm core as they are advected by background cyclonic flows. The increases in PV at upper levels are associated with the evaporation of upper level stratiform precipitation and increases of vertical potential temperature gradient below the maximum stratiform cloud layer. It appears that two key sources of PV at upper and lower levels are crucial for the build up of high PV and a deepening of a cyclonic layer throughout the troposphere.