Evolution of tangential and radial flows of Typhoon Nari (2001) at landfall

In order to understand the evolution of tangential and radial flows of Typhoon Nari (2001) upon its landfall on Taiwan, absolute angular momentum (AAM) and radial wind budgets of Nari are conducted by analyzing the MM5 simulation results from Yang et al. (2008) with high spatial and temporal resolutions (2-km horizontal grid spacing and 2-min output interval). Before landfall, Nari's tangential wind exhibited an evident wavenumber-1 signature, with radial inflow at low levels and outflow at upper levels. During landfall, the horizontal wind field became highly asymmetric. Both radar observation and MM5 simulation indicate that the radial inflow at low levels became thicker and stronger, and the sloping radial outflow jet was maximized at mid levels over the rugged terrains.

From the diagnostics of AAM budget, we find that after landfall the tangential wind was decreased at low levels through the upward transport of weaker tangential wind and further reduced by friction and turbulence with the boundary layer. It is determined from the radial wind budget that the centrifugal force was greater than the pressure gradient force, leading to supergradient winds over Taiwan's topography. The radial inflow at low levels became stronger and thicker after landfall, resulting from the vertical transport of stronger radial inflow upward. Both centrifugal and pressure gradient forces contributed to radially-outward acceleration, resulting in the sloping radial outflow jet at mid levels over the mountains.