To study the direct effect of asymmetric momentum on the genesis of tropical cyclone, the momentum equations in cylindrical coordinates are separated to asymmetric and symmetric momentum equations. Eddy flux is the exchange term between asymmetric and symmetric momentum. Further analysis states that the eddy flux is not only relative with the quantity of asymmetric momentum, but also depends on the asymmetric streamline pattern. If the pattern is cyclonic inflow or anticyclonic outflow, the eddy flux transforms momentum from asymmetric flow to symmetric flow. Otherwise, there is momentum transport from asymmetric to symmetric flow when the asymmetric streamline pattern is cyclonic outflow or anticyclonic inflow.

The evolution of tropical cyclone Bilis (2000, in the western north pacific) from a very weak tropical depression into tropical storm and typhoon is simulated using PSU/NCAR nonhydrostatic, two-way interactive, nested grid mesoscale model MM5V3. The model output data are used to diagnose the effect of eddy flux on the genesis of Bilis. It is found that the eddy flux plays an important role in the formation of Bilis. The asymmetric streamline patterns are cyclonic inflow and anticyclonic outflow in Bilis's early stage. To clarify the effect of asymmetric flow on the genesis of Bilis, we separate the asymmetric azimuthal and radial velocity into number-N waves by fast fourier transformation. It is found that number-2 wave is more significant than number-1 wave, but number-1 wave play more important role in eddy flux.