Characteristics of Convective Processes and Vertical Vorticity from the Tropical Wave to the Tropical Cyclone Stage in a High-Resolution Numerical Model Simulation of Tropical Cyclone Fay (2008)

The statistics of convective processes and vertical vorticity from the tropical wave to the tropical cyclone stage are examined in a high-resolution simulation of Tropical Cyclone Fay (2008). The intensity of vertical velocity approximately follows the truncated lognormal distribution in the model simulation, which is consistent with previous observational studies. The upward motion at the pre-genesis stage is weaker compared to mature hurricanes or midlatitude thunderstorms. The relatively strong upward velocities occupying a small areal fraction make a substantial contribution to the upward mass and moisture fluxes and condensation.

It is also found that upward motion and downward motion both intensify with time, but the former is stronger than the latter, and the mean vertical motion and the mean vertical mass flux thus increase with time. By contrast, the maximum anticyclonic vorticity is comparable to the maximum cyclonic vorticity in magnitude. Both cyclonic vorticity and anticyclonic vorticity intensify with time, but the former covers a larger areal fraction in the lower and middle troposphere and becomes dominant throughout the troposphere after genesis.

Sensitivity test with different model resolutions were carried out to test the robustness of the results. When the horizontal grid spacing is reduced, the size of updrafts decreases and the number of updrafts increases, but the areal fraction of updrafts, the mean vertical velocity and the mean vertical mass flux are rather insensitive to the model resolution, especially in the lower troposphere when the model resolution is 1 km or higher. This may explain why models with relatively coarse resolution can simulate tropical cyclogenesis reasonably well.