Thermodynamic Aspects of Tropical Cyclone Spin-up

The thermodynamic aspects of tropical cyclone (TC) spin-up near the center of the wave pouch, a region of approximately closed Lagrangian circulation within the wave critical layer, are examined through diagnoses of a high-resolution numerical simulation and dropsonde data from a field campaign. It is found that the meso-β area near the pouch center is characterized by high saturation fraction, small difference in equivalent potential temperature (θe) between the surface and the middle troposphere, and a short incubation time scale. Updrafts tend to be more vigorous in this region, presumably due to reduced dry air entrainment, while downdrafts are not necessarily suppressed. The thermodynamic conditions near the pouch center are thus critically important for TC formation.

The balanced responses to convective heating and stratiform heating at the pre-genesis stage are examined using the Sawyer-Eliassen equation. Deep convection is concentrated near the pouch center, and the strong vertical and radial gradients of latent heat release effectively force the transverse circulation and spin up a surface proto-vortex near the pouch center. Stratiform heating induces modest mid-level inflow and very weak low-level outflow, which contributes mid-level spin-up without substantially spinning down the low-level circulation.

The analysis of dropsonde data shows that the mid-level θe increases significantly near the pouch center one to two days prior to genesis but changes little away from the pouch center. This may indicate the impending TC genesis, and also suggests that the critical information of TC genesis near the pouch center may be masked out if spatial average is taken over the pouch scale.