High resolution simulations of tropical cyclogenesis in idealized environments with and without shear

The formation of a tropical cyclone from a weak, mid-level vortex is studied in idealized environments with high-resolution simulations using the WRF model. Environments with no mean flow, mean flow with no wind shear, and mean flow with shear are considered. Even in the absence of wind shear, deep convection must occur near the vortex center for 2 to 3 days before genesis occurs. During this time, the vortex goes through important structural and thermodynamic changes, with the vortex intensifying at middle levels while the inner-core becomes nearly saturated. After near saturation of the core and sufficient contraction of the mid-level vortex, a new, smaller-scale vortex appears rapidly at the surface, which becomes the core of a rapidly developing tropical cyclone.

In the presence of significant wind shear, the process is modified and delayed. Before genesis, the vortex is substantially tilted, with the mid-level circulation pushed 100-200 km ahead of the surface center. Nonetheless, mid-level saturation increases, until a large convective burst relocates the low-level center underneath the mid-level center. Genesis and intensification follow shortly thereafter.