Genesis of Pre-hurricane Felix (2007) and the Role of the Wave Critical Layer

The formation of pre-hurricane Felix (2007) in a tropical easterly wave is simulated using the WRF model with a high-resolution nested grid configuration to permit the representation of cloud system processes. The model simulation starts at the wave stage of the precursor disturbance. It is shown that the low-level convergence associated with the Inter-Tropical Convergence Zone (ITCZ) helps to enhance the wave signal and extend the wave pouch to the boundary layer. A diagnosis of the model simulation indicates a bottom-up development process within the wave critical layer and supports the new “marsupial paradigm” of tropical cyclogenesis advanced recently by Dunkerton, Montgomery and Wang (2009). The region of a closed Lagrangian circulation within the wave critical layer, the so-called wave “pouch”, protects the moist air inside from dry air intrusion, and provides a favorable environment for the formation of vortical hot towers (VHTs). The wave pouch also provides a focal point for diabatic merger of the VHTs and their vortical remnants. A tropical storm forms near the center of the wave pouch via system-scale convergence in the lower troposphere and vorticity aggregation.

The study also examines the evolution of stratiform vs. convective precipitation within the wave pouch. It is shown that moist deep convection is sustained near the wave pouch center. While stratiform precipitation occupies a large area even at the storm stage, moist deep convection makes a comparable contribution to the rain rate at the pre-genesis stage, and a larger contribution than stratiform processes at the storm stage. The convergence profile near the pouch center is found to become dominantly convective with increasing convection. Low-level convergence plays a key role in forming and intensifying the near-surface closed circulation, while the mid-level convergence associated with stratiform precipitation helps to increase the mid-level circulation and build a tropospheric-deep vortex.

Sensitivity tests with different physics options and initial conditions demonstrate the same basic picture of the marsupial paradigm. It suggests that the diabatic merger of VHTs, vorticity aggregation, and thus the genesis location of the tropical storm are largely controlled by the moist wave critical layer dynamics while the genesis time and intensity of the proto-vortex depend on the details of the mesoscale organization, which is less predictable.