Lagrangian boundaries and flow visualization of the genesis of Hurricane Felix (2007)

The genesis of Hurricane Felix (2007) was examined using the high-resolution ECMWF analysis data (0.5 X 0.5 degree), QuikSCAT surface wind and TRMM 3-hourly precipitation. Felix was born in an easterly wave over the Atlantic. ECMWF analysis shows that an inverted trough propagated westward prior to the genesis of Felix and that a closed circulation formed within the trough at the genesis time in the resting frame of reference. In the Lagrangian view that translates with the easterly wave (a so-called “wave gauge” reference frame), a closed circulation was detectable near the surface more than 2 days before the official declaration of genesis by NHC forecasters. It is also evident that a sub-gyre or a mesoscale vortex emerged within the wave critical layer about two days before genesis. The mesoscale vortex was predominantly rotational with high values of Okubo-Weiss parameter and was accompanied by persistent convection with high saturation fraction. It propagated westward along with the parent wave, and later became hurricane Felix. The diagnoses suggest that the wave critical layer along the trough axis serves as a “sweet spot” for genesis in the synoptic-scale disturbance and that the translating frame of reference is an appropriate choice for understanding the transition from an easterly wave to a tropical storm vortex.