Quikscat-Derived Near-Surface Vorticity during Tropical Cyclogenesis

Traditional surface and upper-air observations are often absent over the tropical oceans. This lack of routine in-situ measurement, outside of special field programs, has limited the observational study of tropical cyclogenesis. Remote sensing from satellites, however, can provide information in regions where surface-based observing networks are not present. This study utilizes infrared satellite imagery and QuikSCAT-derived near-surface vorticity from the 2005 hurricane season in the North Atlantic to examine the relationship between deep convection and low-level vorticity during tropical cyclogenesis. QuikSCAT-derived cyclonic relative vorticity is identified in association with developing easterly wave disturbances tracked using NHC products and 3-hourly infrared satellite imagery. Area-averaged vorticity near mesoscale regions of convection within the easterly wave envelope is then computed. In most of the 19 cases examined, the low-level vorticity followed the convective evolution, decreasing or remaining nearly constant during periods of inactive convection and increasing as convective activity increased. A composite of North Atlantic easterly wave disturbances was constructed to characterize the average evolution of near-surface vorticity during tropical cyclogenesis. 48 hours prior to genesis, the average tropical disturbance has a region of cyclonic relative vorticity about 125 km in diameter with peak magnitude of approximately 1x10-4 s-1. During its subsequent evolution, the vorticity of the composite disturbance increases as convection increases until a tropical cyclone forms. These results are considered in the context of prior and future numerical simulations of tropical cyclogenesis.