A satellite-based examination of tropical cyclogenesis in the Atlantic during June and July 2005

An unusually high number of tropical cyclones formed during the early part of the 2005 Atlantic hurricane season. Infrared, water vapor, and visible satellite data are the basis for analyzing the large-scale influences and genesis of tropical cyclones that originated in Africa during June and July 2005.

Of particular interest are the circulations that spawned Hurricanes Dennis, Emily, and Frances. These hurricanes could be traced to mesoscale convective systems that developed in the lee of the Ethiopian highlands. The mesoscale convective systems produced mesoscale convective vortices that underwent several regenerative cycles and propagated with and through tropical easterly waves as they moved over the ocean. The analyses are focused on the transformations that occurred over the eastern Atlantic prior to tropical cyclogenesis.

Preceding Hurricane Dennis was a large tropical wave that exited the west coast of Africa as a mesoscale convective system dissipated at the apex of the wave. That wave was defined by the classic inverted "V" shaped convective cloud band. On 2 July, a weak secondary axis formed ahead of the main easterly wave axis. The bulk of the deep convection remained aligned to the main wave while the new short wave was marked by trade-wind cumulus in the lower troposphere. Within 24 hours, the western-most short-wave had strengthened and a dual short-wave pattern was outlined by deep convective clouds. Two subsequent mesoscale convective “bursts” signaled intensification from tropical waves to tropical depression. The first convective “burst” developed at 50W on the downwind side of the eastern wave. From classical easterly wave theory, the convection is expected to be strongest on the right side of the inverted “V”, close to the apex, rather than close to the ridge. Questions arise as to the role of the ocean in the growth of the convection. In the case of Frances, the precursor tropical wave and vortices appeared to be modulated by interactions with an extra-tropical trough. Unlike Dennis and Frances, the mesoscale vortex that spawned Emily could be more easily tracked. It maintained a more symmetric cloud pattern as while its associated convection intensified and weakened across the continent and the ocean.

Further analysis of the dynamics and thermodynamics will be integrated with the satellite images to better understand the factors that created an above average frequency of early season tropical cyclones over the eastern tropical Atlantic.