A comparison of the tropical disturbances that spawned Hurricanes Dennis, Emily, Katrina

An unusually high number of tropical cyclones formed during the 2005 Atlantic hurricane season. The tropical disturbances from which these cyclones developed are varied and interesting. Infrared, water vapor, and visible satellite data are the basis for analyzing the large-scale influences and genesis of three major hurricanes: Dennis, Emily, and Katrina. The analyses are focused on the transformations that occurred prior to tropical cyclogenesis.

The proliferation of strong tropical waves in late June and early July indicated that 2005 hurricane season could be unusually active. Preceding Hurricane Dennis was a large tropical wave that exited the west coast of Africa with a large mesoscale convective system 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. 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.

The birth of Hurricane Emily could be traced to mesoscale convection that developed in the lee of the Ethiopian highlands. There mesoscale convective systems (MCSs) produced mesoscale convective vortices that underwent several regenerative cycles and propagated with and through tropical easterly waves. The mesoscale vortex that spawned Emily maintained a symmetric cloud pattern once it exited the continent, although the associated convection intensified and weakened through several diurnal cycles. Convection strengthened around the vortex and

The genesis Katrina was influenced by mid-latitude and tropical interactions. The precursor to Hurricane Katrina could be traced to convection along the tail-end of a mid-latitude trough on 14 August. That convection drifted south and then northwestward. Between 17 and 18 August, evidence of a vortex northeast of Puerto Rico could be seen in water vapor image. That upper-level vortex drifted northwestward and then southwestward towards the eastern Bahamas over the next three days. During the same period, a second cyclonic circulation centered at 500hPa over Hispaniola produced several MCSs. A tropical wave entered the eastern Caribbean but weakened as it moved westward. By 21 August, convection over the Caribbean was primarily associated with the mid-level circulation over Hispaniola. Between 22 and 23 August, the northern upper-level circulation merged with the strengthening mid-level circulation. Convection intensified around was now a tropical depression. An interesting feature of the period of intensification was very dry middle-level air mixing into to the west and north of the tropical depression.

These preliminary analyses present a small sample of the variety of factors that influence tropical cyclogenesis. Further analyses include examination of ocean characteristics during the periods of intensification.