The dynamics of heavy rainfall in landfallen tropical systems

Currently precipitation forecasts associated with land-falling tropical cyclones are based on a simple algorithm where the maximum 24-h precipitation (in inches) is forecast by 100/v, where v (in mph) is the translational speed of the cyclone. This algorithm, however, provides little insight as to the precipitation distribution and intensity that can be expected in a land-falling tropical cyclone. Furthermore, several recent cases (Danny 1997, Dennis, Floyd, and Irene 1999) show that precipitation distribution and intensity can be drastically altered by interactions with mid-latitude troughs and jet streaks which often result in extratropical transitions. Occasionally, these interactions produce catastrophic rainfalls as illustrated by hurricane Floyd in September 1999. This talk will present diagnostics of results from case studies of the storms mentioned above, from a Quasi-Geostrophic potential vorticity perspective, designed to elucidate the important dynamical and thermodynamical processes responsible for the modulation of the precipitation distribution and intensity.

Preliminary results suggest that regions of strong baroclinicity are often created as the cold-core anomaly associated with the mid-latitude trough approaches the warm-core anomaly associated with the tropical system. The resulting baroclinic zone can resemble a coastal front, with the significant exception that this baroclinic zone is a very deep feature, often extending throughout the troposphere. As the extratropical transition takes place, cool dry air is wrapped around the southern extent of the cyclone, resulting in a marked decrease in the precipitation on the east side of the cyclone. Meanwhile, a significant increase in both the aerial extent and rate of the precipitation occurs in the northwest quadrant of the cyclone in response to deep ascent associated with warm-air advection and differential cyclonic vorticity advection, sometimes leading to large-scale flooding. Storms failing to transition after making landfall (i.e. Dennis) tend to produce localized flooding relatively near the center of circulation in the absence of any synoptic-scale forcing mechanisms to either focus or expand the precipitation shield.