Atmospheric Processes Assocoated with Heavy Rainfall During the Extra-Tropical Transitions of Ivan and Frances

The remnants of hurricane Ivan produced very heavy rainfall over much of Pennsylvania and southern New York on September 17-18, 2004. This presentation focuses primarily on the evolution and structure of the atmosphere during this event.

It is shown that heavy rain associated with the remnants of Ivan expanded across the Mid-Atlantic area during the afternoon on September 17, 2004, as a potential vorticity maxima at 850-500 hPa, associated with the remnants of Ivan, moved increasingly closer to a potential vorticity maxima at 400-200 hPa, associated with a trough over the upper Midwest. During this time, the rain area expanded into the shape of a large comma, similar to what is typically observed with a well-developed winter storm. An examination of the relationship between the comma-shaped rainfall pattern, large-scale and frontal-scale forcing, and atmospheric instability revealed some structures typically observed with a winter storm. Specifically, bands of heavy rain appeared to form within a large stratiform rainfall area, in a region characterized by elevated moist symmetric instability, within the upward branch of a very strong, thermally direct circulation associated with a sloping frontal zone. The strength of the southeasterly upward branch of the circulation appeared to be enhanced by the presence of a pronounced upper-tropospheric jet entrance region northwest of the storm. Meanwhile, convective bands of heavy rain located southeast of the stratiform rainfall region appeared to form in areas characterized by surface-based instability and low-level convergence. The relationship between rainfall amounts and terrain for this event is also examined, along with the relationship between the environmental thermal structure and the accuracy of WSR-88D precipitation estimates.

Included in this study is a comparison of the forcing for heavy rain with Ivan versus the forcing for heavy rain associated with the remnants of hurricane Frances, which also produced very heavy rains across portions of the Mid-Atlantic and Ohio Valley States (on September 8-9, 2004). The reason for performing a direct comparison between these two storms was to see whether or not the patterns associated with Ivan were truly unique. The rainfall pattern associated with the remains of Tropical Cyclone Frances was quite similar in shape, extent and intensity to the pattern associated with Ivan. In addition, many of the structures observed with Ivan also appeared with Frances, including the tendency for heavy rain bands to form within a stratiform rain area in regions characterized by the juxtaposition of elevated moist symmetric instability and the upward branch of a strong thermally direct circulation associated with a sloping frontal zone.