27th Conference on Hurricanes and Tropical Meteorology


Dynamical structures and precipitation distributions of transitioning tropical cyclones in Eastern Canada, 1979-2004

Shawn M. Milrad, McGill University, Montreal, QC, Canada; and E. Atallah and J. R. Gyakum

Tropical cyclones pose an annual threat to land in the western North Atlantic Basin. However, forecasters often overlook the potential danger posed by tropical cyclones that transition into extratropical cyclones at higher latitudes. Occurring mainly during the late summer and early autumn, these storms can have a large impact on the weather of Eastern Canada, especially in terms of extreme rainfall events. From 1979-2004, thirty-two storms originally classified as tropical systems by the National Hurricane Center, affected Eastern Canada during, or following completion of extratropical transition (ET). Using data from both the NCEP Regional Reanalysis (NARR) and NCEP Global Reanalysis, the dynamical structures of the thirty-two cases are examined utilizing a quasi-geostrophic (QG) approach. Via the Sutcliffe approximation to the QG Omega equation, cases are partitioned into two groups, “intensifying” and “decaying”, with each group comprised of eleven storms. Composite synoptic structures are presented for both partitioned groups, using both a QG and Potential Vorticity (PV) perspective. In addition, analyses of storm-relative precipitation distributions show that storms intensifying (decaying) during or after ET often exhibit a counter-clockwise (clockwise) rotation of precipitation around the storm center. Finally, the performance of the NARR with respect to this study is reviewed, including the lack of accuracy in the reanalysis precipitation over Canada prior to 2003, and the misrepresentation of Hurricane Juan over Nova Scotia in September, 2003. .

Session 4A, Extratropical Transition of Tropical Cyclones II
Monday, 24 April 2006, 3:30 PM-5:30 PM, Big Sur

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