Extended Abstract (168K)13D.1
Influence of the downstream state on extratropical transition: Hurricane Earl (1998) case study
R. J. McTaggart-Cowan, McGill University, Montreal, PQ, Canada
The extratropical transition (ET) of hurricanes in the western North Atlantic Ocean poses problems for operational forecasters and researchers alike. Complex interactions between the remnant tropical vortex and midlatitude upper-level disturbances result in low predictive skill and complicated diagnoses. The research presented here quantifies the influence of features downstream of the transitioning cyclone on the ET and reintensification process. A more complete understanding of the dynamics associated with these events is crucial to enhancing their predictability and will prove extremely valuable to the North American eastern seaboard inhabitants, who can be severely affected by transitioning storms.
Typical Cape Verde hurricanes move westward across the equatorial Atlantic and curve northward near the Caribbean or in the Gulf of Mexico. Subsequent weakening can be attributed to the combined effects of cooler surface temperatures, reduced moisture supply, and increased vertical wind shear. Often, the ex-tropical system dissipates rapidly during recurvature. However, some systems (approximately 25% in the western Pacific Ocean) undergo ET and reintensify rapidly at latitudes as high as 50*N. The presence of an upstream trough has been shown to be necessary for such a rapid redevelopment since it connects typical midlatitude quasigeostrophic forcings with the moist lower-level ex-hurricane dynamics. The nature of the interaction between the tropical and extratropical features, however, is modulated to a certain extent by the atmospheric state downstream of the transitioning system.
Two modes of ET and reintensification have been analyzed during a double-transition event that occurred in September 1998. Hurricane Earl transitioned to an intense baroclinic system in the western North Atlantic; simultaneously, Hurricane Danielle underwent a tropical mode of reintensification in the eastern section of the basin. Dynamic tropopause maps highlight the structural differences associated with the distinct baroclinic and tropical modes of ET and reintensification. In the baroclinic mode, cyclonic potential vorticity (PV) rollup at upper levels aids in the deformation of a near-surface baroclinic zone and associated frontogenesis. The tropical mode is characterized by strong warm advection ahead of and around the core of the system. The isolation of the warm, moist tropical air within 1000 km of the center by stratospheric PV streamers reduces mixing with the cooler surrounding and provides a locally tropical environment during the storm’s redevelopment. It is shown that the presence of a downstream jet/front structure is necessary for baroclinic mode reintensification. The location of the transitioning storm in the entrance or exit to the zonal jet proves to be the crucial factor in determining what mode of reintensification will occur.
Session 13D, Tropical Cyclone Extratropical Transition II (Parallel with Sessions 13A, 13B, and 13C)
Thursday, 2 May 2002, 11:00 AM-12:15 PM
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