Extended Abstract (204K)12D.5
The extratropical transition of hurricane Irene (1999)
Anna Agusti-Panareda, Univ. of Reading, Reading, Berks, United Kingdom
Extratropical transitions (ET) of tropical cyclones (TC) are common phenomena in all ocean basins where TCs recurve polewards. After the TC experiences ET, a rapid deepening can take place resulting in the development of a very large and deep extratropical cyclone.
The ET of hurricane Irene (1999) was an example of such an "explosive" ET. Irene formed in the Caribean and experienced ET as it moved poleward resulting in a low pressure system which deepened 39 hPa in 24 hours (according to the Met Office analyses).
The major question this work tries to address is : what is the role of the hurricane in the explosive extratropical development ? To try to answer this question forecasts have been performed using the Met Office Unified Model from initial states with and without the hurricane. The circulation and temperature anomalies associated with the hurricane have been removed from the initial state using piecewise potential vorticity inversion. The moisture anomaly around the hurricane core has also been removed.
The results show that an explosive extratropical cyclogenesis event takes place regardless of the presence of the hurricane in the initial conditions. However, the hurricane makes a significant difference to the extratropical development as indicated by differences in the track and mslp evolution of the resulting extratropical cyclone. When Irene is present the track of the extratropical cyclone is more zonal and the cyclone deepening rate is twice as fast as when Irene is not present.
Session 12D, tropical cyclone extratropical transition (Parallel with Sessions 12A, 12B, and 12C)
Thursday, 2 May 2002, 8:30 AM-10:30 AM
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