Journal
QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY
Volume 137, Issue 661, Pages 2174-2193Publisher
WILEY-BLACKWELL
DOI: 10.1002/qj.891
Keywords
potential vorticity; Rossby waves; warm conveyor belt; extratropical transition; diabatic processes; hurricane Hanna; extratropical cyclone
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Funding
- German Research Foundation (DFG) as part of the research unit PANDOWAE [FOR896]
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This study highlights the importance of diabatic processes for the complex interaction of weather systems in the North Atlantic-European sector during the week of 714 September 2008. A chain of events occurred including the extratropical transition (ET) of hurricane Hanna, a subsequently developing extratropical cyclone, the formation of an upper-level potential vorticity (PV) streamer that protruded towards Europe and triggered intense rainfall, and the genesis of a Mediterranean cyclone. A PV perspective is adopted along with trajectory calculations to elucidate the diabatic modification of the midlatitude flow. Important diabatic PV modifications occurred at upper levels, associated with the cross-isentropic transport of low-PV air within warm conveyor belts (WCBs). These were diagnosed during the ET of Hanna and the development of the extratropical cyclone near Newfoundland. The WCBs contributed to the amplification of ridges downstream of each cyclone and to the subsequent elongation of Hanna's upstream trough into a PV streamer. This streamer eventually triggered the Mediterranean cyclogenesis. The second major effect of the diabatic processes occurred on smaller scales, in the low and middle troposphere. The remnants of Hanna's tropical PV core advected moist air towards the baroclinic zone leading to condensational PV production in the lower troposphere. In contrast, in the case of the extratropical cyclone, diabatic PV production occurred within its WCB at mid levels. These diagnostic analyses corroborate the potential of diabatic processes associated with extratropical flow systems for the modification of both the low-level vortices and the upper-level Rossby wave guide. Copyright (C) 2011 Royal Meteorological Society
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