Keynote Talk: Tropical and Extratropical Transitions and Precipitation with Landfalling Hurricanes

The tropical eastern Atlantic is a common breeding ground for classical Cape Verde hurricanes that originate from African disturbances. Over the westward Atlantic hurricanes development is less classical and generally occurs poleward of 20°N. Many of the western Atlantic storms originate from remnant higher latitude cold-core baroclinic disturbances that penetrate equatorward into the subtropics. If ascent associated with these baroclinic disturbances can trigger organized deep convection over sufficiently warm seas surface temperatures the initial cold-core disturbance can often transition into a warm-core hurricane by the tropical transition (TT) process (e.g., Diana 1984, Michael 2000, Alex 2004, Catarina 2004). The opposite process, extratropical transition (ET), involves the transition of a hurricane into a cold-core disturbance (e.g., Hazel 1954, Agnes 1972, Lili 1996, Floyd 1999, Irene 1999). In the Atlantic basin 40-50% of hurricanes recurving into higher latitudes experience some form of ET. ET storms, as well as non-transitioning (NT) storms, that make landfall or near landfall are frequently associated with significant mesoscale precipitation events that can lead to major inland flooding (e.g., Floyd 1999) or severe weather outbreaks (e.g., Ivan 2004) In this presentation, TT and ET processes will be reviewed with an emphasis on important mesoscale signatures seen in both developments. During a TT, mesoscale deep convection acts to redistribute potential vorticity (PV) so that the PV associated with the triggering baroclinic disturbance is eroded with consequent reduction in the deep layer shear over the transitioning cyclone. Vortical convective hot towers appear to lead to low-level vorticity concentration and amalgamation during the TT process. During an ET, increasing baroclinicity and shear lead to a disruption of the quasi-symmetric hurricane circulation with the resulting shift of the precipitation shield to the left of the storm track as thermal advection develops. Mesoscale frontal circulations may develop with the resulting concentration of precipitation into banded structures with embedded convective elements. Orographic features may add further complexity to the mesoscale precipitation signature during an ET. Examples of TT and ET will be shown to illustrate the characteristic behavior of Atlantic storms undergoing these processes. In the ET examples, where data is available over land, emphasis will be placed on documenting the observed mesoscale precipitation structure and explaining its distribution within the context of the evolving synoptic-scale circulation pattern. An example of a landfalling NT storm that produced a severe weather outbreak (Ivan 2004) will be presented to contrast with ET events. Finally, mention will be made of a possible western Atlantic TT and genesis experiment (WATTAGE) to acquire critical mesoscale observations to quantify thermodynamic and dynamic processes critical to the TT problem. When ready, this presentation will be posted at: http://www.atmos.albany.edu/student/tomjr/lb1link/abq_tt_et.html.