Cyclogenesis and Tropical Transition in frontal zones

Tropical cyclones can form from many different precursors, including baroclinic systems. The process of an extratropical system evolving into a warm core tropical cyclone is defined by Davis and Bosart (2004) as a Tropical Transition (TT) with a further classification of systems into Weak Extratropical Cylclone (WEC) and Strong Extratropical Cyclone (SEC) to differentiate the type of cyclone transitioning. Because not much is known about the TT process, it is difficult to predict which systems will make the transition and which will not. It is hoped that the description of a common type of TT occurring along a front will aid forecasters in locating systems which might undergo TT.

QuikSCAT and other satellite data are used to locate TT cases forming along fronts and track the their transformation into tropical systems. 17 of these systems are identified as SEC TT, and their evolution from a frontal wave to a tropical system is described in 5 stages. A wind speed and SST relationship thought to be necessary for this type of transition is discussed. Several storms also forming along a front and not transitioning into tropical systems are included for comparison in the wind speed and SST relationship.

Model data from MM5 simulations of three storms, Noel (2001), Peter (2003) and Gaston (2004) are analyzed to illustrate the 5 stages of frontal TT. Both frontogenesis functions and geopotential height anomalies show the baroclinicity of each front prior to transition and temperature anomalies track and the magnitude of the warm core as it forms.