Adjoint-derived forecast sensitivity study of the extratropical transition of Floyd (1999)

Adjoint-derived forecast sensitivity studiess have been widely applied to problems related to extratropical cyclone intensity, but few extant studies have used adjoint output to understand the dynamics of extratropical transition (ET) to changes in a forecast model trajectory of an event. In this presentation, the results of an adjoint-derived forecast sensitivity study of Hurricane Floyd (1999) will be used to diagnose the sensitivity to initial conditions (and model forecast state) of the structure and intensity of Floyd and its subsequent ET state. Interpretations of the forecast sensitivities in the 36 hours prior to ET, suggest that the vorticity in a mesoscale coastal front downstream of the Floyd, rather than the vorticity of Floyd itself, is key to understanding the structure and intensity of the extratropically transitioned cyclone. The results suggest that during the cold-season, forecasts of eastern U.S. cyclogenesis events, may benefit from well analyzed lower-tropospheric vorticity fields in the vicinitiy of the nascent costal front.