The Separate Roles of Geostrophic Vorticity and Deformation in the Midlatitude Occlusion Process

Separate vector expressions for the rate of change of direction of the potential temperature gradient vector resulting from the geostrophic vorticity and geostrophic deformation, referred to as QVR and QDR, respectively, are derived. The evolution of the thermal structure and forcing for quasigeostrophic vertical motion in an occluded cyclone are investigated by examining the distributions of QVR and QDR and their respective convergences. The dynamics of two common structural transformations observed in the evolution of occluded cyclones are revealed by consideration of these separate forcings. First, the tendency for the sea level pressure minimum to deepen northward and/or westward into the cold air west of the triple point is shown to be controlled by the convergence of QVR, which is mathematically equivalent to thermal wind advection of geostrophic vorticity, a well-accepted mechanism for forcing of synoptic-scale vertical motion. Second, the lengthening of the occluded thermal ridge and surface occluded front are forced by the nonfrontogenetic geostrophic deformation, which rotates the cold frontal zone cyclonically while it rotates the warm frontal zone anticyclonically. The net result is a squeezing together of the two frontal zones along the thermal ridge and a lengthening of the occluded thermal ridge. The associated convergence of QDR along the axis of the the thermal ridge also forces vertical motion on a frontal scale. This vertical motion accounts for the clouds and precipitation often observed to extend from the triple point westward to the sea level pressure minimum in the northwest quadrant of occluding cyclones. 1