Depiction of Upper/Lower Vortex Interaction Associated with Extratropical Cyclogenesis

Abstract

Using observed data and model simulations, an attempt is made to verify that baroclinic instability can be viewed as an interaction and mutual amplification of a pair of upper- and lower-tropospheric potential vorticity (PV) perturbations. This view has recently been advanced by Hoskins et al. and McIntyre. Two episodes from March and May 1989, encompassing two lee-cyclogenetic events east of the Rocky Mountains and one over the central Gulf states, are selected to demonstrate this concept. In order to depict the low-level PV maximum which forms infinitely thin sheets of infinite PV, the display of PV in cartesian space is replaced by a display of absolute vorticity in isentropic space. Perspective computer drawings of iso-surfaces of absolute vorticity in three-dimensional space are found to be a convenient means of conveying the desired information.