Quasigeostrophic Forcing of Ascent in the Occluded Sector of Cyclones and the Trowal Airstream

Abstract

A numerical model-based analysis of the quasigeostrophic forcing for ascent in the occluded quadrant of three cyclones is presented based upon a natural coordinate partitioning of the Q vector into its along-and acrossisentrope components, Q s and Q n , respectively. The Q n component describes the geostrophic contribution to the rate of change of the magnitude of p (traditional frontogenesis), whereas the Q s component describes the geostrophic contribution to the rate of change of direction of p (rotational frontogenesis). It is shown that convergence of Q s simultaneously creates the isobaric thermal ridge characteristic of the thermal structure of occluded cyclones and provides the predominant dynamical support for ascent within the occluded quadrant. The absence of significant Q n convergence there suggests that quasigeostrophic (Q-G) frontogenesis plays a subordinate role both in forcing vertical motions and in affecting three-dimensional structural changes in the occluded sector of post-mature phase midlatitude cyclones.