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

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 alongand acrossisentrope components, Qs and Qn, respectively. The Qn component describes the geostrophic contribution to the rate of change of the magnitude of =pu (traditional frontogenesis), whereas the Qs component describes the geostrophic contribution to the rate of change of direction of =pu (rotational frontogenesis). It is shown that convergence of Qs 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 Qn 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. A cyclonically ascending, cloudand precipitation-producing airstream that originates in the warm-sector boundary layer and flows through the trowal portion of the occluded structure is supported by the upward vertical motions implied by the identified Q-G forcing. This airstream is referred to as the ‘‘trowal airstream’’ and it is shown to be responsible for the production of the ‘‘wrap around’’ cloud and precipitation commonly associated with occluded systems. The relationship of the trowal airstream to previously identified cloud and precipitation producing airflows in cyclones is discussed.