Diagnostic Evaluation of Vertical Motion Forcing Mechanisms by Using Q-Vector Partitioning

Abstract

Q-vector partitioning has proven to be a useful tool for the understanding of the frictionless, adiabatic processes responsible for the generation of synoptic-scale vertical motion in the extratropical atmosphere. Partitioning of Q into components parallel and normal to the isotherms on an isobaric surface is standard practice in studies dealing with vertical motion and frontogenesis. This paper is concerned with vertical motion only and examines the consequences of projecting Q onto isohypses, instead of isotherms, on an isobaric surface. Specifically, the Q vector is partitioned in the natural coordinate system that follows the geostrophic wind. The novelty with this partitioning is that it naturally leads to the evaluation of different vertical motion forcing mechanisms, among which are those related to flow curvature and to confluence or diffluence. This evaluation is illustrated by applying the new Q-vector partition to a gridded analysis of a real weather situation. An important conclusion is that the thermal advection by horizontal geostrophic shear is as significant to the forcing of vertical motion as the geostrophic confluence/diffluence. While this result has previously been obtained in the study of frontal dynamics, this is the first application of this finding to the synoptic scale.