DIAGNOSTIC EVALUATION OF VERTICAL MOTION FORCING MECHANISMS BY USING Q-VECTOR PARTITIONING

Q-vector partitioning has proven to be a useful tool for the understan ding of the frictionless, adiabatic processes responsible for the gene ration of synoptic-scale vertical motion in the extratropical atmosphe re. Partitioning of Q into components parallel and normal to the isoth erms on an isobaric surface is standard practice in studies dealing wi th vertical motion and frontogenesis. This paper is concerned with ver tical motion only and examines the consequences of projecting Q onto i sohypses, instead of isotherms, on an isobaric surface. Specifically, the Q vector is partitioned in the natural coordinate system that foll ows the geostrophic wind. The novelty with this partitioning is that i t naturally leads to the evaluation of different vertical motion forci ng mechanisms, among which are those related to flow curvature and to confluence or diffluence. This evaluation is illustrated by applying t he new Q-vector partition to a gridded analysis of a real weather situ ation. An important conclusion is that the thermal advection by horizo ntal geostrophic shear is as significant to the forcing of vertical mo tion as the geostrophic confluence/diffluence. While this result has p reviously been obtained in the study of frontal dynamics, this is the first application of this finding to the synoptic scale.