Layer‐wise attribution of vertical motion and the influence of potential‐vorticity anomalies on synoptic development

A methodology is presented for attributing vertical motion to potential vorticity (PV) in a explicit manner using the quasi‐geostrophic omega equation. The methodology is then used to determine the contribution individual PV anomalies make to the synoptic development. It is shown how the total vertical motion may be decomposed into self‐generating and interaction terms. The former correspond to vertical motion that arises from one PV region alone, whilst each of the interaction terms represent vertical motion due to the interaction of two different regions of PV. The method is applied to a number of cyclones studied in the Fronts and Atlantic Storm‐Tracks Experiment in order to determine the dynamical role of individual PV regions in cyclogenesis. For purposes of comparison, cyclones were chosen to be representative of the types (A, B and C) of an extended Petterssen–Smebye scheme devised by Deveson, Browning and Hewson. In terms of our new PV method, the Type A and B cyclones exhibit only subtle differences, with the upper‐level layer of PV playing a larger role in the case of the Type B cyclone. A much larger difference is apparent between the Type A and B cyclones on the one hand, and the Type C cyclone on the other: in the Type C cyclone, the upper‐level PV self‐generating term plays an important role, whereas in the Type A and B cases the influence of the upper‐level PV is largely restricted to the interaction terms. In all of the cases, the vertical motion attributable to the interaction between the low‐level PV and boundary thermal gradient was found to be large. Hence, from the viewpoint of instantaneous attribution, we conclude that the low‐level PV is always important.