Using piecewise potential vorticity inversion to diagnose frontogenesis. Part I: A partitioning of the Q vector applied to diagnosing surface frontogenesis and vertical motion

Abstract

The technique of piecewise potential vorticity (PV) inversion is used to identify the nondivergent wind fields attributed to upper-, middle-, and lower-tropospheric PV anomalies in addition to the irrotational wind with the goal of diagnosing the respective wind fields' frontogenetic potentialities. Frontogenesis is diagnosed using a piecewise separation of the Q vector into parts associated with the partitioned wind field. Partitioned geostrophic Q vectors are used to diagnose the vertical motion attributed to the upper-, middle-, and lower-tropospheric PV anomalies. Insight gained from this new diagnostic technique is demonstrated by examining a particular case of extratropical marine cyclogenesis resulting from the interaction of an upper-tropospheric short-wave trough with a surface thermal wave. In the early stages of development, the largest contributor to surface frontogenesis was associated with winds attributed to the lower-tropospheric thermal wave. As the cyclone matured, the contributions of the upper-tropospheric PV and near-surface potential temperature anomalies to surface frontogenesis increased. Winds attributed to the upper-tropospheric PV were frontogenetical north of the thermal ridge axis and fron-tolytical south of the thermal ridge in the warm sector. The upper-tropospheric PV acted to amplify the thermal ridge while simultaneously narrowing the warm sector. The patterns of geostrophic Q vectors associated with the upper-tropospheric PV suggest that ascent should be favored in the narrowing surface thermal ridge. The contribution to surface frontogenesis due to lower- and middle-tropospheric PV, whose increase is imputed to latent heat release, was variable during the evolution of the cyclone - suggesting that the location of diabatically generated PV anomalies relative to frontal zones can have a significant impact on frontogenesis and associated frontal precipitation distribution. Throughout the evolution of the cyclone, the irrotational wind was frontogenelical along the warm and cold fronts with the magnitude of the irrotational frontogenesis increasing as the surface cyclone amplified. For the case considered, partitioned geostrophic Q vector "forcing" for vertical motion revealed approximately equal contributions from the upper-tropospheric PV anomalies and the near-surface thermal perturbations. Characteristic patterns of Q vectors and Q-vector divergence are identified and presented for cases including an upper trough interacting with a surface baroclinic zone and a propagating surface edge wave.