Piecewise Frontogenesis from a Potential Vorticity Perspective: Methodology and a Case Study

Abstract

Output from a numerical simulation by the University of Wisconsin Nonhydrostatic Modeling System is used to investigate the evolution of the surface frontal structure of an intense winter cyclone that occurred off the east coast of North America. Using the model data as input to the piecewise potential vorticity (PV) inversion method developed by Davis and Emanuel, the total surface frontogenesis is partitioned into contributions from discrete PV anomalies. These contributions are calculated using the nondivergent balanced winds associated with PV anomalies in the upper, interior, and surface layers as well as the balanced irrotational winds in what is termed piecewise frontogenesis. The cyclone of interest developed into a nearly stationary, well-occluded storm characterized by a cutoff PV anomaly at the tropopause and near-surface frontogenesis concentrated in the warm frontal zone. During the open wave stage of the cyclone life cycle, this frontogenesis was produced largely by the balanced flow, particularly that portion of it associated with the upper-level PV anomaly. Throughout the remainder of the cyclone life cycle, the upper-level contribution to the frontogenesis retained a distribution similar to that of the total frontogenesis. After the cyclone occluded, however, the frontogenesis forced by the unbalanced portion of the flow became increasingly significant despite the persistent smallness of the unbalanced winds. In fact, a substantial portion of the lower-tropospheric frontogenesis observed in the occluded quadrant, including that along a developing bent-back front, was eventually associated with the unbalanced flow. These results suggest a potentially significant role is played by unbalanced motions in the development and maintenance of lowertropospheric frontal structure in midlatitude cyclones.