A Model-aided Study of the Origin and Evolution of the Anomalously High Potential vorticity in the Inner Region of a Rapidly Deepening Marine Cyclone

High spatial and temporal resolution fields generated by a mesoscale prediction model are used to study a case of rapid marine cyclogenesis (26 mb in 12 h) within the context of potential vorticity (PV) thinking. The case, which occurred on 23 February 1987 near the east coast of the United States, was well simulated by the model, as was verified by ship reports and satellite imagery. Three components of the cyclone were investigated: 1) a surface thermal anomaly (or surrogate PV anomaly), 2) a low-level, diabatically produced PV anomaly in and near the frontal cloud band, and 3) a dry PV anomaly of upper-tropospheric and lower-stratosphoric origin. Trajectory tracing revealed that the surface thermal anomaly was caused primarily by northward transport of warm air into the central region of the low. Although surface sensible heat flux also contributed to the warm anomaly, a sensitivity experiment, run with the flux withheld, showed that it had little impact on the depth of the storm. Furthermore, it was found from trajectory tracing that the region of positive anomaly in the frontal cloud mass formed with extreme rapidity as air from the boundary layer ascended the warm frontal surface and entered the cloud. Differential condensational heating in the vertical was likely responsible for much of the sudden increase. Some of the diabatically produced PV was evident in the wake of the storm, after emerging from the cloud. Finally, from examination of a large number of trajectories, it was found that the anomalous PV of high-level origin had a complex behavior as it approached the low from upstream. Initially the anomalous PV was contained in a tropopause depression and in a fold that protruded downward along the south flank of the depression. Stratospheric air in the depression subsided to mid-to upper levels and advanced directly on the low center. Some of the air in the tropopause fold followed a cyclonically curved trajectory, first subsiding and then rising to midlevels as it neared the low. Other air in the fold sank to low levels along an anticyclonically curved path and was left behind. At 18 h into the development, the low-level, diabatically produced PV, the midlevel PV from the tropopause fold, and the upper-level PV from the tropopause depression were aligned vertically in a column located 100 km upstream of the surface-low center.