A POLAR-LOW DEVELOPMENT OVER THE BERING SEA - ANALYSIS, NUMERICAL-SIMULATION, AND SENSITIVITY EXPERIMENTS

A polar low that developed over the western Bering Sea on 7 March 1977 and tracked across St. Paul Island is investigated using observations and the Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model Version 5 (MM5). A series of fine-mesh (20 k m) simulations are performed in order to examine the structure of the cyclone and the airflow within it and to determine the physical proces ses important for its development. Observations show that the low form ed near the ice edge in a region of moderate low-level baroclinicity a nd cold-air advection when an upper-level trough, or lobe of anomalous ly large potential vorticity (PV), broke off from a migratory, upper-l evel cold low over Siberia and advanced into the region. A full physic s model experiment, initialized 24 h prior to the appearance of the po lar low, produced a small, intense cyclone having characteristics simi lar to the observed low. The simulated low more closely resembled an e xtratropical cyclone than a typical circularly symmetric hurricane, po ssessing a thermal structure with frontlike features and an asymmetric precipitation shield. Although the simulated low developed southeast of, and earlier than, the observed low, the basic similarity of the ob served and modeled systems was revealed by a comparison of the sequenc e of weather elements at a point in the path of the simulated low with the sequence of observations from nearby St. Paul Island, Alaska. A s eries of experiments was performed to test the sensitivity of the simu lated polar low development to various physical processes. Four experi ments of 48-h duration each were initialized 24 h before the low appea red. In the first experiment, in which surface fluxes were turned off, the low failed to develop. In the second experiment, in which the flu xes were switched on after a 24-h delay, only a weak low formed. In th e third experiment, in which the ice edge was shifted a degree of lati tude to the north, thus increasing the overwater fetch of the cold air , the low's evolution was slightly altered but the final outcome was l ittle changed. A fourth high-horizontal resolution experiment (6.67-km spacing) displayed more plentiful and sharper mesoscale features but on the storm scale yielded results that were similar to those of the f ull-physics run. A full-physics experiment initialized 24 h later, at the time the low first appeared, and run for 24 h, produced a system o f similar intensity to that in the 48-h full-physics run but somewhat better positioned. Corresponding sensitivity experiments showed that w ith both surface fluxes and latent hearing omitted, the low weakened a nd nearly died away. Experiments retaining only surface fluxes in one case and only latent heating in the other, produced similar cyclones o f moderate depth. The results suggest that the development of some, if not most, polar lows can be regarded as fundamentally similar to that of midlatitude ocean cyclones. In both cases a mobile upper-level PV anomaly interacts with a low-level thermal or PV anomaly produced by t hermal advection and/or diabatic heating. The polar low lies at the en d of the spectrum of extratropical cyclogenesis in which concurrent su rface fluxes of sensible and latent heat and the immediately ensuing c ondensation hearing in organized convection dominate the development o f the low-level anomaly.