ORIGIN AND STRUCTURE OF A NUMERICALLY SIMULATED POLAR LOW OVER HUDSON-BAY

The PSU-NCAR mesoscale model (MM4) is used to simulate a polar low tha t developed over Hudson Bay in December 1988. The structure and charac teristics of the simulated low are documented, and results are present ed of sensitivity experiments aimed at elucidating the physical mechan isms involved in the cyclogenesis. The low formed over an ice-free reg ion in the eastern bay as an amplifying upper-level cold trough advanc ed into the region. The model depicted the polar low as a small, relat ively shallow system embedded within the larger cold low. It resembled a miniature hurricane in structure but lacked hurricane-force winds. The lapse rate near its center was moist neutral to 550 mb (4 km); ant icyclonic outlow occurred at and immediately below that level. The sen sitivity experiments revealed that fluxes of heat and moisture from th e region of open water and the associated condensation heating in deep organized convection were essential to the development. Sensible heat ing alone produced a relatively weak low and no low formed in an exper iment with a completely ice-covered bay. The feedback between the surf ace fluxes and wind speed enhanced the intensification, especially in an experiment with the sea surface temperature raised by 8 degrees C. Winds of minimal hurricane intensity were attained in the latter exper iment when the Feedback effect was included but not when it was disall owed. A sizable impact of the ice-edge configuration was found. It is concluded that the Hudson Bay polar low formed as a consequence of lat ent heat release in deep organized convection that formed when an uppe r-level cold low moved over a relatively warm body of open water from which large fluxes of heat and moisture took place. Baroclinic forcing appeared to play little direct role in the low development. Instead, the configuration of the upstream ice boundary provided an important i nitiating and organizing mechanism.