Origin and structure of a numerically simulated polar low over Hudson Bay

Abstract

The PSU-NCAR mesoscale model (MM4) is used to simulate a polar low that developed over Hudson Bay in December 1988. The structure and characteristics of the simulated low are documented, and results are presented of sensitivity experiments aimed at elucidating the physical mechanisms involved in the cyclogenesis. The low formed over an ice-free region in the eastern bay as an amplifying upper-level cold trough advanced into the region. The model depicted the polar low as a small, relatively 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); anticyclonic outlow occurred at and immediately below that level. The sensitivity experiments revealed that fluxes of heat and moisture from the region of open water and the associated condensation heating in deep organized convection were essential to the development. Sensible heating alone produced a relatively weak low and no low formed in an experiment with a completely ice-covered bay. The feedback between the surface fluxes and wind speed enhanced the intensification, especially in an experiment with the sea surface temperature raised by 8 °C. Winds of minimal hurricane intensity were attained in the latter experiment when the feedback effect was included but not when it was disallowed. A sizable impact of the ice-edge configuration was found. It is concluded that the Hudson Bay polar low formed as a consequence of latent heat release in deep organized convection that formed when an upper-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 initiating and organizing mechanism.