Md. Albright et al., ORIGIN AND STRUCTURE OF A NUMERICALLY SIMULATED POLAR LOW OVER HUDSON-BAY, Tellus. Series A, Dynamic meteorology and oceanography, 47(5), 1995, pp. 834-848
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.