The effects of interactions between surface forcings in the development of a model-simulated polar low in Hudson Bay

Abstract

A 30-km version of the Canadian Regional Climate Model is used to simulate a polar low developmentin early December 1988 over the Hudson Bay. This polar low is quantitatively analyzed in detail, in theinitial and mature stages of its development, in order to understand physically howsea surface conditionsinfluence this mesocyclone. This analysis is realized via the description of the effects of differentatmospheric forcings (i.e. thermal and vorticity advection, and turbulent and convective fluxes) on thepolar low development (called the direct effects) using the diagnostic equations of omega and vorticitytendency. Also, the effects of forcing interactions on subsequent cyclone development (called theindirect effects) is analyzed via the diagnostic equations of vorticity and thermal advection tendencies.In the early stage of development, a low-level cyclogenesis appears over the northwestern Hudson Bayessentially due to diabatic forcings in the context of low-level cold air advection. Progressively, thesynergetic effect of time rate of changes in advection terms, resulting from surface diabatic and stressforcings, favours low-level cyclogenesis and baroclinicity over open water near the sea-ice margin, whose shape is determinant for the deepening and tracking of the polar low. In the mature stage, thegrowth in advection terms becomes the main factor of cyclone intensification with the increase inlow-level convection. Forcings are maximum near the surface and differ substantially from the verticalstructure found in classical extratropical cyclones. In the upper troposphere they appear to play asecondary role in this polar low development. Finally, the polar low studied here is primarily the resultof combined forcing interactions near the sea-ice edge, which are responsible for vorticity and thermaladvection changes at low levels. It is also found that the indented sea-ice shape is a favourable factorfor the local surface cyclogenesis due to the formation of local Laplacians of diabatic and thermalforcings.