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

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 the initial and mature stages of its development, in order to understand physically how sea surface conditions influence this mesocyclone. This analysis is realized via the description of the effects of different atmospheric forcings (i.e. thermal and vorticity advection, and turbulent and convective fluxes) on the polar low development (called the direct effects) using the diagnostic equations of omega and vorticity tendency. 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 Bay essentially due to diabatic forcings in the context of low-level cold air advection. Progressively, the synergetic effect of time rate of changes in advection terms, resulting from surface diabatic and stress forcings, 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, the growth in advection terms becomes the main factor of cyclone intensification with the increase in low-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 a secondary role in this polar low development. Finally, the polar low studied here is primarily the result of combined forcing interactions near the sea-ice edge, which are responsible for vorticity and thermal advection 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 thermal forcings.