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A 30-km version of the Canadian Regional Climate Model is used to simulate a polar low development in 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 the indirect 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 vertical structure 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 factor for the local surface cyclogenesis due to the formation of local Laplacians of diabatic and thermal forcings.

Balance omega equations have recently been used to try to improve the characterization of balance in variational data assimilation schemes for numerical weather prediction (NWP). Results from Fisher and Fillion et al. indicate that a quasigeostrophic omega equation can be used adequately in the definition of the control variable to represent synoptic-scale balanced vertical motion. For high-resolution limited-area data assimilation and forecasting (1-10-km horizontal resolution), such a diagnostic equation for vertical motion needs to be revisited. Using a state-of-the-art NWP forecast model at 2.5-km horizontal resolution, these issues are examined. Starting from a complete diagnostic partial differential equation for omega, the rhs forcing terms were computed from model-generated fields. These include the streamfunction, temperature, and physical time tendencies of temperature in gridpoint space. To accurately compute one term of secondorder importance (i.e., the ageostrophic vorticity tendency forcing term), a special procedure was used. With this procedure it is shown that Charney's balance equation brings significant information in order to deduce the geostrophic time tendency term. Under these conditions, results show that for phenomena of length scales of 15-100 km over convective regions, a diagnostic equation can capture the major part of the model-generated vertical motion. The limitations of the digital filter initialization approach when used as in Fillion et al. with a cutoff period reduced to 1 h are also illustrated. The potential usefulness of this study for mesoscale atmospheric data assimilation is briefly discussed.