An improved formulation to represent cloudy boundary layers, appropriate for low-order turbulence models, has been developed to replace the current scheme used in the Canadian MC2 mesoscale model. Our current approch uses a TKE-based turbulence scheme coupled somewhat artificially with a shallow convection scheme to describe cloud-topped convective boundary layers. The new formulation is based on a unified cloudiness-turbulence approach along the lines proposed by Bechtold et al. (1995), that allows for a more physical representation of turbulent fluxes under various clear and cloudy conditions and for a more detailed treatment of microphysics in partly cloudy layers. The subgrid-scale cloudiness scheme gives a unified description of stratiform and shallow, non-precipitating cumulus convection. Comparisons between model forecasts and detailed aircraft observations of various types of cloudy boundary layers from recent field experiments, such as MERMOZ-II and FIRE.ACE, will be presented at the Symposium.