A unified model of the planetary boundary layer and shallow cumulus convection

While cumulus clouds are simply moist, buoyant thermals rooted in the boundary layer, conventional atmospheric models treat the boundary layer and cumulus convection separately. Here we apply a multiple updraft eddy diffusivity-mass flux (EDMF) approach to couple the planetary boundary layer and shallow cumulus convection, as a natural and physically appealing unification of these two processes. While EDMF has been used successfully in the past to model the convective boundary layer, the single bulk updraft used in most implementations is insufficient to accurately resolve the probability distribution of sub-grid turbulence and to achieve scale independence. In our approach, an ensemble of buoyant thermals is released at the surface and each one is followed to its cloud top, with the statistics of this ensemble determining the convective transport at each altitude. Preliminary results are presented from several test cases over a variety of climate conditions