Towards a formulation of the impact of mesoscale orographic variability on a cloud-topped PBL for use in GCMs.

We simulate the behavior of the planetary boundary layer (PBL) over complex orography with a cloud resolving model, focusing on cases with high PBL-cloud incidence. The goal is to asses the impact that the mesoscale topographic variability has on the evolution of a cloud-topped PBL (CT-PBL), and ultimately formulate a parameterization of such an effect for use in General Circulation Models (GCMs).

The fluxes carried by mesoscale circulations induced by inhomogeneities of the lower boundary are currently neither resolved nor parameterized in GCMs. In particular, orographic thermal circulations add to the obvious geometrical effect that topographic variability has on the PBL-cloud field. Moreover, our experience with the UCLA-AGCM suggests that this effect can be crucial in preventing the PBL from falling to an unrealistically stable, deep-cloud regime. The nature of the feedbacks between cloud’s albedo and ground temperature makes the consideration of horizontal inhomogeneities in the cloud field essential.

Here, we report on a subset of a larger systematic set of experiments we are performing to cover the parameter space as broadly as possible. The simulated diurnal cycle of a cloud-topped PBL under different atmospheric conditions over flat terrain are first presented. For each of these control experiments, different configurations of the bottom topography (with the same mean height) are added. The effect of the topography on the qualitative evolution of the PBL throughout the day is shown. Furthermore, the quantitative impact of terrain-induced fluxes on the diurnally-changing thermodynamic properties of the PBL are also assessed.