Abstract
The influence of some simple modifications to the physical parameterizations in the current GLAS climate GCM is examined. The aim of these modifications was to eliminate strong occasional bursts of 2 - δt oscillations in the PBL fluxes. The PBL of the current GLAS climate model was modified by invoking concepts of ensemble averaging of PBL eddies in a grid cell of the GCM. This resulted in smoothly varying bulk aerodynamic friction and heat transport coefficients. An arbitrary function to account for diffusion of moisture from stomatal cavities found in vegetation was also incorporated. Simultaneously some modifications to the cloud parameterizations were made.
Two integrations, one with the old model and the other with the modified model, were made to simulate 47 days, starting from the NMC analysis for June 15, 1979. Their comparisons showed that the surface fluxes and cloudiness in the modified model simulations are far better. The planetary albedo in the modified model is also realistic. However, some weaknesses still persist, including an ITCZ (Inter-tropical convergence zone) that is too far northward in Sahelian Africa, polar regions that are too cold, and a rather strong ITCZ. It is pointed out that these weaknesses are primarily caused by model deficiencies, e.g., the cloud parameterization and the uniformly prescribed land surface roughness height.
In another simulation with the modified model using a realistic value of surface roughness for deserts, the precipitation in the Sahara Desert reduced significantly, which effectively pushed the ITCZ southward to a more realistic location as compared to observations.
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Sud, Y.C., Smith, W.E. Ensemble formulation of surface fluxes and improvement in evapotranspiration and cloud parameterizations in a GCM. Boundary-Layer Meteorol 29, 185–210 (1984). https://doi.org/10.1007/BF00206830
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DOI: https://doi.org/10.1007/BF00206830