LOCAL VERSUS NONLOCAL BOUNDARY-LAYER DIFFUSION IN A GLOBAL CLIMATE MODEL

The results of a local and a nonlocal scheme for vertical diffusion in the atmospheric boundary layer are compared within the context of a g lobal climate model. The global model is an updated version of the NCA R Community Climate Model (CCM2). The local diffusion scheme uses an e ddy diffusivity determined independently at each point in the vertical , based on local vertical gradients of wind and virtual potential temp erature, similar to the usual approach in global atmospheric models. T he nonlocal scheme determines an eddy-diffusivity profile based on a d iagnosed boundary-layer height and a turbulent velocity scale. It also incorporates nonlocal (vertical) transport effects for heat and moist ure. The two diffusion schemes are summarized, and their results are c ompared with independent radiosonde observations for a number of locat ions. The focus herein is on the temperature and humidity structure ov er ocean, where the surface temperatures are specified, since the boun dary-layer scheme interacts strongly with the land-surface parameteriz ation. Systematic differences are shown in global-climate simulations, with CCM2 using the two schemes. The nonlocal scheme transports moist ure away from the surface more rapidly than the local scheme, and depo sits the moisture at higher levels. The local scheme tends to saturate the lowest model levels unrealistically, which typically leads to clo uds too low in the atmosphere. The nonlocal scheme has been chosen for CCM2 because of its more comprehensive representation of the physics of boundary-layer transport in dry convective conditions.