Local Versus Nonlocal Boundary-Layer Diffusion in a Global Climate Model

Abstract

The results of a local and a nonlocal scheme for vertical diffusion in the atmospheric boundary layer are compared within the context of a global climate model. The global model is an updated version of the NCAR Community Climate Model (CCM2). The local diffusion scheme uses an eddy diffusivity determined independently at each point in the vertical, based on local vertical gradients of wind and virtual potential temperature, similar to the usual approach in global atmospheric models. The nonlocal scheme determines an eddy-diffusivity profile based on a diagnosed boundary-layer height and a turbulent velocity scale. It also incorporates nonlocal (vertical) transport effects for heat and moisture. The two diffusion schemes are summarized, and their results are compared with independent radiosonde observations for a number of locations. The focus herein is on the temperature and humidity structure over ocean, where the surface temperatures are specified, since the boundary-layer scheme interacts str... Abstract The results of a local and a nonlocal scheme for vertical diffusion in the atmospheric boundary layer are compared within the context of a global climate model. The global model is an updated version of the NCAR Community Climate Model (CCM2). The local diffusion scheme uses an eddy diffusivity determined independently at each point in the vertical, based on local vertical gradients of wind and virtual potential temperature, similar to the usual approach in global atmospheric models. The nonlocal scheme determines an eddy-diffusivity profile based on a diagnosed boundary-layer height and a turbulent velocity scale. It also incorporates nonlocal (vertical) transport effects for heat and moisture. The two diffusion schemes are summarized, and their results are compared with independent radiosonde observations for a number of locations. The focus herein is on the temperature and humidity structure over ocean, where the surface temperatures are specified, since the boundary-layer scheme interacts str...