NONLOCAL BOUNDARY-LAYER VERTICAL DIFFUSION IN A MEDIUM-RANGE FORECASTMODEL

In this paper, the incorporation of a simple atmospheric boundary lave r diffusion scheme into the NCEP Medium-Range Forecast Model is descri bed. A boundary layer diffusion package based on the Troen and Mahrt n onlocal diffusion concept has been tested for possible operational imp lementation. The results from this approach are compared with those fr om the local diffusion approach, which is the current operational sche me, and verified against FIFE observations during 9-10 August 1987. Th e comparisons between local and nonlocal approaches are extended to th e forecast for a heavy rain case of 15-17 May 1995. The sensitivity of both the boundary layer development and the precipitation forecast to the tuning parameters in tile nonlocal diffusion scheme is also inves tigated. Special attention is given to the interaction of boundary lay er processes with precipitation physics. Some results of parallel runs during August 1995 are also presented.