A COMPARISON OF BOUNDARY-LAYER DIFFUSION SCHEMES IN UNSTABLE CONDITIONS OVER LAND

We compare the results of a local and a nonlocal scheme for vertical d iffusion in the atmospheric boundary layer with observations at the 20 0 m tower at Cabauw. This is done for a 12 h period during daytime on 31 May 1978, which is characterised by strong insolation, clear skies, moderately strong winds and weak advection. The local diffusion schem e uses an eddy diffusivity determined independently at each point alon g the vertical based on local vertical gradients of wind and virtual p otential temperature, similar to the usual approach in atmospheric mod els. The nonlocal scheme determines an eddy diffusivity profile based on a diagnosed boundary-layer height and a turbulent velocity scale. I t also incorporates nonlocal (vertical) transport effects for heat and moisture. The boundary-layer diffusion schemes are forced with the lo cally observed fluxes for heat and moisture. The outputs of the scheme are compared with the observed mean structure along the Cabauw tower, and the radiosonde profile at a nearby location (De Bilt). Overall, t he nonlocal scheme transports moisture away from the surface more rapi dly than the local scheme, and deposits the moisture at higher levels. The local scheme tends to saturate the lowest model levels unrealisti cally in comparison with the observations. We also compare the outputs of the two diffusion schemes with the results of a transilient model simulation. Subsequently, we study the impact on the model behaviour b y varying important parameters in both diffusion schemes and we invest igate the sensitivity to uncertainty in the environmental conditions. Finally, we study the interaction of the diffusion schemes with a simp le surface flux scheme.