THE EFFECTIVE ROUGHNESS LENGTH FOR SCALER TRANSFER IN NEUTRAL CONDITIONS OVER HILLY TERRAIN

Numerical weather prediction models are required to represent the area -averaged effects of hills on temperature and water vapour in the atmo spheric boundary layer. The transfer of these scalars within the bound ary layer is affected by buoyancy. However, we have simplified the pro blem by simulating the behaviour of a passive scalar in a neutrally st ratified turbulent boundary layer over hills. The numerical model used for these simulations was fully nonlinear and flows were simulated ov er a range of slopes, including ones where separation occurred. The ar ea-averaged effects of hills are small over low slopes but significant at steeper slopes due to the impact of the separated region. The area -averaged scalar concentration varies logarithmically with height, but in a region that is shallower than the logarithmic wind region. We ha ve found that, using an effective roughness length for scalar transfer , Z(OS)(eff), the effects of hills on area-averaged scalars may be par ametrized. Z(OS)(eff) is smaller than the local value of roughness len gth for scalar transfer, and for hills of certain wavelengths and roug hness it is three orders of magnitude smaller. A simplified interpreta tion of the physical processes simulated by the numerical model has be en used to derive an expression for Z(OS)(eff) in terms of hill charac teristics only. This parametrization should be useful for representing the influence of hills on scalar transfer in numerical weather predic tion and climate models.