The effective roughness length for scalar transfer in neutral conditions over hilly terrain

Abstract

Numerical weather prediction models are required to represent the area-averaged effects of hills on temperature and water vapour in the atmospheric boundary layer. The transfer of these scalars within the boundary layer is affected by bouyancy. However, we have simplified the problem by simulating the behaviour of a passive scalar in a neutrally stratified turbulent boundary layer over hills. The numerical model used for these simulations was fully nonlinear and flows were simulated over a range of slopes, including ones where separation occurred. The area-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 have found that, using an effective roughness length for scalar transfer, ZOSeff, the effects of hills on area-averaged scalars may be parametrized. ZOSeff is smaller than the local value of roughness length for scalar transfer, and for hills of certain wavelengths and roughness it is three orders of magnitude smaller. A simplified interpretation of the physical processes simulated by the numerical model has been used to derive an expression for ZOSeff in terms of hill characteristics only. This parametrization should be useful for representing the influence of hills on scalar transfer in numerical weather prediction and climate models.