In this short note we discuss a long-standing problem in modelling the atmospheric boundary layer (ABL) over complex terrain: namely, an excessive use of the Monin–Obukhov length scale LMO. This issue becomes increasingly relevant with the ever-increasing resolution of numerical weather-prediction and climate models, which typically use LMO in one way or another for parametrizing the surface layer, or at least for formulating the lower boundary conditions. Hence, inevitably, the models under-represent a significant part of the mesoscale flow variability.
We focus here on the stable ABL over land: in particular, sloped cooled flows. However, a qualitatively similar reasoning applies to the corresponding unstable ABL. We show that for sufficiently stratified flows over moderately sloped surfaces, Monin–Obukhov scaling is inadequate for describing the basic ABL dynamics, which is often governed by katabatic and drainage flows. Copyright © 2007 Royal Meteorological Society