Boundary-layer decoupling over cold surfaces as a physical boundary-instability

In the stable atmospheric boundary layer (SBL), the interaction of processes in numerical weather prediction (NWP) models seems to test their representation more stringently than their separate 'validation'. Some SBL schemes derived from micrometorological research seem to allow a 'decoupling' behaviour when implemented in NWP. That is, turbulence dies out from the ground upwards. Such 'decoupling' of the surface from atmospheric fluxes can permit dramatic and possibly unrealistic falls in surface temperature. This study traces the mechanism of model decoupling, asks whether this behaviour is in any sense correct and considers the implications. It is shown that decoupling can occur in idealized single-column models, originating from an unstable boundary-mode. This behaviour can depend critically on parameters such as surface roughness and soil thermal diffusivity as well as turbulence. But the turbulence dependence arises through the response of the boundary layer as a whole, and not just the surface-layer scheme. Such decoupling arises from the 'physics', rather than the finite-difference schemes, and appears to occur sometimes in the real atmosphere.