Modelling of transition boundary layers

Parametrizations of the boundary layer used in GCMs often make a clear distinction between the stable and unstable regimes. Transitional effects and the way in which parametrizations switch between the two regimes have not been explored so widely.

The present study is aimed at assessing the representation of transitional boundary layers in the Met Office's NWP model in comparison with observational data obtained from the Meteorological Research Unit at Cardington (UK) and with large-eddy simulations of the transition. A single-column version of the NWP model is forced with surface conditions from the observational site and the evolution of the profiles of wind speed and temperature is compared against observational ascents with a balloon made through the evening transition. Large-eddy simulations with the same forcings provide insight into the timescales for transition and the physical mechanisms involved.

Sensitivity studies with the single-column model are carried out to assess the impact of modifications to the parametrizations: these include changes in the vertical resolution of the model, simple representations of persisting turbulence and an assessment of the impact of more sophisticated treatments of radiative effects.