The boundary layer, also called the atmospheric or planetary boundary layer, is the part of the atmosphere which is directly influenced by the surface. Accurate modelling of processes in the boundary layer is important for the forecasting of conditions at or near the surface, such as the formation and dissipation of fog or extremes of temperature. Boundary layer processes are also important in modelling atmospheric dispersion, an activity which is also carried out in the Met Office.

The characteristics of the boundary layer depend on the underlying surface and the time of day and exhibit great variability. A key distinction is made between unstable boundary layers, where the surface transfers heat to the atmosphere, and stable boundary layers, where the transfer of heat is reversed. Further distinctions can be made depending on the presence of clouds or convection. Over land there is a strong diurnal cycle with unstable conditions during the day and stable conditions at night.

To improve our modelling of the boundary layer in forecasting models we rely on detailed observations and more sophisticated models of the physical processes in the boundary layer. Constraints on computing time and memory make it impossible to resolve small-scale turbulent motions in forecasting or climate models, so parametrizations are required. Large-eddy models, allow us to resolve such features and provide insights into boundary layer processes and data on which to base parametrizations.

Current work is aimed at improving the parametrizations of turbulent transfer in unstable boundary layers, modelling the transitions between stable and unstable conditions and improving our understanding of mixing in stable boundary layers.

As an example of this work, the animation below shows the evolution of the temperature and the wind in a large-eddy simulation of an idealised stable boundary layer driven by cooling at the surface. Note the evolution of an S-shaped profile in the potential temperature and the development of a jet with higher wind speeds at the top of the boundary layer.