UMBRELLA - UM Boundary Layer Representation including land-atmosphere interactions

The atmospheric boundary layer (BL) is the layer of the atmosphere near the Earth's surface which is directly influenced by the surface. The BL plays a key role in the atmosphere - controlling the exchange of heat, moisture and other atmospheric constituents (both natural and anthropogenic) between the surface and the free troposphere. The boundary layer is also crucial in the initiation of convection and so impacts on the location and timing of convective rainfall. Turbulence is a key characteristic of the BL but is generally too small scale to be resolved in weather and climate models and so needs to be parametrised. With increased computational resources it is now feasible to run operation weather forecasts at km scales where convection can be resolved explicitly (though not necessarily well resolved). As we move towards sub-km scale the models will also begin to resolve large scale boundary structures. This regime where we partly resolve these key processes is known as the grey zone. Modelling the boundary layer in the grey zone requires re-evaluation and modification of the boundary layer parametrisation schemes and their coupling to convection (parametrised or explicit).

This project will firstly evaluate the currently available boundary layer schemes and their coupling to convection in the Met Office Unified Model (MetUM) at different resolutions across the grey zone as well as testing any new developments produced as part of this programme. Secondly it will study the coupling between surface heterogeneity, boundary layer structures and convection to understand the key role of heterogeneity. Previous studies largely focus on homogeneous case studies, despite the fact that most land is actually not flat and covers a variety of land use and surface conditions. Including surface heterogeneity will allow a more thorough evaluation of the current boundary layer schemes as well as providing a physical underpinning for more realistic parametrisations which are aware of the underlying variability in the surface characteristics.

The project will make use of the latest field observations from the upcoming Wescon field campaign as well as data from long term observational sites in the UK and US. We will also utilise the latest Meteosat third generation satellite observations to evaluate the model in the tropics. We will combine these observations with idealised large eddy simulations and more realistic simulations with the MetUM to study the behaviour of the boundary layer over the diurnal cycle at homogeneous sites and for heterogeneous regions (e.g. regions with variable surface temperature and moisture, or gentle topography).