The impact of spin-up and resolution on the representation of a clear convective boundary layer over London in order 100 m grid-length versions of the Met Office Unified Model

With a number of operational centres looking forward to the possibilities of city scale NWP and climate modelling it is important to understand the behaviour of order 100 m models over cities. A key issue is how to handle the representation of partially resolved turbulence in these models. In this article we compare the representation of a clear convective boundary-layer case in London in 100 and 50 m grid-length versions of the Unified Model (MetUM) with observations. Comparison of Doppler lidar observations of the vertical velocity shows that convective overturning in the boundary layer is broadly well represented in terms of its depth and magnitude. The role of model resolution was investigated by comparing a 50 m grid-length model with the 100 m one. It is found that, although going to 50 m grid length does not greatly change many of the bulk properties (mixing height, heat flux profiles, etc.) the spatial structure of the overturning is significantly different. This is confirmed with spectral analysis which shows that the 50 m model resolves significantly more of the energetic eddies, and a length-scale analysis that shows the 50 and 100 m models produce convective structures 2-3 times larger than observed. We conclude that, for the MetUM, model grid-lengths of order 100 m may well be sufficient for predicting many bulk and statistical properties of convective boundary layers; however, the details of the spatial structures around convective overturning in these situations are likely to be still under-resolved. Spin-up artefacts emanating from the inflow boundary of the model are investigated by comparing with a smaller 100 m grid-length domain which is more dominated by such effects. These manifest themselves as along-wind boundary-layer rolls which produce a less realistic comparison with the lidar observations. A stability analysis is presented in order to better understand the formation of these rolls.