Improving the Representation of Convective Storms in the Unified Model at 1.5 km to 50 m Gridlength

Convective storms are a crucially important forecasting problem in the UK, not least because of the flooding they can cause. In the last few years many operational weather centres have begun to run at “convection permitting” resolutions, with the UK Met Office currently running a 1.5 km forecast model. While there is evidence that precipitation forecasts at this grid length are more accurate than lower resolution forecasts, it is clear that there are still significant shortcomings in the nature of the simulated convective cells. Cells in the model tend to be too large and too intense, and tend not to organise into Mesoscale complexes as observed, indicating that convection is not fully resolved at this grid length as well as illustrating our lack of understanding of the nature of small-scale mixing and microphysical processes. The DYMECS (Dynamical and Microphysical Evolution of Convective Storms) and COPE (COnvective Precipitation Experiment) projects provide a large database of observations of convective storms that can be used to evaluate NWP models. In this study we perform simulations of some of the DYMECS and COPE cases with the Met Office Unified Model (UM) at horizontal grid lengths ranging from 1.5 km to 50 m, which allows us to apply a statistical approach to evaluate the properties and evolution of the simulated storms over a range of conditions. Here we present results comparing the storm morphology in the model and reality which show that the simulated storms become smaller as grid length decreases and that the grid length that fits the observations best changes with the size of the observed cells. We also show that the modelled storms are very sensitive to some aspects of the model configuration, such as the subgrid mixing scheme and the introduction of a grey-zone convection scheme.