Experiences with a 1.5km version of the Met Office Unified Model for short range forecasting

The Met Office has implemented a 1.5km version of the Unified Model over the UK for forecasting up to 36 hours ahead. This is intended, in due course, to replace the 4km UK model. Part of the motivation for this is to improve forecasts of convective rainfall. Although deep convection is still very under-resolved at 1.5km it shows a significant improvement over 4km in many cases. Some simulations at higher horizontal resolution (e.g.500m) will also be presented as a way of understanding the behaviour at 1.5km. The benefits of running at 1.5km gridlength for other weather types, including orographic rainfall, will be shown.

The UK 1.5km model is a variable resolution model with 1.5km gridlength over the UK but increasing to 4km at the edges of the domain. This enables us to push the boundaries of the model further away from the area of interest at lower cost and also to reduce the resolution mismatch with the driving (12km) model. Comparison of results from the variable resolution domain with those from an equivalent set of nested fixed resolution models will be shown.

Finally some of the future plans for the model will be mentioned including potential physics changes and the possibility of obtaining probabalistic forecasts by running a small ensemble of 1.5km models.