Numerical Weather Prediction at the Met Office: Recent Progress and Future Direction (Invited Presentation)

The Met Office Unified Model (UM) applies a "seamless" modelling approach, with the same dynamical core and, where possible, the same parameterization schemes used across a broad range of spatial and temporal scales. The model is suitable for numerical weather prediction (NWP), seasonal forecasting and climate modelling with forecast times ranging from a few days to hundreds of years. Furthermore, the UM can be used both as a global and a regional model.

The UM is run operationally, in a number of configurations, for NWP at the Met Office. A 17 km grid length global configuration provides the medium-range weather forecast and also supports the nested higher resolution regional models with boundary data. The variable resolution UK model (UKV), which is nested inside the global model, provides detailed short-range forecasts over the UK. The UKV has a high resolution inner domain (1.5 km grid length) over the area of forecast interest, separated from a coarser grid (4 km) near the boundaries by a variable resolution transition zone. This variable resolution approach allows the boundaries to be moved further away from the region of interest, reducing unwanted boundary effects on the forecasts. Along with the deterministic UKV model, a 2.2 km grid length UK ensemble forecasting system, nested within a 33 km grid length global ensemble, provides information on the uncertainty in short-range forecasts. The ensemble is obtained by perturbing both the initial conditions and also some aspects of the physical processes within the model.

A recent upgrade to the Met Office supercomputer facility led to discussions on how to best use the increased resources, with increased resolution, increasing the size of the regional domain, more ensemble members and longer forecast lead times all being considered. This presentation will discuss recent changes to the operational system, including the expansion of the UKV domain to improve spin-up, the introduction of hourly 4D-Var to improve nowcasting, extending the lead time of the UKV and the UK ensemble out to 5 days and the centring of the UK ensemble around the UKV analysis, as well as looking forward to future physics upgrades.