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Met Office Numerical Weather Prediction models
The Met Office Unified Model is run operationally, in a number of configurations, for weather forecasting at the Met Office.
A global configuration provides the medium-range weather forecast and also supports the nested higher resolution regional models with boundary data. More detailed short-range forecasts are provided by these high-resolution models which are able to represent certain atmospheric processes more accurately, as well as having a more detailed representation of surface features such as coastlines and orography.
The latest addition is a variable resolution UK model (UKV) which has a high resolution inner domain (1.5 km grid boxes) 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.
Regional and UK ensemble forecasting systems, supported by a global ensemble, provide information on the uncertainty in short-range forecasts. The solution spread is obtained by perturbing both the initial conditions and also some aspects of the physical processes within the model. A medium-range global ensemble supports probabilistic weather forecasting to two weeks ahead.
| Model | Grid length in mid-latitudes | Grid points | Vertical levels | Forecast length | Run times (UTC) | Initial conditions |
| Global | 25 km | 1024 x 769 | 70 (lid ~80 km) | 144 hrs (for 00z and 12z) | 00, 06, 12, 18 | Hybrid Incremental 4D-Var with MOGREPS Ensemble |
| North Atlantic European (NAE) | 12 km | 600 x 360 | 70 (lid ~80 km) | 48 hrs | 00, 06, 12, 18 | Incremental 4D-Var |
| UKV | 1.4 km inner 4 km outer | 744 x 928 | 70 (lid ~40 km) | 36 hrs | 03, 09, 15, 21 | Incremental 3D-Var |
| MOGREPS Global ensemble | 33 km | 800 x 601 | 70 (lid ~80 km) | 72 hrs (12 members) 9 hrs (32 members for Hybrid analysis) | 00, 06, 12, 18 | Global analysis + 44 member ETKF perturbations (of which 12 run to full forecast length at each cycle) |
| MOGREPS medium-range ensemble | 60 km | 432 x 325 | 70 (lid ~80 km) | 15 days | 00, 12 | Global analysis + 22 member ETKF perturbations + 1 Stochastic physics member |
| MOGREPS Regional ensemble | 18 km | 400 x 240 | 70 (lid ~80 km) | 54 hours | 00, 06, 12, 18 | NAE analysis + 12 member perturbations interpolated from global ensemble |
| MOGREPS UK ensemble | 2.2 km | 532 x 654 | 70 (lid ~40 km) | 36 hours | 03, 09, 15, 21 | 12 member, perturbations interpolated from global ensemble |
The Met Office also has the capability to rapidly relocate regional models to any area of interest worldwide. These Crisis Area Models (CAMs) are run in support of allied military operations and disaster relief.
The weather forecasting configurations of the Unified Model differ from the climate configurations in a number of respects.
- Data assimilation - in weather forecasting an accurate representation of the initial state of the atmosphere is vital and so assimilation of the latest observations is required.
- Resolution - the shorter forecast length allows the model to be run at higher resolution allowing more accurate representation of atmospheric dynamics and surface features.
- Coupling - processes which are not thought to have a significant impact on weather prediction timescales such as deep ocean circulation, atmospheric chemistry and the carbon cycle are not currently modelled. For example, while climate projections are typically run from coupled ocean-atmosphere models, the current NWP configurations use a fixed SST from the OSTIA analysis system.
