Numerical weather prediction models
The Unified Model is run operationally in a number of configurations for weather forecasting at the Met Office.
Global and regional configurations
Global configurations of the Unified Model provide the medium-range weather forecasts and also support the nested higher resolution regional models with boundary data. Both deterministic and ensemble global weather model configurations are now interactively coupled to an ocean model. More detailed short to medium-range forecasts are provided by nested 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 deterministic UK forecast uses a variable resolution model 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.
UK ensemble forecasting, supported by a global ensemble, provides information on the uncertainty in forecasts up to five days ahead. The solution spread is obtained by perturbing both the initial conditions and also some aspects of the physical processes within the model.
|
Model |
Grid length in mid-latitudes |
Vertical levels |
Forecast length |
Run times (UTC) |
Initial/boundary conditions |
|---|---|---|---|---|---|
|
Deterministic Global atmosphere fully coupled to 1/4 degree ocean |
10 km (2560 x 1920 grid points) |
70 |
6 days |
00, 06, 12, 18 |
Hybrid Incremental 4D-Var with flow-dependent background errors from 44-member global ensemble (3D-Var for ocean initialisation). |
|
Ensemble Global atmosphere fully coupled to 1/4 degree ocean |
20 km (1280 x 960 grid points) | 70 (lid ~80 km) |
Control member + 17 perturbed members to 7 days. 36 member ensemble generated by time-lagging over 12 hours (latest 2 cycles). |
00, 06, 12, 18 |
44-member ensemble of 4D-ensemble-Var. Model perturbations from Additive Inflation from historical analysis increments, Stochastic Kinetic Energy Backscatter (SKEB), Stochastic Perturbation of physics Tendencies (SPT), sea surface temperature (SST) and soil-moisture perturbations. |
|
Deterministic UK atmosphere (UKV) |
1.5 km inner domain (622 x 810 grid points); 4 km outer domain (950 x 1025 grid points) |
70 |
120 hours |
03,15 |
Hourly 4D-Var. Lateral boundary conditions from deterministic Global model. AMM15 shelf seas model supplies hourly sea surface temperatures for the lower boundary condition on sea points. |
|
54 hours |
00, 06, 09, 12, 18, 21 |
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|
12 hours |
All other hours |
||||
|
Ensemble UK atmosphere |
2.2 km inner domain; 4km outer domain (740 x 752 grid points) |
70 |
Control member + 2 perturbed members to 5 days |
05, 11, 17, 23 |
UKV analysis + perturbations interpolated from global ensemble. Lateral boundary conditions from corresponding global ensemble member. AMM15 shelf seas model supplies hourly sea surface temperatures for the lower boundary condition on sea points |
|
3 perturbed members to 5 days |
All other hours |
||||
|
18 member ensemble generated by time-lagging over 6 hours (latest 6 cycles) |
Every hour |
Crisis Area Models (CAMs)
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.
Differences compared to climate configurations
The weather forecasting configurations of the Unified Model differ from the Met Office seasonal and climate models 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 - Our global NWP configurations now run with a coupled ocean-atmosphere in the same way as climate projections. The atmosphere and ocean models use the same land-sea mask which includes partial land/sea fractions at coastal grid points. For larger in-land bodies of water that are not resolved by the ocean model we use fixed surface temperatures (and where relevant ice fractions) updated daily from the OSTIA analysis system.
- Complexity - Processes which are not thought to have a significant impact on weather prediction timescales such as atmospheric chemistry and the carbon cycle are not currently modelled.
