Numerical weather prediction models

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. More detailed short- to medium-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 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. A medium-range global ensemble supports probabilistic weather forecasting out to seven days ahead.

Model	Grid length in mid-latitudes	Vertical levels	Forecast length	Run times (UTC)	Initial conditions
Deterministic Global	10 km (2560 x 1920 grid points)	70 (lid ~80 km)	6 days (for 00z and 12z)	00, 06, 12, 18	Hybrid Incremental 4D-Var with flow-dependent background errors from 44-member global ensemble.
Ensemble Global (MOGREPS-G)	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), SST and soil-moisture perturbations.
Deterministic UK (UKV)	1.5 km inner domain (622 x 810 grid points); 4 km outer domain (950 x 1025 grid points)	70 (lid ~40 km)	120 hours	03,15	Hourly 4D-Var
			54 hours	00, 06, 09, 12, 18, 21
			12 hours	All other hours
Ensemble UK (MOGREPS-UK)	2.2 km inner domain; 4km outer domain (740 x 752 grid points)	70 (lid ~40 km)	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.
			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 - 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.