Unified Model

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The Unified Model is the collection of Numerical Weather Prediction computer models used by the United Kingdom Met Office. It includes the main suite of a Global Model, a UK and North Atlantic model and a high resolution UK model, in addition to a variety of Crisis Area Models and other models that can be run on demand. The models are grid based, rather than wave based and are run on a variety of supercomputers. Data are provided by observations (human and automatic), satellites, radar, radiosonde weather balloons, wind profilers and a background field from previous model runs. The produced data are verified against actual data for initial conditions and the first two hours and problems are worked towards in subsequent model runs, rather than force the model to accept a real value that may make the system unstable. The models are written in Fortran (originally 77 but working towards 90)^[1] and use height as the vertical variable ^[2]. Because most developments of interest are at near to the ground the vertical layers are closer together near the surface.^[3]

Global Model

Approximately 40 km resolution (at mid latitudes, 0.5625 x 0.375 degrees) with 50 vertical levels^{[citation needed]}. Covers the entire globe and 144 hours in the future twice a day. The Global model provides boundary information for the North Atlantic European (NAE) model, for which additional shorter runs (48 hours) are produced twice a day.

North Atlantic and European

The North Atlantic and European model (NAE) has 38 levels with a 12 km resolution^{[citation needed]}. It is run out to 48 hours from start. Because the UK is at a northern latitude the computer transposes the model area to an equatorial location so the grid points give an area that is more square. This reduces the load on the model, allowing it to run more quickly.

UK 4km

38 Vertical levels, 4 km horizontal resolution. Run out to 36 hours.^{[citation needed]}

CAM

The Crisis Area Model is a 12 km model that can be run for any area of the world should the need arise. This can include military use (the MMU use this on deployed operations) or environmental catastrophes.

Mountain Wave Models

This high resolution model provides information on mountain waves for a variety of locations around the UK and other areas of interest to the Met Office.

Global Wave Model

This models sea waves around the world.

1.5-4 km variable resolution UKV model

A trial model, not operational at the time of writing. This is run every 3 hours using starting conditions from the 12-km North Atlantic and European (NAE) model. The resolution is 1.5 km over the UK, and 4 km over surrounding areas.

All of the models use varying resolutions of topography with greater accuracy at higher resolutions. The limiting factor with all models is that for a weather event to be recorded by the model it must be at least three grid points in size. Thus for the global model at 40 km, a weather system must be at least 120 km to be modelled. This means smaller phenomena such as small depressions, smaller hurricanes and large thunderstorms are too small for the model to catch them. As the resolution increases smaller events can be caught, the 1.5 km model for example, is reputedly capable of modelling individual showers.

UKCA

United Kingdom Chemistry & Aerosols (UKCA) is a sub-model of the UM that deals with trace gas and aerosol chemistry within the model. This includes calculating the concentrations of climatically relevant gases such as methane and ozone, as well as the composition and evolution of aerosols. As with most of the UM, the UKCA was written in a collaboration between the UK Met Office and UK Academia.

References

External links

• Unified Model Papers
• Introduction to the UM by NCAS