Atmospheric numerical modelling is the process of solving a set of equations to obtain an objective forecast of the future state of the atmosphere. The equations describe the evolution of many variables (e.g. temperature, wind speed, humidity and pressure) and together define the state of the atmosphere.

Numerical models may be very complicated, consisting of numerical integration schemes, physical parametrizations and data output schemes.

Components of a numerical model system

Prior to running any numerical model one requires an initial condition and depending upon the model type one may also require a number of boundary conditions.

In numerical weather prediction we obtain our initial condition by analysing the current state of the atmosphere. Observations incorporated into the model, using data assimilation provide the initial condition. Surface fields also play a significant role in the numerical model. Whether a grid point is over land or sea, what type of vegetation is prevalent etc, will impact upon how the model interacts with the surface boundary condition.

All numerical models of the atmosphere are based upon the same set of governing equations, described here in non-mathematical terms. Where numerical models differ, is how the equations are solved; what approximations and assumptions are made and how one represents the physical processes in the physical parametrizations. Many processes in the atmosphere, for example radiation, convection and precipitation to name a few, often occur at a scale too small to be directly resolved by the numerical model and thus need to be parametrized.

The Met Office numerical formulations are summarised in the following factsheet.

The Unified Model

The numerical model employed at the Met Office is called the Unified Model; the same model formulation is for all models from climate scale to mesoscale:

• Climate modelling
• seasonal forecasting
• and operational NWP forecasts

A ported version of the Unified Model (PUM) may be used by research centres and other meteorological institutes under the PUM license.

More about the Unified Model

Computers

To run a numerical model for weather forecasting or climate studies requires a lot of computing power.

The Met Office has readily adapted to the changing architecture of supercomputers. Early computers were scalar processors, then came along vector processors and now they are massively parallel processors.

More about computers