Medium-range ensemble system

As we forecast more than two days ahead the potential for rapid growth in forecast error increases dramatically. For operational medium-range (3-15 days) ensemble forecasting, the Met Office uses the Ensemble Prediction System (EPS) run by the European Centre for Medium-Range Weather Forecasts (ECMWF).

ECMWF

ECMWF is an international organisation supported by many European states, including the UK, which specialises in numerical weather prediction for medium-range prediction. 

The ECMWF EPS consists of 51 forecasts run using the ECMWF global forecast model with a horizontal resolution of around 50 km. One member, called the control forecast, is run directly from the ECMWF analysis — our best guess at the initial state of the atmosphere. Initial conditions for the other 50 members are created by adding small perturbations to the original ECMWF analysis, to represent uncertainties in the initial state. Small random variations are also made to the forecast model, to represent uncertainties in how the forecast model represents atmospheric processes.

What the Met Office does

The Met Office uses the ECMWF ensemble, along with our own model and models from other forecast centres, to assess the most likely outcome in the medium-range forecast, plus the uncertainty in that forecast. ECMWF ensemble forecasts are then processed further to generate an range of probability forecasts for our forecasters and for customers.

Research suggests that forecast skill may be improved by combining ensemble forecasts produced using several different models. To test this, the MOGREPS ensemble system has been extended to produce a 15-day global forecast. The model resolution is approximately 90 km and it has 38 vertical levels. While MOGREPS-global is run out to three days at the Met Office, the MOGREPS-15 system is run using the ECMWF supercomputer.

Our research on medium-range ensemble forecasting is being carried out in conjunction with the THORPEX (The Observing System Research and Predictability Experiment) programme. THORPEX is an international research programme set up to accelerate improvements in the accuracy of one to 14-day weather forecasts for the benefit of society, the economy and the environment.

The output from all the ensemble systems can be used in many different ways to quantify the uncertainty in the forecast.