Medium-range multimodel ensemble combination and calibration

As part of its contribution to The Observing System Research and Predictability Experiment (THORPEX), the Met Office has developed a global, 15 day multimodel ensemble. The multimodel ensemble combines ensembles from the European Centre for Medium-Range Weather Forecasts (ECMWF), Met Office and National Centers for Environmental Prediction (NCEP) and is calibrated to give further improvements. The ensemble post-processing includes bias correction, model-dependent weights and variance adjustment, all of which are based on linear-filter estimates using past forecast-verification pairs, calculated separately for each grid point and forecast lead time. Verification shows that the multimodel ensemble gives an improvement in comparison with a calibrated single-model ensemble, particularly for surface temperature. However, the benefits are smaller for mean-sea-level pressure (mslp) and 500 hPa height. This is attributed to the higher degree of forecast-error similarity between the component ensembles for mslp and 500 hPa height than for temperature. The results also show only small improvements from the use of the model-dependent weights and the variance adjustment. This is because the component ensembles have similar levels of skill, and the multimodel ensemble: variance is already generally well calibrated. In conclusion, we demonstrate that the multimodel ensemble (toes give benefit over a single-model ensemble. However, as expected, the benefits are small if the ensembles are similar to each other and further post-processing gives only relatively small improvements. (C) Crown Copyright 2009. Reproduced with the permission of HMSO. Published by John Wiley & Sons Ltd.