Prediction of Extratropical Cyclones by TIGGE Ensemble Prediction Systems

The THORPEX Interactive Grand Global Ensemble (TIGGE) is a World Weather Research project, which includes an archive of ensemble prediction system (EPS) data from different operational weather centers around the world. A cyclone identification and tracking methodolgy has been used to analyse the prediction of extratropical cyclones in both the northern and southern hemispheres for the 6 month period of 1 February 2008 - 31 July 2008. Forecast verification statistics have been produced (using the European Centre for Medium Range Weather Forecasts [ECMWF] analysis as the truth) for cyclone position, intensity and propagation speed, showing large differences between the different EPS. The results show that the ECMWF ensemble mean and control forecast have the highest level of skill for all cyclone properties in both hemispheres, with the Japanese Meteorological Administration (JMA), the National Centers for Environmental Prediction (NCEP), the Met Office (UKMO), and the Canadian Meteorological Centre (CMC) having the next highest level of skill. The results also show that NCEP, the Centro de Previsa o de Tempo e Estudos Climaticos (CPTEC), and the Australian Bureau of Meteorology (BoM) all have faster intensity error growth in the earlier part of the forecast. These EPS are also very underdispersive and significantly underpredict intensities. This is perhaps due to the comparatively low spatial resolutions of these EPSs not being able to accurately model the tilted structure essential to cyclone growth and decay. The Chinese Meteorological Agency (CMA) EPS has a significantly lower level of performance in the southern hemisphere compared to the northern hemisphere and it is suggested that this is caused by larger errors in the initial state, related to the observations and how they are assimilated. Cyclone propagation speed is underpredicted (i.e. the cyclones propagate too slowly) by all EPS in both hemispheres.