|Title||The TIGGE global, medium-range ensembles|
|Publication Type||Technical memorandum|
|Secondary Title||ECMWF Technical Memorandum|
TIGGE, the THORPEX Interactive Grand Global Ensemble, is a World Meteorological Organization research programme, launched in March 2005, with the goal to accelerate improvements in the accuracy of 1-day to 2-week high-impact weather forecasts. This paper introduces a unified framework that can be used to characterize the TIGGE ensembles, compare their characteristics and highlight the rationale behind their design. It presents the configuration of each TIGGE ensemble at implementation time and the time of writing (summer 2014), and discusses the performance of the TIGGE ensembles in predicting synoptic-scales in recent seasons. There are four main conclusions that can be drawn from this work. Firstly, there is not a unique recipe to generate reliable and skilful ensembles, and indeed the nine TIGGE ensembles show that different methods can be used to simulate the effects of the sources of forecast errors. Secondly, one of the TIGGE ensembles, from ECMWF, still outperforms all the others, with differences in predictive skill for synoptic-scale variables in the medium-range of about 1 day, which corresponds to about 7-8 years of development. Thirdly, international projects such as TIGGE and follow-on projects, will continue to play the essential role of providing the resources (e.g. easy access to standardized multi-system data) to help scientists to understand predictability and further advance ensemble techniques. Because of this, they should continue to be supported. Fourthly, ensembles are here to stay, and the future will see ensembles used even more to estimate the probability distribution function of initial and forecast states of ocean, land and atmospheric variables.