|Title||The new ECMWF Variable Resolution Ensemble Prediction System (VAREPS): methodology and validation.|
|Publication Type||Technical memorandum|
|Secondary Title||ECMWF Technical Memorandum|
|Authors||Buizza, R, Bidlot, J-R, Wedi, N, Fuentes, M, Hamrud, M, Holt, G, Palmer, TN, Vitart, F|
On 1 February 2006, the resolution of the European Centre for Medium-Range Weather Forecasts (ECMWF) Ensemble Prediction System (EPS) was increased from TL255L40 to TL399L62. This change was the first of a three-phase upgrading process that will lead to the implementation of the ECMWF Variable Resolution Ensemble Prediction System (VAREPS), a system designed to provide skilful predictions of small-scale, severe-weather events in the early forecast range, and accurate forecast large-scale guidance in the medium forecast range. In this work, first the rationale behind VAREPS is presented, and then average results based on a VAREPS with a truncation at forecast day 7 and 40 vertical levels [i.e. TL399L40(d0-7) and TL255L40(d7-15)] are discussed, and compared to the performance of two constant resolution systems, a TL255L40 and a TL319L40 (this latter one requires similar computing resources to VAREPS). Average results based on 111 cases indicate that VAREPS is more skilful than a TL255L40 EPS, and that VAREPS should be preferred to the constant-resolution TL319L40 EPS, since it provides significantly better forecasts in the early forecast range without loosing accuracy in the long forecast range. The differences between VAREPS and the other two systems are, on average, small, but statistically significant in the early forecast range. The discussion of some specific events indicate that that these differences can be very large, and can lead to substantial improvements in the prediction of severe weather events, such as the ones linked with hurricanes, or intense precipitation. Results have also shown that VAREPS will be able to provide some skilful forecasts beyond forecast day 10. VAREPS will further increase the value of the ECMWF probabilistic forecasting system, and deliver more accurate predictions of small-scale, severe weather events in the early forecast range and skilful probabilistic predictions of larger scale features in the medium forecast range.