The new ECMWF VAREPS (Variable Resolution Ensemble Prediction System)

The European Centre for Medium-Range Weather Forecasts (ECMWF) Variable Resolution Ensemble Prediction System (VAREPS) is a system designed to provide skilful predictions of small-scale, severe-weather events in the early forecast range, and accurate large-scale forecast guidance in the extended forecast range (say beyond forecast day 7). In this work, first the rationale behind VAREPS is presented, and then the performance of VAREPS with a truncation at forecast day 7, i.e. T(L)399L40(d0-7) and T(L)255L40(d7-15), is discussed and compared to the performance of two constant resolution systems, a T(L)255L40 and a T(L)319L40 (this latter one requires similar computing resources to VAREPS). Average results based on up to 111 cases indicate that VAREPS has a higher forecast-time-integrated skill, and it provides better forecasts in the early forecast range without losing accuracy in the long forecast range. In the early forecast range, the differences in forecast performance can be very large and responsible for substantial improvements in the prediction of weather variables such as surface wind, significant wave height and total precipitation, as was shown in two case-studies. Average results have also shown that the VAREPS extension to 15 days (the old EPS system was run operationally only up to forecast day 10) will provide users with some skilful extended-range forecasts. Copyright (C) 2007 Royal Meteorological Society.