Scale-dependent verification of ensemble forecasts

A scale-dependent verification of the ECMWF ensemble prediction system (EPS) in the Northern Hemisphere is presented. The relationship between spread and skill is investigated alongside probabilistic forecast skill for planetary, synoptic and subsynoptic spectral bands. Since the ECMWF model is a spectral model, the three spectral bands have been isolated using total and zonal wavenumber filters. Diagnosed overdispersiveness of ECMWF EPS in the short range is primarily due to excessive amounts of spread on synoptic scales. Diagnosed underdispersiveness of the ensemble beyond day 5 of the forecast can be explained by too little spread on both synoptic and planetary scales. Copyright (C) 2008 Royal Meteorological Society.