Extended Abstract

perturbations. Blending combines the large-scale uncertainty generated by ECMWFSingular Vector (SV) with the small-scale uncertainty generated by breeding withALADIN model. The combined perturbation has the feature that its large-scale part ofthe perturbation is from ECMWF SV, and the small scale part is from ALADINbreeding. Through breeding the blended perturbation attempts to give the bestestimate of the actual errors in the initial analysis based on the past information ofthe flow, whereas the SV contain future information of possible forecast error. Theinitial perturbation is now consistent with the LBC perturbation in large scale part,both of them from ECMWF SV perturbation. The small-scale uncertainty in theanalysis is more detailed and accurate due to the higher resolution and morebalanced orographic/surface forcing of the LAM breeding.Figure 1. Verification of ALADIN-LAEF 12h accumulated precipitation forecast ofexperiments DOWN (in red, dynamical downscaling), BRED (in green, ALADINBreeding) and BLEND (in blue, ALADIN Blending). Left: Continuous Rank ProbabilityScore; Right: ensemble spread, averaged statistics over central Europe and a twomonthsummer period.The Blending technique is a spectral technique using standard non-recursive lowpassDolph-Chebyshev digital filter. The detailed description and discussion ofBlending, in particular the technical implementation in ALADIN-LAEF, are given inBellus et al. (2010).Figure 1 shows the comparison of Blending and breeding on ALADIN-LAEFprecipitation forecasts for a two months period. Regarding the probabilistic scoremeasured by Continuous Rank Probability Score, the outperformance of Blending toBreeding is found, in particular, in the late forecast hours after 30h. It is obviously dueto the impact of the LBC perturbation. Blending has the large scale perturbationswhich are consistent with the LBC perturbations. Breeding initializes a similar spreadin the early hours, but the growth of perturbation is slower than Blending. Thegeneration of perturbation by breeding conflicts somehow with the LBC perturbationfrom ECMWF SV, the benefit of using Blending comes clear in the later lead timeafter 30 hours.3. How can we generate the surface perturbation in LAMEPS?-327-