|Title||On ensemble prediction using singular vectors started from forecasts.|
|Year of Publication||2005|
|Secondary Title||Technical Memorandum|
The impact on the ECMWF Ensemble Prediction System of using singular vectors computed from 12-hour forecasts instead of analyses has been studied. Results are based on 34 cases in Nov/Dec 1999 and 28 cases in Sep 2003. The similarity between singular vectors started from a 12-hour forecast and singular vectors started from an analysis is very high for the extra-tropical singular vectors in each of the 62 cases and for both hemispheres. Consistently, ensemble scores and spread measures show close to neutral impact on geopotential height in the extra-tropics. The sensitivity of the singular vectors to the choice of trajectory is larger in the tropics than in the extra-tropics. However, the spread in tropical cyclone tracks is not significantly decreased if singular vectors computed from 12-hour forecasts are used. The computation of singular vectors from forecasts could be used to disseminate the ensemble forecasts earlier or to allocate more resources to the nonlinear forecasts. Furthermore, it greatly facilitates the implementation of computationally more demanding configurations for the singular vector based initial perturbations.