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Ensemble forecasts require many alternative analyses and forecasts and each ensemble member requires its own global perturbation.  Importantly, the analysis and subsequent forecast of each ensemble member must be truly independent of all the others.  The process of deriving independent perturbed analyses for ENS members is to use:

• A 50 member Ensemble of Data Assimilations (EDA) is calculated over the globe.  The differences of each member from the EDA mean gives 50 different sets of global EDA perturbations.
• Sets of Singular Vectors (SVs) are separately calculated over the Northern and Southern Hemispheres, and over the tropics between 30°N and 30°S.  These are linearly combined (using coefficients randomly sampled from a Gaussian distribution) to give 100 different sets of global SV perturbations.

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• The 50 sets of EDA perturbations and 100 sets of SVs are added together to give 100 sets of global perturbations for model initialisation, which are in practice added to the HRES analysis as follows:
  • ENS member 1 Analysis = HRES Analysis + (EDA member 1 - EDA mean) + SV Perturbation 1
    
  • ENS member 2 Analysis = HRES Analysis + (EDA member 2 - EDA mean) + SV Perturbation 2
  •         and so on until
  • ENS member 49 Analysis = HRES Analysis + (EDA member 49 - EDA mean) + SV Perturbation 49
  • ENS member 50 Analysis = HRES Analysis + (EDA member 50 - EDA mean) + SV Perturbation 50
  • ENS member 51 Analysis = HRES Analysis + (EDA member 1 - EDA mean) + SV Perturbation 51
  • ENS member 52 Analysis = HRES Analysis + (EDA member 2 - EDA mean) + SV Perturbation 52
  •         and so on until
  • ENS member 99 Analysis = HRES Analysis + (EDA member 49 - EDA mean) + SV Perturbation 99
  • ENS member 100 Analysis = HRES Analysis + (EDA member 50 - EDA mean) + SV Perturbation 100

Need to decide on whether HRES or Control is used

Quality of the individual perturbed analyses

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