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The 50 ensemble member forecasts are made from slightly different initial conditions (perturbations) which are designed to represent the uncertainties inherent in the analysis.  The initial perturbations are constructed using the singular vector (SV) technique, and perturbations generated from the ensemble of data assimilations (EDA).  

Points to note:

• SV- and EDA-based perturbations Perturbations based on singular vectors (SV) and perturbations based on ensemble of data assimilations (EDA) have different characteristics:
  • Geographically, EDA-based perturbations are less localized than SV-based perturbations. In  In particular, they have a larger amplitude over the tropics.
  • Spectrally, EDA-basaed perturbations are smaller in scale.
  • Vertically, EDA-based perturbations are more barotropic than SV-based perturbations; SV-based perturbations show westward tilt with height typical of baroclinically unstable structures.
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  • At initial time, SV-based perturbations have a larger amplitude in potential energy than kinetic energy; EDA-based perturbations have a similar amplitude in potential and kinetic energy.
  • EDA perturbations grow less rapidly.
An EDA-based ensemble underestimates the ensemble spread.
• More reliable and accurate forecasts are obtained with a combination of EDA- and SV-based perturbations (operational since Jun 2010).

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