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• Perturbations based on singular vectors (SV) and perturbations based on ensemble of data assimilations (EDA) have different characteristics:
  • Geographically, EDA-perturbations based perturbations on EDAs are less localized localised than SV- based perturbations based on SVs.  In particular, they have a larger amplitude over the tropics.
  • Spectrally, EDA-perturbations based perturbations on EDAs are smaller in scale.
  • Vertically, EDA-perturbations based perturbations on EDAs are more barotropic than SV-based perturbations; SV-based perturbations perturbations based on SVs.  Perturbations based on SVs 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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