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HRES (ECMWF forecasts cycle 43r3 and earlier) was not coupled with NEMO but retained the initial sea-surface temperature anomalies throughout the forecast period.  HRES tended to deepen relatively slow-moving tropical cyclones too much.  This was due to the lack of ocean/atmosphere coupling in HRES which often kept the ocean too warm when it should be cooling.  In reality the strong winds and slow movement would induce turbulent mixing of the very warm surface waters with cooler waters from deeper in the ocean, reducing sea-surface temperatures, and hence inducing less deepening of the storm.  ENS was and continues to be coupled with the ocean and didn't suffer from this problem.

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• 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 particular, they have a larger amplitude over the tropics.
  • Spectrally, EDA-basaed based 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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