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• during the execution of the forecast.
• during creation of the Ensemble of Data Assimilations (EDA) as part of the representation of uncertainties in the forecast initial conditions.

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• parametrisation scheme assumptions which incorporate bulk descriptions of sub-grid scale processes active within an individual grid box or column,.
• approximations that are necessary to describe poorly constrained processes.

Since the The physics schemes operate through an entire grid box column, the .  The perturbations act to preserve the shape of the unperturbed vertical profile of tendencies by multiplying a column of tendencies with a single random number.

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• tends to focus its perturbations above the boundary layer.  Physics-related uncertainties in near-surface weather parameters (e.g. temperature, visibility) may be under-represented.   The ensemble spread for such parameters may be too small.
  
• perturbations do not explicitly depend on the current synoptic pattern.  Occasionally the ensemble may show a very small risk of extreme weather beyond what is synoptically reasonable (e.g. convective .  For example: convective heating from the seas in winterin winter-time cold NW'ly outbreaks may be damped so that air-masses look masses look unrealistically cold in one or two members).

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