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SPPT randomly perturbs the tendencies from the physical parameterisation schemes.  This is done to represent uncertainties in the effects of under-resolved processes that the atmospheric physics parametrisation parameterisation schemes aim to describe.  These uncertainties arise from either or both:

• parametrisation parameterisation scheme assumptions. These 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.

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• tends to focus its perturbations above the boundary layer.  Physics-related uncertainties in near-surface weather parameters  Uncertainties in weather parameters near the surface (e.g. temperature, visibility) may not be under-well 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.  For example: convective heating from the seas in winter-time cold NW'ly outbreaks may be damped so that air-masses look unrealistically cold in one or two members).

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