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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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