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Since the physics schemes operate through an entire grid box column, 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.
Some Limitations of SPPT
Forecasters should be aware that the The current SPPT scheme:
- tends to focus its perturbations above the boundary layer.
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- Physics-related uncertainties in near-surface weather parameters (e.g. temperature, visibility) may be under-represented. The
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- ensemble spread for such parameters may be too small.
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- perturbations do not explicitly depend on the current synoptic pattern.
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- Occasionally the
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- ensemble may
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- show a very small risk of extreme weather beyond what is synoptically reasonable (e.g. 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).
It should also be stressed that overall stochastic perturbations undoubtedly do deliver clearcut improvements in the ensemble performance.
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