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The Stochastically Perturbed Parametrization (SPP) scheme has been developed over several years to improve the physical consistency characteristics of the perturbed forecasts, such as spread.  The The Stochastically Perturbed Parametrization (SPP) scheme, introduced in Cy49r1 in autumn 2024, replaced the previously used Stochastically Perturbed Parametrization Tendency (SPPT).   

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• the creation of the Ensemble of Data Assimilations (EDA) as part of the representation of uncertainties in the forecast initial conditions.
• all these ensemble forecast systems (medium, sub-seasonal, seasonal).

The ensemble control member is unperturbed and does not use these "stochastic physics" perturbations during execution.  

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• on 200hPa and 925hPa.
• on synoptic scale evolution.
• on 2m temperature and 2m specific humidity (SPPT tended to focus its perturbations above the boundary layer).
• increases spread of near surface fields and precipitation (increases capture of extremes).
• decreases spread of mid/upper troposphere fields (users should asses important features).
• increased spread in tropical cyclones (beneficial for strong systems).
• positive impact in MJO at week 3-4 in monthly forecasts.  

Occasionally In the past with SPPT the ensemble may used to sometimes show a very small risk of extreme weather beyond what is synoptically reasonable (e.g. winter maritime convective heating may be was dampened and some members may be were unrealistically cold as a result), but SPP has improved this aspect.

It should be stressed that overall stochastic perturbations undoubtedly do deliver clear improvements in the particular aspects of ensemble performance.

Additional Sources of Information

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