Uncertainties within the IFS ensemble system are currently represented by a stochastic perturbation technique (SPPT). This represents uncertainties due to the model integration, and is used during:
SPPT randomly perturbs the tendencies from the physical parameterisation schemes. This is done to represent uncertainties in the effects of under-resolved processes. These uncertainties arise from either or both:
The physics schemes operate through an entire grid box column. The shape of the unperturbed column of tendencies is preserved by multiplying by a single random number.
SPPT perturbs tendencies but not fluxes and this may cause certain inconsistencies and different behaviour between perturbed and unperturbed forecasts.
The current SPPT scheme:
It should be stressed that overall stochastic perturbations undoubtedly do deliver clear improvements in the ensemble performance.
(Note: In older material there may be references to issues that have subsequently been addressed)