Stochastic representations of model uncertainties at ECMWF: state of the art and future vision

Abstract

Members in ensemble forecasts differ due to the representations of initial uncertainties and model uncertainties. The inclusion of stochastic schemes to represent model uncertainties has improved the probabilistic skill of the ECMWF ensemble by increasing reliability and reducing the error of the ensemble mean. Recent progress, challenges and future directions regarding stochastic representations of model uncertainties at ECMWF are described in this article. The coming years are likely to see a further increase in the use of ensemble methods in forecasts and assimilation. This will put increasing demands on the methods used to perturb the forecast model. An area that is receiving greater attention than 5–10 years ago is the physical consistency of the perturbations. Other areas where future efforts will be directed are the expansion of uncertainty representations to the dynamical core and other components of the Earth system, as well as the overall computational efficiency of representing model uncertainty.

Exchange Processes in the Atmospheric Boundary Layer Over Mountainous Terrain

“…An underdispersive ensemble, instead, underestimates forecast uncertainty. A widely used approach to increase ensemble spread, the SPPT (stochastically-perturbed parameterized tendencies) scheme [240,241], superimposes column-wise multiplicative noise, with pre-determined spatial and temporal correlation scales, to the parameterized tendencies of dynamic and thermodynamic variables. This approach proved successful at improving reliability, especially for near-surface variables, but is not free of drawbacks.…”

“…Current attempts at improving stochastic parameterizations are directed at developing SPP (stochastically perturbed parameter) schemes [241][242][243], where stochastic perturbations are imposed at the parameter level instead of the tendency level. Knowledge of physical processes is instrumental in the development of this type of schemes and suggests what parameters might be more appropriately subject to stochastic perturbation.…”

Climate SPHINX: evaluating the impact of resolution and stochastic physics parameterisations in the EC-Earth global climate model

“…The new scheme removes the imbalance in P −E, which is now equal to that in the run where the SPPT scheme is disabled. This fix has subsequently been implemented at ECMWF (Leutbecher et al, 2017).…”

Medium- and Extended-Range Ensemble Weather Forecasting

“…An alternative is to use a single parametrisation, but to perturb some of the key parameters in the scheme (Bowler et al 2008). Other schemes represent the uncertainty from the sub-grid scale by stochastically perturbing the tendencies from the parametrisation schemes, as, for example, in the Stochastically Perturbed Parametrisation Tendency scheme (SPPT, Leutbecher et al 2017;Buizza et al 1999). Backscatter schemes are designed to simulate the transfer of energy from the unresolved sub-grid scales to the larger scales that are resolved by the model (Shutts 2005;Berner et al 2009).…”