|Title||Addressing model error through atmospheric stochastic physical parameterisations: Impact on the coupled ECMWF seasonal forecasting system|
|Year of Publication||2014|
|Authors||Weisheimer, A, Corti, S, Palmer, TN, Vitart, F|
|Secondary Title||Technical Memorandum|
The finite resolution of general circulation models of the coupled atmosphere-ocean system and the effects of sub-grid scale variability present a major source of uncertainty in model simulations on all time scales. ECMWF has been at the forefront of developing new approaches to account for these uncertainties. In particular, the stochastically perturbed physical tendency scheme and the stochastically perturbed backscatter algorithm for the atmosphere are now used routinely for global numerical weather prediction. The European Centre also performs long-range predictions of the coupled atmosphere-ocean climate system in operational forecast mode and the latest seasonal forecast System 4 has the stochastically perturbed tendency and backscatter schemes implemented in a similar way as for the medium range weather forecasts. Here we present results of the impact of these schemes in System 4 by contrasting the operational performance on seasonal time scales during the retrospective forecast period 1981-2010 with comparable simulations that do not account for the representation of model uncertainty. We find that the stochastic tendency perturbation schemes helped to reduce excessively strong convective activity especially over the maritime continent and the tropical Western Pacific leading to reduced biases of the outgoing longwave radiation, cloud cover, precipitation and near-surface winds. Positive impact was also found for the statistics of the Madden-Julian Oscillation showing an increase in the frequencies and amplitudes of Madden-Julian Oscillation events. Further, the errors of El Niño Southern Oscillation forecasts become smaller while increases in ensemble spread lead to a better calibrated system if the stochastic tendency is activated. The backscatter scheme has overall neutral impact. Finally, evidence for noise-activated regime transitions has been found in a cluster analysis of mid-latitude circulation regimes over the Pacific-North America region.