Results from the CCI’s Climate Modelling User Group on forecast errors in the seasonal prediction system EC-Earth will be presented at the CMIP6 Model Analysis Workshop (25-28 March, Barcelona).
The workshop is organized by the WCRP Working Group on Coupled Modelling CMIP Panel and European Commission Horizon 2020 project PRIMAVERA.
Abstract: An anatomy of the forecast errors in the seasonal prediction system with EC-Earth
Authors: Ruben Cruz-García, Pablo Ortega, Juan Acosta, François Massonnet and Francisco Doblas-Reyes
Initialization is an essential step when performing climate predictions, and the use of the latest high-quality observations and their assimilation in the model realm is of paramount importance. Less attention has been paid to other essential aspects of initialization that are equally important. For example, incompatibilities between the initial conditions (ICs) products used for the different model components can cause important initialization shocks, hindering the prediction capacity during the first weeks of the forecast.
In this study, we investigate the different contributions to forecast errors in a seasonal prediction system with EC-Earth where sea ice is initialized via Ensemble Kalman filter assimilation of ESA-derived sea ice concentrations, that was produced within the context of the Climate Model User Group (CMUG) Climate Change Initiative (CCI) from ESA. The largest source of forecast error in Arctic sea ice appears in regions of high observational uncertainty, which hinders the efficiency of the assimilation protocol. We also investigated the development of the systematic error and its competing effect with the shock that emerges from the incompatibility between ocean and sea ice ICs. After 26 (21) days the systematic model error becomes the largest contributor to the forecast error for the May (November) initialized forecasts, with the initial-incompatibility dominating in the previous days. Moreover, the development of both errors is sensitive to the month of initialization: the shock is more pronounced in November than in May. The major differences between both months relate to the systematic error, which is much higher in November, and also to the direction of the shock with respect to the seasonal trend. In both cases the shock leads to sea ice melting, but, unlike in May, in November it happens in a context of sea ice expansion. This opposing effect during November might be accentuating the generation of the drift.
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