Deep learning for post-processing global probabilistic forecasts on sub-seasonal time scales

Sub-seasonal weather forecasts are becoming increasingly important for a range of socio-economic activities. However, the predictive ability of physical weather models is very limited on these time scales. We propose four post-processing methods based on convolutional neural networks to improve sub-seasonal forecasts by correcting systematic errors of numerical weather prediction models. Our post-processing models operate directly on spatial input fields and are therefore able to retain spatial relationships and to generate spatially homogeneous predictions. They produce global probabilistic tercile forecasts for biweekly aggregates of temperature and precipitation for weeks 3-4 and 5-6. In a case study based on a public forecasting challenge organized by the World Meteorological Organization, our post-processing models outperform the bias-corrected forecasts from the European Centre for Medium-Range Weather Forecasts (ECMWF), and achieve improvements over climatological forecasts for all considered variables and lead times. We compare several model architectures and training modes and demonstrate that all approaches lead to skillful and well-calibrated probabilistic forecasts. The good calibration of the post-processed forecasts emphasizes that our post-processing models reliably quantify the forecast uncertainty based on deterministic input information in form of ECMWF ensemble mean forecast fields only.