Abstract
Bridging the gap between weather forecasting and climate prediction, subseasonal to seasonal (S2S) forecasts are of great importance yet currently of relatively poor quality. Using the S2S Prediction Project database, the study evaluate products derived from four operational centers of CMA, KMA, NCEP and UKMO, and superensemble experiments including the straightforward ensemble mean (EMN), bias-removed ensemble mean (BREM), error-based superensemble (ESUP) and Kalman filter superensemble (KF), in forecasts of the surface air temperature with lead times of 6*–30 days over Northeast Asia in 2018. Validations after the pre-processing of a 5-day running mean suggest that the KMA model shows the highest skill for either the control run or the ensemble mean. The non-equal weighted ESUP is slightly superior to BREM, whereas they both show larger biases than EMN after the lead time of 22 days. The KF forecast constantly outperforms the others, decreasing mean absolute errors by 0.2–0.5°C relative to EMN. Forecast experiments of the 2018 Northeast Asia heat wave reveals that the superensembles remarkably improve the raw forecasts featuring biases >4°C. The prominent advancement of KF is further confirmed, showing the regionally averaged bias of ≤2°C and the hit rate of 2°C reaching up to 60% at the lead time of 22 days. The superensemble techniques, particularly the KF method dynamically adjusting the weights in accordance with the latest information available, are capable of improving forecasts of spatiotemporal patterns of surface air temperature on the subseasonal timescale, which could extend the skillful prediction lead time of extreme events such as heat waves to about 3 weeks.
