Model output statistics for enhancing dynamically downscaled temperature prediction from FuXi-ENS over South Korea

While machine learning (ML) weather models are emerging as promising tools for predicting weather conditions on a global scale, their coarse resolution and systematic biases limit practical applicability in regional contexts. This paper assesses the role of model output statistics (MOS) in post-processing temperature ensemble forecasts that are dynamically downscaled from the state-of-the-art ML weather model FuXi-ENS. The dynamically downscaled forecasts for July with a one-month lead time are obtained from a Weather Research and Forecasting modeling system optimized for South Korea and its complex geographic features. A Joint-Gaussian (JG) approach is applied to post-process the downscaled forecasts and is benchmarked against widely used quantile mapping (QM) under the framework of leave-one-year-out cross-validation. The results show that although dynamically downscaled forecasts reasonably capture spatial variability and correlate with observations, they are usually subject to warm biases, leading to underperformance relative to reference climatological forecasts. The QM effectively corrects the systematic biases but, as a deterministic mapping, inadequately calibrates the over-confident ensemble spread, typically limiting its positive skill to a one-week lead time. The JG improves upon QM by explicitly accounting for the forecast-observation dependency relationship, thereby yielding reliable ensemble spreads and extending positive skill to two-week lead times. As forecasts become non-informative at extended lead times, the JG forecasts reliably characterize predictive uncertainties by reverting toward the marginal distribution of observations, ensuring coherent predictive performances. Overall, this paper highlights the value of combining statistical post-processing with dynamical downscaling to transform ML-based global predictions into actionable regional information.