Benefits of Refined 10-Day Effective Angular Momentum Forecasts for Earth Rotation Parameter Prediction

Effective angular momentum forecasts are widely used as an important input to provide short-term polar motion and UT1-UTC predictions. So far, the Earth system modelling group at GFZ provides effective angular momentum forecasts based on model prediction runs of the atmosphere, ocean, and terrestrial hydrosphere that reach only 6-days into the future. The oceanic and land-surface model forecasts are forced with operational 6-day high-resolution deterministic numerical weather predictions provided by the European Center for Medium-range Weather Forecasts. Those atmospheric forecasts extend also further into the future but have a reduced sampling rate of only 6 hours and their prediction skill decreases rapidly after roughly one week. Here, we present a test set of 454 10-day EAM forecasts that we used within GFZ's EAM Predictor method to calculate Earth rotation predictions. Compared to the 6-day forecasts, the introduction of the 10-day forecasts leads to slight improvements in y-pole and UT1-UTC predictions for 10 to 30 days ahead. Introducing in addition neural network models trained on the errors of the effective angular momentum forecasts when compared to their subsequently available analysis runs, the benefit of extended EAM forecasts for Earth rotation prediction can be enhanced. A reduction of the mean absolute errors for polar motion and length-of-day prediction at a forecast horizon of 10 days of 26.8% in x-pole, 15.5% in y-pole,27.6% in UT1-UTC, and 47.1% in ΔLOD was achieved. This promising test application of extended effective angular momentum forecasts based on geophysical models persuaded GFZ to publish 10-day instead of 6-day forecasts since October 2024.