Informal Seminar: Assessing the impact of model biases on sub-seasonal forecast skill

The impact of model biases on sub-seasonal forecast skill has been evaluated in the ECMWF sub-seasonal forecasting system using a relaxation experiment where the nudging was performed towards bias corrected integrations of the same model. The relaxation was performed regionally or globally but only for the very large scale. Results indicate that this methodology significantly reduces model biases over the areas where the nudging was applied as expected. Experiments with global relaxation or with a relaxation applied only over the North Pole or only over the Tropics display significantly improved forecast skill at a lead time of 3 to 4 weeks, particularly over the northern Extratropics, where sub-seasonal forecast skill has been stagnant at this time range over the past 20 years in the ECMWF operational sub-seasonal forecasting system. Sensitivity experiments indicate that the largest positive impact is obtained with a 2-day relaxation timescale in the Tropics and 5-day in the Extratropics.  An analysis of the North Pole relaxation experiment shows that the polar relaxation impacts biases outside the region of nudging, such as the representation of the sub-tropical jet stream in the ECMWF model, reducing mid-latitude geopotential height bias and leading to a slight improvement in MJO teleconnections over the northern Extratropics. These results indicate that there is a large potential for improving dynamical sub-seasonal forecasting skill in weeks 3 and 4 in Extratropical regions, which could be achieved using online or offline bias correction.