GloFAS Seasonal v3.1 forecast skill is evaluated using the Continuous Ranked Probability Skill Score (CRPSS) for ECMWF SEAS5 36-year reforecasts against a climatology benchmark forecast with respect to GloFAS-ERA5 river discharge reanalysis (also known as forced simulations (sfo) ) as proxy observations at n=5997 GloFAS diagnostic river points. Further details on the evaluation methodology can be found here: GloFAS forecast skill .
Overall, GloFAS Seasonal is skilful at the global scale for the majority of catchments across all lead times analysed (Figure 1). The CRPSS against climatology decays exponentially as a function of lead time reaching a minimum of 0.01 (interquartile range of -0.09 to 0.09) by week 16. The CRPSS at week 1 = 0.92 (interquartile range of 0.97 to 0.84), week 4 = 0.30 (0.15, 0.48), and at week 8 = 0.10 (-0.03, 0.15).
Figure 1: Skill of GloFAS Seasonal 3.1 with global median Continuous Ranked Probability Skill Score (CRPSS) for reforecasts against a climatology benchmark from 1- to 16-week lead time with respect to GloFAS-ERA5 river discharge reanalysis at 5997 diagnostic river points. The interquartile range of CRPSS values at each lead time are shown by semi-transparent bands.
The spatial distribution of skill is shown in Figure 2. GloFAS Seasonal is skilful compared to the climatology benchmark forecast in all catchments at a 1-week lead time. This reduces to 91%, 74% and 55% by lead times 4-, 8-, and 16-weeks, respectively. The regions of lowest skill against climatology include Northwest Canada and Alaska, North and central Africa, the Northwest coast of South America, and central and southern Asia.
Figure 2: Skill of GloFAS Seasonal 3.1 with global median Continuous Ranked Probability Skill Score (CRPSS) for reforecasts against a climatology benchmark for lead times (1- (a), 4- (b), 8- (c), and 16-weeks (d)) with respect to GloFAS-ERA5 river discharge reanalysis at 5997 diagnostic river points. Optimum value of CRPSS is 1. Blue (red) dots show catchments with positive (negative) skill.