Ratio limits · September initialization · ONDJ and its months

The operational correction limits the spread ratio r = σ_obs/σ_model to 0.5–2. This study runs the same screening and confirmation for that correction with wider limits (1/3–3, 1/4–4) and none, all with the same fallback (r = 1 where the model spread is below 1 mm), to see whether the limits are worth changing. ✓ = better than the operational limits with Holm-adjusted p < 0.05 across the 3 alternatives × 5 targets and not worse over the test years. Run at commit fb7425f.

Confirmed better than the operational limits — published (blended) probabilities: none; rainfall amounts: MVS, ratio limits 1/3-3 (Jan), MVS, ratio limits 1/4-4 (Jan) and Variance Scaling (unbounded) (Jan).

Probabilities

Blended tercile RPSS through the full pipeline, outer leave-one-year-out 1993–2016
Method	Oct	Nov	Dec	Jan	ONDJ
Mean-Variance Scaling (operational, 0.5–2)	+0.013	+0.040	+0.003	+0.001	+0.039
MVS, ratio limits 1/3-3	+0.013	+0.041	+0.004	+0.001	+0.039
MVS, ratio limits 1/4-4	+0.013	+0.041	+0.004	+0.001	+0.039
Variance Scaling (unbounded)	+0.013	+0.041	+0.004	+0.001	+0.039

Amounts

Amount CRPSS, leave-one-year-out 1993–2016
Method	Oct	Nov	Dec	Jan	ONDJ
Mean-Variance Scaling (operational, 0.5–2)	-0.002	+0.084	+0.028	-0.048	+0.070
MVS, ratio limits 1/3-3	-0.004	+0.082	+0.031	-0.013 ✓	+0.063
MVS, ratio limits 1/4-4	-0.004	+0.082	+0.031	-0.008 ✓	+0.063
Variance Scaling (unbounded)	-0.004	+0.082	+0.031	-0.007 ✓	+0.063

Where the operational limits act

Share of the corrected area, mean over the leave-one-year-out folds, 1993–2016
Target	Lower limit active (r would be < 0.5)	Upper limit active (r would be > 2)	Spread–error, operational	Spread–error, no limits
Oct	1%	12%	0.90	0.93
Nov	5%	9%	1.01	1.02
Dec	17%	6%	0.96	1.00
Jan	26%	28%	0.83	1.00
ONDJ	2%	22%	0.96	1.01