Ratio limits · May initialization · JJAS 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 (Jul, Aug, Sep), MVS, ratio limits 1/4-4 (Jul, Aug, Sep) and Variance Scaling (unbounded) (Jul, Aug, Sep).

Probabilities

Blended tercile RPSS through the full pipeline, outer leave-one-year-out 1993–2016
Method	Jun	Jul	Aug	Sep	JJAS
Mean-Variance Scaling (operational, 0.5–2)	-0.000	+0.014	+0.012	+0.003	+0.027
MVS, ratio limits 1/3-3	-0.000	+0.015	+0.012	+0.003	+0.027
MVS, ratio limits 1/4-4	-0.001	+0.015	+0.012	+0.003	+0.027
Variance Scaling (unbounded)	-0.001	+0.015	+0.012	+0.004	+0.027

Amounts

Amount CRPSS, leave-one-year-out 1993–2016
Method	Jun	Jul	Aug	Sep	JJAS
Mean-Variance Scaling (operational, 0.5–2)	-0.096	-0.007	-0.028	-0.060	-0.028
MVS, ratio limits 1/3-3	-0.094	-0.004 ✓	-0.025 ✓	-0.059 ✓	-0.027
MVS, ratio limits 1/4-4	-0.093	-0.003 ✓	-0.024 ✓	-0.059 ✓	-0.027
Variance Scaling (unbounded)	-0.093	-0.000 ✓	-0.021 ✓	-0.058 ✓	-0.027

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
Jun	7%	8%	0.82	0.82
Jul	32%	5%	0.92	0.93
Aug	28%	2%	0.93	0.93
Sep	14%	1%	0.89	0.89
JJAS	14%	1%	0.83	0.83