Impact of Ensemble Size on Significance of Climate Change,

Variability, and Extremes

Jeff Yin, NCAR

11th CCSM Workshop

22 June 2006

Examples of the Impact of Ensemble Size

• Climate change (2080-2099 – 1980-1999):– Stationary waves/zonal means– Midlatitude storm tracks– Seasonal maximum wind speed

• Climate variability (NAO):– Seasonal maximum wind speed– Percentiles of wind speed

• 95% significance level shown

2080-2099 minus 1980-1999:Total Z300

2080-2099 minus 1980-1999:Total Z300

2080-2099 minus 1980-1999:Total Z300

2080-2099 minus 1980-1999:Stationary wave Z300

2080-2099 minus 1980-1999:Stationary wave Z300

2080-2099 minus 1980-1999:Stationary wave Z300

2080-2099 minus 1980-1999:Zonal mean Z300

2080-2099 minus 1980-1999:Zonal mean Z300

2080-2099 minus 1980-1999:2-8 day EKE200

2080-2099 minus 1980-1999:2-8 day EKE200

2080-2099 minus 1980-1999:Seasonal maximum daily WS850

2080-2099 minus 1980-1999:Seasonal maximum daily WS850

2080-2099 regression on NAO:Seasonal maximum daily WS850

2080-2099 regression on NAO:Seasonal maximum daily WS850

Positive NAO, 2080-2099:99%ile of WS850

Positive NAO, 1870-1999:99%ile of WS850

Positive NAO, 1870-1999:99.9%ile of WS850

Summary

• Ensemble size needs depend on problem

• For differences of 20-year periods:– 1 member not enough (even for zonal mean)– 4+ members for stationary waves– 4+ members for storm tracks, extreme winds

• For relationships with NAO:– 4+ members for seasonal max wind speed– > 300 years for 99%ile of wind speed