Impact of Ensemble Size on Significance of Climate Change, Variability, and Extremes

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