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