1. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Decadal Variability in Water Vapour Richard Allan, Tony Slingo Environmental Systems Science Centre, University of Reading Mark Ringer Hadley Centre, Met Office

2. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 INTRODUCTION –Can we determine water vapour feedback from observations of present day climate? –How does water vapour respond to changes in surface temperature? –Can we use reanalyses? –How do the results link in present day changes in cloudiness?

3. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Previous studies Observational determination of water vapour feedback (e.g. Raval and Ramanathan 1989; Cess 1989; Soden et al. 2002) Theoretical/ Modelling studies (e.g. Manabe and Wetherald 1967; Ingram 2002)

4. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Climate sensitivity Ts= Q, =-1/( BB + WV + + Cld +….), WV ~-( d OLR/ wv)(dwv/dTs) WV Cld Theory, Measurement Observations

5. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 OLR Sensitivity to Water Vapour

6. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Interannual variability of Column Water vapour (Allan et al. 2003, QJ, p.3371) 1980 1985 1990 1995 See also Soden (2000) J.Clim 13

7. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 CWV Sensitivity to Ts dCWV/dTs = 3.5 kgm -2 K -1 for HadAM3 and Satellite Microwave Observations over tropical oceans Corresponds to ~9%K -1 in agreement with Wentz & Schabel (2000) who analysed observed trends What about upper tropospheric moisture?

8. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Can we use reanalyses? Renalyses are currently unsuitable for detection of subtle trends associated with water vapour feedbacks

9. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Upper tropospheric moisture Clear-sky OLR sensitive to Ts and RH 6.7 mm cloud cleared radiance sensitive to upper tropospheric Relative Humidity Explicitly simulate 6.7 mm radiance in HadAM3 Modified satellite-like clear-sky diagnostics

10. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Sensitivity of OLRc to UTH

11. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Sensitivity of OLRc to UTH

12. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Interannual monthly anomalies: tropical oceans g a =1-(OLRc/ Ts 4 ) (Allan et al. 2003, QJ, p.3371)

13. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 (+additional forcings) (Allan et al. 2003, QJ, p.3371)

14. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Large changes in OLR from 7 independent satellite instruments (Wielicki et al, 2002)

15. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Allan & Slingo 2002, GRL, 29(7) OLR Clear-sky OLR RSW +Altitude Correction?

16. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 - Even considering the latest corrections to the ERBS WFOV data, models still appear to underestimate the variation of tropical mean cloudiness - This is despite the apparent agreement between models and observations that tropical mean Relative Humidity varies only slightly on a decadal time-scale

17. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Summary (1) Climate model simulates low-level water vapour changes Need to accurately measure upper tropospheric water vapour Reanalyses are not an option at present Satellite measurements of 6.7 m radiances RH –Future use of GERB+SEVIRI

18. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Summary (2) Simulations of satellite brightness temperatures: Consistent decadal variability suggests small DRH realistic –can multiple satellite intercalibration artificially remove decadal trends in the UTH radiances? –Changes in atmospheric T also influences T6.7 decadal fluctuations BUT…Satellite measurements suggest that cloudiness variations are much larger than produced by climate models. –Uncertainty remains over precise decadal variability of radiation –More wide-field of view instruments required? (simple yet well-calibrated for variability)

19. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 EOF analysis of spatio- temporal variability in water vapour radiance

20. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Climatological mean over 60 o S-60 o N oceans

21. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Info on upper tropospheric water vapour T 6.7 500mb omega Clear-sky OLR

22. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Water vapour feedback: recent advances (1) Insensitive to resolution (Ingram 2002, J Climate, 15, 917-921) (2) Consistent with observations following post-Pinatubo cooling (Soden et al 2002, Science, 296, 727)

23. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Is water vapour feedback really consistent between models? dOLRc/dTs ~ 2 Wm -2 K -1 dOLR/dTs uncertain (Cess et al. 1990, JGR, 95, 16601) Allan et al. 2002, JGR, 107(D17), 4329. doi: 10.1029/2001JD001131. - Temperature lapse rate (Gaffen et al 2000, Science, 287, 1242) - Tropical Cloudiness (Wielicki et al, 2002, Science, 295, 841)

24. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 Clear-sky sampling: interannual variability Light blue: Type I (weighted by clear-sky fraction) Dark Blue: Type II (unweighted mean)

25. WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004 OLR sensitivity to RH