WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

Decadal Variability in Water Vapour

Richard Allan, Tony SlingoEnvironmental Systems Science Centre, University of Reading

Mark RingerHadley Centre, Met Office

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?

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)

WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

Climate sensitivity

Ts=Q,

=-1/(BB +WV ++Cld+….),

WV~-(OLR/wv)(dwv/dTs)

WV Cld

Theory, Measurement

Observations

WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

OLR Sensitivity to Water Vapour

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

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?

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

WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

Upper tropospheric moisture

Clear-sky OLR sensitive to Ts and RH 6.7 m cloud cleared radiance sensitive to

upper tropospheric Relative Humidity

Explicitly simulate 6.7 m radiance in HadAM3

Modified “satellite-like” clear-sky diagnostics

WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

Sensitivity of OLRc to UTH

WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

Sensitivity of OLRc to UTH

WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

Interannual monthly anomalies: tropical oceans

ga=1-(OLRc/Ts4)

(Allan et al. 2003, QJ, p.3371)

WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

(+additional forcings)

(Allan et al. 2003, QJ, p.3371)

WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

Large changes in OLR from 7 independent satellite instruments (Wielicki et al, 2002)

WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

Allan & Slingo 2002, GRL, 29(7)

OLR

Clear-sky OLR

RSW

+Altitude Correction?

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

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

WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

Summary (2) Simulations of satellite brightness

temperatures: Consistent decadal variability suggests small RH 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)

WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

EOF analysis of spatio-temporal variability in water vapour radiance

WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

Climatological mean over 60oS-60oN oceans

WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

Info on upper tropospheric water vapour

T6.7

500mb omega

Clear-sky OLR

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)

WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

Is water vapour feedback really consistent between models?

dOLRc/dTs ~ 2 Wm-2K-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)

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)

WGCM CFMIP/IPCC Climate Sensitivity Meeting, Exeter, April 2004

OLR sensitivity to RH