IPCC Workshop on Climate Sensitivity, Paris, July 2004 Exploiting observations of water vapour to investigate simulations of water vapour feedback processes Richard Allan, Tony Slingo Environmental Systems Science Centre, University of Reading Mark Ringer Hadley Centre, Met Office

IPCC Workshop on Climate Sensitivity, Paris, July 2004 INTRODUCTION – How does water (vapour/liquid/ice) respond to global warming? » Cruicial for Climate Sensitivity – Direct feedback: water vapour »Fundamental test of models »Can we determine water vapour feedback from observations of present day climate? »Can we use reanalyses? »do results link into present day changes in cloudiness?

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Previous studies Observational determination of water vapour feedback (e.g. Raval and Ramanathan 1989; Cess 1989; Soden et al. 2002; Forster & Collins 2004) Theoretical/ Modelling studies (e.g. Manabe and Wetherald 1967; Held and Soden 2000; Ingram 2002, Minschwaner and Dessler 2004)

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Climate sensitivity Ts~ Q, =-1/( BB + WV + + Cld +….), WV ~-( d OLR/ wv)( wv/ Ts) WV Cld Theory, Measurement Observations

IPCC Workshop on Climate Sensitivity, Paris, July 2004 dOLRc/dwv theoretical calculations

IPCC Workshop on Climate Sensitivity, Paris, July 2004 OLR Sensitivity to Water Vapour

IPCC Workshop on Climate Sensitivity, Paris, July 2004 dwv/dTs Theoretical basis (Clausius Clapeyron) –dwv/dTs ~ %K -1 (Wentz&Schabel 2000) Can only measure dwv/dTs

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Interannual variability of Column Water vapour (Allan et al. 2003, QJRMS, p.3371) See also Soden (2000) J.Clim 13 SST CWV

IPCC Workshop on Climate Sensitivity, Paris, July 2004 CWV Sensitivity to SST dCWV/dTs = 3.5 kgm -2 K -1 for HadAM3 and Satellite Microwave Observations (SMMR, SSM/I) over tropical oceans Corresponds to ~9%K -1 in agreement with Wentz & Schabel (2000) who analysed observed trends But what about moisture away from the marine Boundary Layer?

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Can we use reanalyses? Reanalyses are currently unsuitable for detection of subtle trends associated with water vapour feedbacks Allan et al. 2004, JGR, accepted

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Observations of clear-sky OLR and the greenhouse parameter Clear-sky OLR sensitive to Ts and RH dOLR c /dT s as proxy for water vapour feedback e.g. Cess et al. (1990) – agreement between climate model dOLRc/dTs

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Observed and modelled changes in OLRc: ENSO Soden (2000): ERBS vs AMIP multi model ensemble –Tropics, AMIP I dOLRc (Wm -2 )

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Interannual monthly anomalies: tropical oceans HadAM3 vs ERBS, ScaRaB and CERES dg a /dT s ~ 4 x K -1 Raval and Ramanathan (1989) get ~3.4x10 -3 K -1 spatially g a =1-(OLRc/ Ts 4 ) (Allan et al. 2003, QJRMS, p.3371)

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Does dOLRc/dTs indicate consistent water vapour feedback? Consider GFDL & HadAM3 AMIP experiments Interannual variability dOLR c /dT s ~ 2 Wm -2 K -1 BUT: differing height dependent Temperature and water vapour response –(Allan, Ramaswamy, Slingo, JGR 2002)

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Does dOLRc/dTs~2 Wm -2 K -1 indicate consistent water vapour feedback? Allan et al. 2002, JGR, 107(D17), HadAM3 GFDL dTa(p)/dTsdq(p)/dTs HadAM3 GFDL

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Water vapour / T-lapse rate Compensation between water vapour and temperature lapse rate feedback in models –e.g. Colman (2003) Sensitivity to convective parametrizations?

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Relative Humidity Feedback (include main component of WV into BB ?) Ts~ Q, =-1/( BB + RH + Cld +….), RH ~-( d OLR/ RH)( wv/ RH) BB : constant RH, Theory, Measurement Observations

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Evaluation of upper tropospheric humidity feedback in HadAM3 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

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Sensitivity of OLRc to UTH

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Interannual monthly anomalies of 6.7 micron radiance: HadAM3 vs HIRS (tropical oceans) (Allan et al. 2003, QJRMS, p.3371) Small changes in T_6.7 (or RH) in model and obs (dUTH/dTs ~ 0 ?)

IPCC Workshop on Climate Sensitivity, Paris, July 2004 (+additional forcings) (Allan et al. 2003, QJRMS, p.3371)

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Small changes in RH but apparently larger changes in tropical cloudiness? (Wielicki et al, 2002)

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Following: Wielicki et al. (2002); Allan & Slingo (2002) +Altitude and orbit corrections (40S-40N) Clear LW LW SW

IPCC Workshop on Climate Sensitivity, Paris, July 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

IPCC Workshop on Climate Sensitivity, Paris, July 2004 Summary reanalyses not yet suitable for analysis of climate sensitivity Climate model captures: –low-level water vapour changes »Sensitivity: dCWV/dTs~3.5 kgm -2 K -1 –Decadal variation in clear-sky OLR »Sensitivity: dOLRc/dTs~2 Wm -2 K -1 –Small decadal changes in free-tropospheric RH »Sensitivity: dOLRc/dRH ~ – 0.5 to –1 Wm -2 % -1 »dRH/dTs ~ 0 % K -1 –But satellite data suggests larger variation in radiation budget due to cloud compared to models