IPCC Workshop on Climate Sensitivity, Paris, July 2004
Exploiting observations of water vapour to investigate simulations of water vapour
feedback processes
Richard Allan, Tony SlingoEnvironmental Systems Science Centre, University of Reading
Mark RingerHadley 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~-(OLR/wv)(wv/Ts)
WV Cld
Theory, Measurement
Observations
IPCC Workshop on Climate Sensitivity, Paris, July 2004
OLRc/wv
theoretical calculations
IPCC Workshop on Climate Sensitivity, Paris, July 2004
OLR Sensitivity to Water Vapour
IPCC Workshop on Climate Sensitivity, Paris, July 2004
wv/Ts
Theoretical basis (Clausius Clapeyron) wv/Ts ~ 6 - 7.5%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)
1980 1985 1990 1995 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 feedbacksAllan 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 dOLRc/dTs 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 2
0
-2 1983 1985 1987 1989
dOLRc
(Wm-2)
IPCC Workshop on Climate Sensitivity, Paris, July 2004
Interannual monthly anomalies: tropical oceans
HadAM3 vs ERBS, ScaRaB and CERES
dga/dTs ~ 4 x 10-3 K-1 Raval and Ramanathan (1989) get ~3.4x10-3 K-1 spatially
ga=1-(OLRc/Ts4)
(Allan et al. 2003, QJRMS, p.3371)
1980 1985 1990 1995
IPCC Workshop on Climate Sensitivity, Paris, July 2004
Does dOLRc/dTs indicate consistent water vapour
feedback?
Consider GFDL & HadAM3 AMIP experiments
Interannual variability
dOLRc/dTs ~ 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), 4329.
HadAM3
GFDL
dTa(p)/dTs dq(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~-(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 m cloud cleared radiance sensitive to upper tropospheric Relative Humidity
Explicitly simulate 6.7 m 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 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
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