Exploiting%20observations%20of%20water%20vapour%20to%20investigate%20simulations%20of%20water%20vapour%20feedback%20processes%20Richard%20Allan,%20Tony%20Slingo%20Environmental%20Systems%20Science%20Centre,%20University%20of%20Reading%20%20%20Mark%20Ringer%20Hadley%20Centre,%20Met%20Office

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Exploiting observations of water vapour to
investigate simulations of water vapour feedback
processesRichard Allan, Tony SlingoEnvironmenta
l Systems Science Centre, University of
ReadingMark RingerHadley Centre, Met Office
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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?

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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)
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Climate sensitivity
DTsl DQ, l-1/(bBB bWV bGbCld.), bWV-(dOLR/
dwv)(dwv/dTs) bG ?? bWV ??bCld
Theory, Measurement
Observations
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dOLRc/dwvtheoretical calculations
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OLR Sensitivity to Water Vapour
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dwv/dTs

• Theoretical basis (Clausius Clapeyron)
• dwv/dTs 6 - 7.5K-1 (WentzSchabel 2000)
• Can only measure dwv/dTs

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Interannual variability of Column Water vapour
(Allan et al. 2003, QJRMS, p.3371)
SST
CWV
1980 1985
1990 1995
See also Soden (2000) J.Clim 13
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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 9K-1 in agreement with Wentz
  Schabel (2000) who analysed observed trends
• But what about moisture away from the marine
  Boundary Layer?

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Can we use reanalyses?
Reanalyses are currently unsuitable for detection
of subtle trends associated with water vapour
feedbacks Allan et al. 2004, JGR, accepted
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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

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Observed and modelled changes in OLRc ENSO

• Soden (2000) ERBS vs AMIP multi model ensemble
• Tropics, AMIP I

2 0 -2
dOLRc (Wm-2)
1983 1985 1987 1989
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Interannual monthly anomalies tropical
oceansHadAM3 vs ERBS, ScaRaB and CERESdga/dTs
4 x 10-3 K-1 Raval and Ramanathan (1989) get
3.4x10-3 K-1 spatially
ga1-(OLRc/sTs4)
1980 1985 1990 1995
(Allan et al. 2003, QJRMS, p.3371)
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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)

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Does dOLRc/dTs2 Wm-2 K-1 indicate consistent
water vapour feedback?
HadAM3
GFDL
HadAM3
GFDL
dTa(p)/dTs dq(p)/dTs
Allan et al. 2002, JGR, 107(D17), 4329.
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Water vapour / T-lapse rate

• Compensation between water vapour and temperature
  lapse rate feedback in models
• e.g. Colman (2003)
• Sensitivity to convective parametrizations?

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Relative Humidity Feedback(include main
component of bWV into bBB ?)
DTsl DQ, l-1/(bBB bRH bCld.), bRH-(dOLR/dRH
)(dwv/dRH) bBB constant RH, G
Theory, Measurement
Observations
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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

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Sensitivity of OLRc to UTH
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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 ?)
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(additional forcings)
(Allan et al. 2003, QJRMS, p.3371)
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Small changes in RH but apparently larger changes
in tropical cloudiness? (Wielicki et al, 2002)
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Altitude and orbit corrections (40S-40N)
Clear LW
LW
SW
Following Wielicki et al.
(2002) Allan Slingo (2002)
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- 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
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Summary

• reanalyses not yet suitable for analysis of
  climate sensitivity
• Climate model captures
• low-level water vapour changes
• Sensitivity dCWV/dTs3.5 kgm-2 K-1
• Decadal variation in clear-sky OLR
• Sensitivity dOLRc/dTs2 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