implications of trends and variability in low-level water vapour richard p. allan department of...
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Implications of trends and variability in low-level water vapour
Richard P. AllanDepartment of Meteorology/NCAS climate, University of Reading
Thanks to: Brian Soden, Viju John, William Ingram, Peter Good, Igor Zveryaev and Mark Ringer
http://www.met.reading.ac.uk/~sgs02rpa [email protected]
Implications of increasing low-level water vapour
• Water vapour feedback– Small but positive contribution from low-level water vapour
• LW continuum• SW absorption
• Hydrological cycle feedback– Extreme precipitation (surface specific humidity)
– Global mean precipitation/evaporation (radiative constraint, boundary layer adjustment)
– Contrasting wet/dry response (moisture transport)
– Circulation
Clausius Clapeyron
• Strong constraint upon low-altitude water vapour over the oceans
• Land regions?
Global changes in water vapour
Updated from O’Gorman et al. (2012) submitted; see also John et al. (2009) GRL
Declining RH over land?
Simmons et al. (2010) JGR
Land T
global T
Land RH
• Stalling of ocean temperatures in 2000s
• Continued warming of land• Reduced relative humidity
over land?
Radiative transfer
• Surface net longwave radiation determined by low level water vapour (self-) continuum absorption
Surface longwave radiation
Downward
Upward
Surface net longwave and water vapour
Allan (2009) J . Climate
ER
A40
N
CE
P
SR
B
SS
M/I
• Surface net longwave strongly dependent on column water vapour
• Increased water vapour enhances ability of atmosphere to cool to the surface
tropical oceans
NCAS-Climate Talk 15th January 2010
Rad
iativ
e co
olin
g, c
lear
(W
m-2K
-1)
Allan (2009) J. Clim
Models simulate robust response of clear-sky radiation to warming (~2 Wm-2K-1) and a resulting increase in precipitation to balance (~2 %K-1)
e.g. Allen and Ingram (2002) Nature, Lambert and Webb (2008) GRL; Stephens & Ellis (2008) J. Clim
Feedback on atmospheric
radiative cooling
see also O’Gorman et al. (2012) Survey. Geophys. submitted
+–• radiative cooling
due to water vapour increases (fixed RH)
• increased water vapour at lowest levels enhances radiative cooling to the surface
Previdi (2010) Environ. Res. Lett.
NCAS-Climate Talk 15th January 2010
CC Wind Ts-To RHo
Muted Evaporation changes in models are explained by small changes in Boundary Layer:1) declining wind stress2) reduced surface temperature lapse rate (Ts-To)3) increased surface relative humidity (RHo)
Richter and Xie (2008) JGR
Evaporation
Changes in net atmospheric radiative cooling and precipitation
Updated from O’Gorman et al. (2012) submitted; see also John et al. (2009) GRL
AMSRE
Extreme Precipitation
1979-2002• Clausius-Clapeyron constraint– e.g.Trenberth et al. (2003) BAMS; Pall et al.
(2007) Clim Dyn
• Changes in intense rainfall also constrained by moist adiabat– O’Gorman and Schneider (2009) PNAS
• Low-level water vapour constraint• Does extra latent heat release within
storms enhance rainfall intensity above Clausius Clapeyron?– e.g. Lenderink and van Meijgaard (2010)
Environ. Res. Lett.; Haerter et al. (2010) GRL
Changes in Extreme Precipitation Determined by changes in low-level water vapour and updraft velocity
Above: O’Gorman & Schneider (2008) J Clim
Aqua planet experiment shows extreme precipitation rises with surface q, a lower rate than column water vapour
Right: Gastineau and Soden (2009) GRL Reduced frequency of upward motion
offsets extreme precipitation increases.
Increases in the frequency of the heaviest rainfall with warming: daily data from models and microwave satellite data (SSM/I)
Reduced frequency Increased frequencyAllan et al. (2010) Environ. Res. Lett.
• Increase in intense rainfall with tropical ocean warming• SSM/I satellite observations at upper range of substantial
model spread (see also O’Gorman and Schneider 2009 PNAS)
Turner and Slingo (2009) ASL: dependence on convection scheme?
Observational evidence of changes in intensity/duration (Zolina et al. 2010 GRL)
Links to physical mechanisms/relationships required (Haerter et al. 2010 GRL)
Contrasting precipitation response expected
Pre
cipi
tatio
n Heavy rain follows moisture (~7%/K)
Mean Precipitation linked to
radiation balance (~3%/K)
Light Precipitation (-?%/K)
Temperature e.g.Held & Soden (2006) J. Clim; Trenberth et al. (2003) BAMS; Allen & Ingram (2002) Nature
Contrasting precipitation response in wet and dry regions of the tropical circulation
Updated from Allan et al. (2010) Environ. Res. Lett.
descent
ascentModelsObservations
Pre
cipi
tatio
n ch
ange
(%
)
Sensitivity to reanalysis dataset used to define wet/dry regions
Implications for moisture transport and
P-E patterns
Projected (top) and estimated (bottom)
changes in Precipitation minus Evaporation d(P-E)
Held and Soden (2006) J Climate
See also Muller & O’Gorman (2011) NCC
~
First argument:P ~ Mq.
So if P constrained to rise more slowly than q, this implies reduced M
Second argument:ω=Q/σ.
Subsidence (ω) induced by radiative cooling (Q) but the magnitude of ω depends on (Гd-Г) or static stability (σ).
If Г follows MALR increased σ. This offsets Q effect on ω.See Held & Soden (2006) and Zelinka & Hartmann (2010) JGR
P~Mq
Tropical Circulation
Models/observations achieve muted precipitation response by reducing strength of Walker circulation. Vecchi and Soden (2006) Nature; see also Gastineau & Soden (2011) GRL
P~Mq
Tropical Circulation
Moisture transports from ERA Interim
• Moisture transport into tropical ascent region
• Use ERA Interim
• Significant mid-level outflow
Zahn and Allan (2011) JGR; see also Sohn and Park (2010) JGR
Instantaneous field
• Low-level water vapour– Powerful Clausius Clapeyron constraint over ocean– Agreement between ground-based and satellite observations– Ocean source of land moisture (e.g. Gimeno et al. 2011 GRL)– Decadal variation in ocean/land temperature and relative
humidity over land? (e.g. Simmons et al. 2010 JGR)
• Radiative cooling and Precipitation– radiative impact of temperature and water vapour increases
changes in mean precipitation and evaporation– Low level water constrains intense precipitation, but large
model uncertainty in the tropics (e.g. O’Gorman & Schneider)– Moisture budget constraint can explain contrasting wet/dry
tropical responses. (Held & Soden 2006 J Clim)
Conclusions
Outstanding issues• Decadal variability:
– surface temperature and relative humidity over land– Atmospheric circulation
• Observing system:– satellite sensors: retrieval, drift & calibration– surface radiation budget
• Changes in extreme precipitation in tropics are uncertain• Changes in boundary layer humidity:
– Importance for surface fluxes– Links to low-altitude cloud feedback?
Clausius-Clapeyron
Radiative transfer
Surface Evaporation
Atmospheric radiative cooling
Extreme Precipitation
Wet get wetter, dry get drier
Lower tropospheric
humidity
Water vapour
feedback
Cloud feedback?
Global Precipitation
Qr (K/day): 0 1 2
P (hPa)100 500
900Free/upper
tropospheric humidity
Water vapour open issues, needs/possibilities for coordinated efforts, upcoming satellite missions
• Moisture transports and atmospheric circulation• Water vapour reanalysis? Surface radiation budget?• Upper tropospheric water vapour datasets
– Retrieval/drifts/calibration• Links to cloud feedback
– High cloud and FAT/PHAT hypothesis– Low cloud and boundary layer processes
• Changes in relative humidity over land• Low level water vapour and extreme precipitation
events: dynamics and thermodynamics
• Evidence for recent increased strength of tropical Hadley/Walker circulation since 1979?– Sohn and Park (2010) JGR
Walker circulation index (top) and sea level pressure anomalies (bottom) over equatorial Pacific (1948-2007)
Hadley circulation index over 15oS-30oN band
Is the mean state important?
• Models appear to overestimate water vapour– Pierce et al. (2006) GRL;
John and Soden (2006) GRL– But not for microwave data?
[Brogniez and Pierrehumbert (2007) GRL]
• This does not appear to affect feedback strength– John and Soden (2006)
• What about the hydrological cycle?– Symptomatic of inaccurate
simulation?
Pierce et al. (2006)
GRL
Does low-level moisture rise at 7%/K?Specific humidity trend correlation (left) and time series (right)
Willett et al. (2007) Nature
Robust relationships globally.
Less coherent relationships regionally/over land/at higher altitudes?
Evidence for reductions in RH over land (Simmons et al. 2009 JGR) which are physically plausible.
Land OceanWillett et al. (2008) J Clim
Links to radiative cooling
• 7-8%/K rises in global column moisture• Associated with rises in clear-sky longwave
radiative cooling
Current trends in wet/dry regions of tropical oceans
• Wet/dry trends remain– 1979-1987 GPCP
record may be suspect for dry region
– SSM/I dry region record: inhomogeneity 2000/01?
• GPCP trends 1988-2008
– Wet: 1.8%/decade– Dry: -2.6%/decade– Upper range of model
trend magnitudes
Models
DR
Y
WE
T
Allan et al. (2010) Environ. Res. Lett.