Implications of trends and variability in low-level water vapour Richard P. Allan Department 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]
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Implications of trends and variability in low-level water vapour Richard P. Allan Department of Meteorology/NCAS climate, University of Reading Thanks.
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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
• 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