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

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 r.p.allan@reading.ac.uk

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.