© University of Reading [email protected] Hadley Centre Workshop: Improving predictions...

© University of Reading 2008 [email protected]

Hadley Centre Workshop: Improving predictions of the large-scale hydrological cycle

Agreement/discrepancies between models andobservations on the large scale hydrological cycle

Richard Allan

ESSC/National Centre for Earth Observation, University of Reading, UK

Thanks to:

Brian Soden

Viju John



• What do we want to predict? – P-E, P, P-events; large-scale, regional– For agriculture, health, infrastructure, etc

• Physical constraints– Thermodynamic, dynamic– Radiation, water vapour, surface conditions

• How can observations help?– Observing system; monitoring, variability





Surface Temperature (K)

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 & Ingram, 2002; Lambert & Webb (2008) GRL



• But moisture observed & predicted to increase at greater rate ~7%K-1

• Thus convective rainfall expected to increase at a faster rate than mean precipitation (e.g. Trenberth et al. 2003 BAMS)

1979-2002



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. see Held and Soden (2006) J. Clim



What do observations tell us?

Response of Moisture and Clear-sky Longwave Radiation to warming appears robust in models and observations



…despite inaccurate mean state

Left: Pierce et al. (2006) GRL; see also John and Soden (2006) GRL



Does 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 (right)?

See also Wang et al. (2008) GRL

Land OceanWillett et al. (2008) J Clim



Does extreme precipitation follow moisture & Clausius Clapeyron?- Intensification of heaviest rainfall with warming in obs/models

Allan and Soden (2008) Science



Large spread in response of heaviest precipitation to warming between models and satellite data.

Changes in extreme vertical motion may be key…



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, in prep: Reduced frequency of upward motion

offsets extreme precipitation increases.



How are Observed Mean Precipitation and Evaporation Changing?

Yu and Weller (2007) BAMS

(Wentz et al. 2007, Science)Observed responses appear larger than models and closer to Clausius Clapeyron



Muted Evaporation changes in models are not explained simply by declining wind stress

Richter and Xie (2008) JGR in press; also Lorenz et al. J Climate under review

Simulated changes in Evaporation are sensitive to small changes in the boundary layer

CC Wind Ts-To RHo



IPCC 2007 WGI

Do observations confirm contrasting precipitation responses in the wet and dry regions? : “the rich get richer” or the wet get wetter?

Rainy season: wetter

Dry season: drier Chou et al. (2007) GRL

Precip trends, 0-30oN



Contrasting precipitation response in ascending and descending portions of the tropical circulation

GPCP Models

ascent

descent

Allan and Soden (2007) GRL

Pre

cipi

tatio

n ch

ange

(m

m/d

ay)



TRMM

ASCENT ALL DESCENT

• Contrasting ascent/descent reponse robust• Magnitude of responses sensitive to time period/dataset

Viju John et al. in prep



Could changes in aerosol be driving recent changes in the hydrological cycle? e.g. Wild et al. (2008) GRL

Wielicki et al. (2002) Science; Wong et al. (2006) J. Clim; Loeb et al. (2007) J. Clim

Mishchenko et al. (2007) Science



Precipitation response depends upon forcing

Andrews et al. J Climate in press:



Conclusions• Low level moisture responses robust:

– Less clear over land. Inaccurate mean state?• Precipitation extremes linked to moisture

– Moisture response at lowest level? Changes in updraft velocity?– Differences between individual models and observations

• Mean and regional precipitation response: a tug of war – Slow rises in radiative cooling– Faster rises in low-level moisture – Wet get wetter and dry get drier– Who cares about drought/flooding over the ocean?

• Recent Precipitation Responses appear larger in observations than models.– Could aerosol be influencing decadal variability in the hydrological cycle?– Are observing systems up to monitoring changes in the water cycle?

• Understanding changes in near surface conditions may be important

• AGU Fall meeting: GC08 Dynamic and Thermodynamic Controls of the Global Water Cycle in the 20th and 21st Centuries, Thurs 18th Dec

• Future NERC projects• PREPARE project proposal



Extra Slides





• Vecchi and Soden (2006) Nature

• Evidence for weakening of Walker circulation in models and observations



How should precipitation respond to climate change?

Allen and Ingram (2002) Nature



Changes in precipitation: “the rich get richer”?

precip trends

0-30oN

Rainy season: wetter

Dry season: drier

Chou et al. (2007) GRL



• Water vapour – Instrumental in determining amount of warming and changes in

precipitation– Agreement: tropical ocean, UTH– Land? Moistening processes/cloud?

• Precipitation– How should it change– Extreme precip– Regional responses

• Evaporation• Sensible Heat• Aerosol



Unanswered questions

• How does UTH really respond to warming?• Do we understand the upper tropospheric moistening processes?• Is moisture really constrained by Clausius Clapeyron over land?• What time-scales do feedbacks operate on?• Apparent discrepancy between observed and simulated changes

in precipitation– Is the satellite data at fault?

– Are aerosol changes short-circuiting the hydrological cycle?

– Could model physics/resolution be inadequate?

• Could subtle changes in the boundary layer be coupled with decadal swings in the hydrological cycle?

• How do clouds respond to forcing and feedback including changes in water vapour?



Is moisture at higher levels constrained by Clausius Clapeyron?

Soden et al. (2005) Science

Moi

sten

ing

Trend in water vapour radiance channels: 1983-2004

Observations

Model

Constant RH model

Constant water vapour model



Soden et al. (2002) Science; Forster/Collins (2004) Clim Dyn; Harries/Futyan (2006) GRL

What time-scales do different processes operate on?



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– Held and Soden (2006),

John and Soden (2006)• What about the

hydrological cycle?– Inaccurate mean state?

Pierce et al. (2006)

GRL



Reduction in UTH with warming

Lindzen (1990) BAMS

Minschwaner et al. (2006) J Clim

Mitchell et al. (1987) QJRMS



Evaporation cannot explain moistening

John and Soden (2006) GRL; Luo and Rossow (2004)

350

250

180

120

90

63

45

30

g m-3



Interanual Variability:Response of water vapour & clear-sky LW radiation at the surface and TOA in models, reanalyses & observations

Water vapour

Surface clear LW Clear-sky OLR

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