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Agreement/discrepancies between models and observations 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

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slide1

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

slide2
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
slide4

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

Surface Temperature (K)

slide5
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
Contrasting precipitation response expected

Heavy rain follows moisture (~7%/K)

Mean Precipitation linked to radiation balance (~3%/K)

Precipitation 

Light Precipitation (-?%/K)

Temperature 

e.g. see Held and Soden (2006) J. Clim

slide7

What do observations tell us?

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

slide8

…despite inaccurate mean state

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

does moisture rise at 7 k
Does moisture rise at 7%/K?

Specific humidity trend correlation (left) and time series (right)

Willett et al. (2008) J Clim

LandOcean

Willett et al. (2007) Nature

Robust relationships globally.

Less coherent relationships regionally/ over land (right)?

See also Wang et al. (2008) GRL

slide10
Does extreme precipitationfollow moisture & Clausius Clapeyron?- Intensification of heaviest rainfall with warming in obs/models

Allan and Soden (2008) Science

slide11

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

Changes in extreme vertical motion may be key…

slide12
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.

slide13

How are Observed Mean Precipitation and Evaporation Changing?

(Wentz et al. 2007, Science)

Observed responses appear larger than models and closer to Clausius Clapeyron

Yu and Weller (2007) BAMS

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

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

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

CC Wind Ts-To RHo

slide15

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

IPCC 2007 WGI

Precip trends, 0-30oN

Rainy season: wetter

Dry season: drier

Chou et al. (2007) GRL

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

ascent

Precipitation change (mm/day)

descent

GPCP

Models

Allan and Soden (2007) GRL

slide17
Contrasting ascent/descent reponse robust
  • Magnitude of responses sensitive to time period/dataset

ASCENT ALL DESCENT

TRMM

Viju John et al. in prep

slide18

Mishchenko et al. (2007) Science

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

precipitation response depends upon forcing
Precipitation response depends upon forcing

Andrews et al. J Climate in press:

conclusions
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
slide23

Vecchi and Soden (2006) Nature

  • Evidence for weakening of Walker circulation in models and observations
changes in precipitation the rich get richer
Changes in precipitation: “the rich get richer”?

precip trends

0-30oN

Rainy season: wetter

Dry season: drier

Chou et al. (2007) GRL

slide26
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
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
Is moisture at higher levels constrained by Clausius Clapeyron?

Trend in water vapour radiance channels: 1983-2004

Observations

Model

Moistening

Constant RH model

Constant water vapour model

Soden et al. (2005) Science

slide29

What time-scales do different processes operate on?

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

is the mean state important
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
Reduction in UTH with warming

Minschwaner et al. (2006) J Clim

Lindzen (1990) BAMS

Mitchell et al. (1987) QJRMS

evaporation cannot explain moistening
Evaporation cannot explain moistening

g m-3

350

250

180

120

90

63

45

30

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

slide33

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