richard p. allan environmental systems science centre, university of reading, uk

[email protected] © University of Reading 2007www.nerc-essc.ac.uk/~rpa

Observed and simulated changes in water vapour, precipitation and the clear-sky

longwave radiation budget of the surface and atmosphere

Richard P. Allan

Environmental Systems Science Centre, University of Reading, UK


Earth’s energy balance

Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p.94


Earth’s energy balance

Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p.94

Precip: +78 Wm-2

SW heating +67 Wm-2

LW cooling -169 Wm-2


Increased moisture enhances atmospheric radiative cooling to surface

ERA40 NCEP

Allan (2006) JGR 111, D22105

SNLc = clear-sky surface net down longwave radiation

CWV = column integrated water vapour

dSNLc/dCWV ~ 1 ─ 1.5 W kg-1

dCWV (mm)


Tropical ocean variability

SST

Water vapour

Clear net LW down at surface


Increases in water vapour enhance clear-sky longwave radiative cooling of atmosphere to the surface

This is offset by enhanced absorption of shortwave radiation by water vapour

Changes in greenhouse gases, aerosol and cloud alter this relationship…

Tropical oceans


Sensitivity test: tropical oceans

Clear-sky Longwave shortwave

TOA SFC ATM ATM

1K increase in tropospheric T, constant RH

Greenhouse gas changes from 1980 to 2000 assuming different rates of warming


Increase in atmospheric LW cooling over tropical ocean descent ~4 ─ 5 Wm-2K-1

AMIP3

CMIP3 non-volcanic

CMIP3 volcanic

Reanalyses/ Observations


• Increased moisture (~7%/K) increased convective

precipitation

• Increased radiative cooling smaller mean rise in

precipitation (~3%/K)

• Implies reduced precipitation away from convective regimes (less light rainfall?)

• Locally, mixed signal from the above


• Method: Analyse separately precipitation over the ascending and descending branches of the tropical circulation– Use reanalyses to sub-sample observed data– Employ widely used precipitation datasets– Compare with atmosphere-only and fully coupled

climate model simulations


GPCP CMAP AMIP3

Tropical Precipitation Response

Allan and Soden, 2007, GRL

Model precipitation response smaller than the satellite observations (see also Wentz et al. 2007 Science; Chou et al. 2007 GRL, etc)


Projected changes in Tropical Precipitation

Allan and Soden, 2007, GRL


Could changes in aerosol and their indirect effect on cloud be driving changes in the tropical hydrological

cycle through the surface radiation budget?

Mishchenko et al. (2007) Science; Wild et al. (2005) Science


Summary

• Global water and energy cycles coupled• Theoretical changes in clear-sky radiative

cooling of atmosphere implies “muted” precipitation response

• Models simulate muted response, observations show larger response

• Possible artifacts of data?• Possible mechanisms (aerosol, cloud)• Implications for climate change prediction


Extra slides…


Calculated trends

• Models understimate mean precipitation response by factor of ~2-3

• Models severely underestimate precip response in ascending and descending branches of tropical circulation


Tropical Subsidence regions dP/dt ~ -0.1 mm day-1 decade-1

OCEAN LAND

AMIP SSM/I GPCP CMAP


Are the results sensitive to the reanalysis data?

• Changes in the reanalyses cannot explain the bulk of the trends in precipitation


Observed increases in evaporation over ocean larger than climate model simulations

Yu and Weller (2007) BAMS

- increased surface humidity gradient (Clausius Clapeyron)

- little trend in wind stress changes over ocean (Yu and Weller, 2007; Wentz et al., 2007) although some evidence over land (Roderick et al. 2007 GRL)


Links to precipitation

richard p. allan environmental systems science centre, university of reading, uk

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