[email protected]© university of reading 2007rpa present day changes in tropical...

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

Present day changes in tropical precipitation extremes

in models and observations

Richard P. Allan

Environmental Systems Science Centre, University of Reading

With thanks to:

Brian Soden (RSMAS, University of Miami)

Viju John (Hadley Centre)


Climate Impacts How the hydrological cycle responds to global warming is crucial for society (e.g. water supply, agriculture, severe weather)

Motivation


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


How does clear-sky radiative cooling respond to warming?

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

Clear-sky net cooling increases at ~3 Wm-2K-1


AMIP3

CMIP3 non-volcanic

CMIP3 volcanic

Reanalyses/ Observations

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


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

See Lambert and Webb (2008)

CMIP3 MODELS: Tropical oceans


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; Lambert and Webb, 2008

Lambert and Webb (2008) submitted


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

(e.g. Soden et al. 2005, Science)

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

1979-2002


Held and Soden (2006) J. Clim

∆P

(%

)

“heavy rain”: ~7 % K-1

∆T (K)

Mean: ~2 % K-1

“light rain”: –XX % K-1

7 % K

-1

Contrasting precipitation response expected


Changes in precipitation: “the rich get richer”?

precip trends

0-30oN

Rainy season: wetter

Dry season: drier

Chou et al. 2007 GRL


IPCC 2007 WGI


• Method: Analyse separately precipitation over the wet ascending and dry 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

Is this contrasting precipitation response borne out by observations?


GPCP CMAP

AMIP3

• Model precipitation response smaller than the satellite observations

see also discussion in: Wentz et al. (2007) Science,Yu and Weller (2007) BAMS,Roderick et al. (2007) GRL,Chou et al. (2007) GRL,Zhang et al. (2007) NatureTrenberth and Dai (2007) GRLLambert and Webb (2008)

Tropical Precipitation ResponseAllan and Soden, 2007, GRL


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

OCEAN LAND

AMIP SSM/I GPCP CMAP


Projected changes in Tropical Precipitation

Allan and Soden, 2007, GRL


Sensitivity to SST (top) and CWV (bottom): more consistent

with models

AMIP3

CMIP3 non-volcanicCMIP3 volcanic

Reanalyses/ Observations

Tropical ocean ascent


Are observed trends sensitive to instrument/ algorithm?(Viju John)

Tropical ocean ascent


Can precipitation response to ENSO be used as a test of model sensitivity?


Histograms of the frequency of precipitation in bins of intensity (e.g. 0-10%, …, 80-90%, 90-95%, 99-100%).

Test model precipitation response to ENSO (+B.Soden)

Changes in tropical precipitation frequency


• Based on response to warming during ENSO, models:– Underestimate increases in frequency of heaviest precipitation– Produce spurious decrease in frequency of moderate

precipitation and increase frequency in lightest rainfall


Satellite data suggests that mean precipitation and evaporation changes appear to be closer to Clausius Clapeyron (7%/K), larger than the model estimates

Yu and Weller (2007) BAMS

(Wentz et al. 2007, Science)This appears to require super-Clausius Clapeyron changes in moist-region precipitation?


• Vecchi and Soden (2006) Nature

• Evidence for weakening of Walker circulation in models and observations


Could decadal changes in aerosol have short-circuited the global water cycle through direct and indirect effect on cloud radiation?

Mishchenko et al. (2007) Science

Also: Liepert and Prevedi (2008) submitted to J Clim


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


Precipitation also linked to clear-sky longwave radiative cooling of the atmosphere


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)


Global precipitation (P) changes constrained by atmospheric net radiative cooling (Q)

• Changes in Q expected to be ~3 Wm-2K-1 (e.g. Allen and Ingram, 2002)

• If so, changes in P with warming ≈3%K-1

• …substantially lower than changes in moisture (~7%K-1)


Global precipitation (P) changes constrained by atmospheric net radiative cooling (Q)

• Changes in Q expected to be ~3 Wm-2K-1 (e.g. Allen and Ingram, 2002)

• If so, changes in P with warming ≈3%K-1

• But convective rainfall supplied by moisture convergence which increases at rate ~7%K-1

e.g. Allen and Ingram (2002) Nature; Trenberth et al. (2003) BAMS


Are the results sensitive to the reanalysis data?

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


Tropical ocean variabilitySST

Water vapour

Clear LW net down at surface


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)


Linear fit

dSNLc/dTs ~ 3.5±1.5 Wm-2K-1

dCWV/dTs ~ 3.0±1.0 mm K-1

CMIP3 non-volcanic CMIP3 volcanic

Reanalyses/ Obs AMIP3

Models, reanalyses and observations show increased surface net downward longwave with warming due to increased water vapour


ERA40 NCEP-1 AMIP ensemble

ERBS/ScaRaB/CERES GISS_E_R volcanic ensemble

Clear-sky outgoing longwave radiation (Wm-2)


Clear-sky atmospheric longwave cooling

Precipitation─ SSM/I AMIP3 GISSvolc

─ OBS ─ ERA40 --- NCEP

Radiative cooling/Latent heating


Summary• Global water and energy cycles coupled• Satellite data and models agree on rate of moisture

increase with temperature (~7%/K) increased radiative cooling of atmosphere to the surface• Theoretical changes in clear-sky radiative cooling of

atmosphere implies “muted” precipitation response• Models simulate muted response, observations show

larger response• Models severely underestimate precipitation response in

ascending and descending branches of tropical circulation– Possible artifacts of data? – Implications for climate change prediction


Extra slides…


But water vapour is rising at a faster rate (~7%/K)

Convective rainfall draws in moisture from surroundings


Increase in clear-sky longwave radiative cooling to the surface

CMIP3

CMIP3 volcanic

NCEP ERA40

SSM/I-derived~ +0.7 Wm-2 decade-1

∆SNLc (Wm-2)


Links to precipitation


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


Microwave estimates of precipitation and evaporation over the ocean appear to be closer to Clausius Clapeyron (7%/K), larger than the model estimates (Wentz et al. 2007, Science)


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)

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