ESSC Seminar, October 18, 2007© University of Reading 2007 Intensification of the tropical hydrological cycle? Richard P. Allan Environmental

ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.uk

Intensification of the tropical hydrological cycle?

Richard P. Allan

Environmental Systems Science Centre, University of Reading, UK

Thanks to Brian Soden


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

Motivation


IPCC 2007 WGI


Introduction

• Hydrological cycle linked to radiative energy balance

• How will global water cycle respond to warming?

• Method: compare observations and reanalysis products with model simulations of the present day climate over the period 1979-2006


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


Changing character of precipitation

Convective rainfall draws in moisture from surroundings


Changing character of precipitation

• Moisture is observed & predicted to increase with warming ~7%K-1

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

• Thus convective rainfall also expected to increase at this rate (e.g. Trenberth et al. 2003 BAMS)

1979-2002


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


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)


Models also display a “muted” precipitation response (~1-3%K-1)

Held and Soden (2006) J. Clim

∆P

(%

) 7 % K

-1

∆T (K)


Held and Soden (2006) J. Clim

∆P

(%

)

“heavy rain”: ~7 % K-1

∆T (K)

Mean: ~2 % K-1

“light rain”: -? % K-1


But recent results suggest that the “muted” precipitation response is not found in the observations

Is the global water cycle/radiation budget not captured by models?

Wentz et al. (2007) Science


Are model simulated changes in clear-sky radiative cooling robust?

σT4 εAσT4

↓OLR

Ts

T, H2O, GHG ? Cloud

? Aerosol


How does atmospheric net radiative cooling respond to warming?

• Sensitivity test (Edwards/Slingo):

• Assume no change in clouds, aerosol, ozone

(1) Two warming scenarios (C1, C2)

(2) Greenhouse gas increase (1980-99)

(3) Shortwave absorption case


• Sensitivity test (Edwards/Slingo):

• Assume no change in clouds, aerosol, ozone

(1) Two warming scenarios (C1, C2)

(2) Greenhouse gas increase (1980-99)

(3) Shortwave absorption case

How does atmospheric net radiative cooling respond to warming?


Increased moisture enhances atmospheric radiative cooling to surface

ERA40 NCEP

Allan (2006) JGR 111, D22105

dSNLc/dCWV ~ 1 ─ 1.5 W kg-1

SNLc = clear-sky surface net down longwave radiation

CWV = column integrated water vapour


What about in observations, reanalyses and models?

• Method: Compare monthly mean changes in temperature, water vapour and clear-sky longwave radiation, 1979-2006.

• Estimate water vapour driven changes in surface radiation using satellite and climatological ocean data, and an empirical model (Prata 1996)


Tropical ocean variability

SST

Water vapour

Clear net LW down at surface


Increase in clear-sky longwave radiative cooling to the surface

CMIP3

CMIP3 volcanic

NCEP ERA40

SSM/I-derived

~ +1 Wm-2 per decade

∆SNLc (Wm-2)


Tropical Oceans

dCWV/dTs ~2 ─ 4 mm K-1

dSNLc/dTs ~3 ─ 5 Wm-2K-1

AMIP3

CMIP3 non-volcanic

CMIP3 volcanic

Reanalyses/ Observations


Tropical ocean variability

LWc: TOA

LWc: ATM

Precip


AMIP3

CMIP3 non-volcanic

CMIP3 volcanic

Reanalyses/ Observations

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


• 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

RECAP…


• 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

• Model precipitation response smaller than the satellite observations– see also Wentz et

al. (2007) Science

Tropical Precipitation Response

Allan 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


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


Questions

(1) Is the analysis flawed?

(2) Are the observed changes physically plausible? Are the observations wrong?

(3) Can decadal changes in cloud and aerosol explain the discrepancy?

(4) Are the models missing fundamental physics?


(1) Is the analysis flawed?

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


(2a) Are the observations wrong?

• Many of the global precipitation datasets use satellite data– Changes in the observing system– Potential algorithm errors sensitive to temperature

and/or water vapour amount

• Evidence from land and ocean observations suggest models underestimate precipitation and evaporation response to warming– e.g. Wentz et al. 2007 Science; Zhang et al. 2007

Nature, Chou et al. 2007 GRL, …


Zhang et al. 2007 Nature


Surface Evaporation and wind speed

• Surface evaporation depends primarily upon– Wind stress– Surface humidity gradient (expected to change due to

Clausius Clapeyron equation ~7%/K)

• Globally, evaporation must equal precipitation– Therefore a muted precipitation response requires a

muted evaporation response (e.g. muted wind stress)



Changes in Evaporation over ocean

• Observed increases in surface evaporation • larger than climate model simulations; caused by

– increased surface humidity gradient (Clausius Clapeyron) – Little trend in wind stress changes over ocean (Yu and Weller,

2007; Wentz et al., 2007) but some evidence over land (Roderick et al. 2007 GRL)

Yu and Weller (2007) BAMS


(2b) Are the changes plausible?• Increases in tropical precipitation and

evaporation at ~7%/K plausible• But increases in ascent region precip ~ 30%K-1,

much larger than Clausius Clapeyron• Implies intensification of hydrological cycle

• At odds with observations and model simulations of a reduction in Walker circulation (Vecchi and Soden, 2007)


• Vecchi and Soden (2006) Nature

• Evidence for weakening of Walker circulation in models and observations


• Vecchi and Soden (2006) Nature

• Evidence for weakening of Walker circulation in models and observations


(3) What else could explain this apparent discrepancy?

• Changes in land/ocean tropics/extra-tropical transport of heat and moisture?

• Decadal variability in radiation balance and hydrological cycle?– Changes in clouds– Changes in aerosol

ESSC Seminar, October 18, 2007 © University of Reading 2007 www.reading.ac.ukWong et al. 2006 J. Climate, Wielicki et al. 2002, Science

Climate models appear to underestimate variability in radiation budget

Does this relate to clouds and/or aerosol?


Global dimming to Global BrighteningStanhill and Cohen EOS (below), Wild et al., Pinker et al. 2005 Science


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

See Mishchenko et al. (2007) Science


Aerosol Hypothesis?• Could changes in aerosol directly and indirectly (through

cloud) alter the radiation balance and precipitation?

↓ Aerosol




↓ Aerosol




↓ Aerosol

Rad cooling

Solar heating




↓ Aerosol

Rad cooling Precip

Solar, Evap Circulation?


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


References

• Allen and Ingram (2002) Nature

• Trenberth et al. (2003) BAMS

• Wentz et al. (2007) Science

• My meagre contributions:– Allan (2006) JGR– Allan and Soden (2007) GRL


Conclusions• Heavy rainfall and areas affected by drought expected to

increase with warming [IPCC 2007]• Heavy precipitation increases with moisture ~7%K-1

• Mean Precipitation constrained by radiative cooling– Models simulate increases in moisture (~7%K-1) and clear-sky LW

radiative cooling (3-5 Wm-2K-1)

• But large discrepancy between observed and simulated precipitation responses…– Model inadequacies or satellite calibration/algorithm problems?– Changes in evaporation and wind-speed over ocean at odds with

models? (Yu and Weller, 2007 BAMS; Wentz et al. 2007, Science; Roderick et al. 2007 GRL)

• Observing systems: capturing decadal variability problematic


Implications for tropical precipitation (GPCP)?

ERA40 QLWc

GPCP P

OBS QLWc

Pinatubo?


Comparison of AMIP3 models, reanalyses and observations over the tropical coeans


Also considering coupled model experiments including greenhouse gas and natural forcings


Clear-sky vs resolution


Sensitivity study

• Based on GERB- SEVIRI OLR and cloud products over ocean:

• dOLRc/dRes ~0.2 Wm-2km-0.5

• Suggest CERES should be biased low

by ~0.5 Wm-2 relative to ERBS


Links to precipitation

Documents

ESSC Seminar, October 18, 2007© University of Reading 2007 Intensification of the tropical hydrological cycle? Richard P. Allan Environmental