Present day changes in tropical precipitation extremes in

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) r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Motivation Climate Impacts How the hydrological cycle responds to global warming is crucial for society (e. g. water supply, agriculture, severe weather) r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Earth’s energy balance r. p. allan@reading. ac. uk © University of Reading 2007 Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p. 94 www. nerc-essc. ac. uk/~rpa

Earth’s energy balance SW heating +67 Wm-2 r. p. allan@reading. ac. uk Precip: +78 Wm-2 LW cooling -169 Wm-2 Reading 2007 Kiehl and Trenberth, 1997; Also IPCC© University 2007 oftech. summary, p. 94 www. nerc-essc. ac. uk/~rpa

How does clear-sky radiative cooling respond to warming? 1 K increase in tropospheric T, constant RH Greenhouse gas changes from 1980 to 2000 assuming different rates of warming TOA SFC ATM Clear-sky Longwave ATM shortwave Clear-sky net cooling increases at ~3 Wm-2 K-1 r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Increase in atmospheric cooling over tropical ocean descent ~4 Wm-2 K-1 r. p. allan@reading. ac. uk AMIP 3 CMIP 3 volcanic CMIP 3 nonvolcanic Reanalyses/ Observations © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

CMIP 3 MODELS: Tropical oceans Increases in water vapour enhance clearsky 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) r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Models simulate robust response of clear-sky radiation to warming (~2 Wm-2 K-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 r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

• 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 r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Contrasting precipitation response expected “heavy rain”: ~7 % K-1 -1 K Mean: ~2 % K-1 ∆P (%) 7% “light rain”: –XX % K-1 ∆T (K) r. p. allan@reading. ac. uk Held and Soden (2006) J. Clim © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Changes in precipitation: “the rich get richer”? precip trends Chou et al. 2007 GRL 0 -30 o. N Rainy season: wetter Dry season: drier r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

IPCC 2007 WGI r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Is this contrasting precipitation response borne out by observations? • 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 r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Tropical Precipitation Response Allan and Soden, 2007, GRL • 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) Nature Trenberth and Dai (2007) GRL Lambert and Webb (2008) r. p. allan@reading. ac. uk GPCP CMAP AMIP 3 © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Tropical Subsidence regions d. P/dt ~ -0. 1 mm day-1 decade-1 OCEAN AMIP r. p. allan@reading. ac. uk LAND SSM/I GPCP © University of Reading 2007 CMAP www. nerc-essc. ac. uk/~rpa

Projected changes in Tropical Precipitation Allan and Soden, 2007, GRL r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Tropical ocean ascent Sensitivity to SST (top) and CWV (bottom): more consistent with models AMIP 3 CMIP 3 nonvolcanic CMIP 3 volcanic r. p. allan@reading. ac. uk © University of Reading 2007 Reanalyses/ www. nerc-essc. ac. uk/~rpa Observations

Are observed trends sensitive to instrument/ algorithm? (Viju John) Tropical ocean ascent r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Can precipitation response to ENSO be used as a test of model sensitivity? r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Changes in tropical precipitation frequency Histograms of the frequency of precipitation in bins of intensity (e. g. 0 -10%, …, 8090%, 90 -95%, 99 -100%). Test model precipitation response to ENSO (+B. Soden) r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

• 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 r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

• 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 r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

• 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 r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Satellite data suggests that mean precipitation and evaporation changes appear to be closer to Clausius Clapeyron (7%/K), larger than the model estimates (Wentz et al. 2007, Science) This appears to require super. Clausius Clapeyron changes in moistregion precipitation? Yu and Weller (2007) BAMS r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

• Vecchi and Soden (2006) Nature • Evidence for weakening of Walker circulation in models and observations r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

• Vecchi and Soden (2006) Nature • Evidence for weakening of Walker circulation in models and observations r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Mishchenko et al. (2007) Science Also: Liepert and Prevedi (2008) submitted to J Clim Could decadal changes in aerosol have short-circuited the global water cycle through direct and indirect effect on cloud radiation? r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

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 r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Extra slides r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Precipitation also linked to clear-sky longwave radiative cooling of the atmosphere r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Increased moisture enhances atmospheric radiative cooling to surface SNLc = clear-sky surface net down longwave radiation CWV = column integrated water vapour ERA 40 NCEP d. SNLc/d. CWV ~ 1 ─ 1. 5 W kg-1 d. CWV (mm) Allan (2006) JGR 111, D 22105 r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Global precipitation (P) changes constrained by atmospheric net radiative cooling (Q) • Changes in Q expected to be ~3 Wm-2 K-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) r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Global precipitation (P) changes constrained by atmospheric net radiative cooling (Q) • Changes in Q expected to be ~3 Wm-2 K-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 r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Are the results sensitive to the reanalysis data? • Changes in the reanalyses cannot explain the bulk of the trends in precipitation r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Tropical ocean variability SST Water vapour Clear LW net down at surface r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Increased moisture enhances atmospheric radiative cooling to surface SNLc = clear-sky surface net down longwave radiation CWV = column integrated water vapour ERA 40 NCEP d. SNLc/d. CWV ~ 1 ─ 1. 5 W kg-1 d. CWV (mm) Allan (2006) JGR 111, D 22105 r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Linear fit Models, reanalyses and observations show increased surface net downward longwave with warming due to increased water vapour d. CWV/d. Ts ~ 3. 0± 1. 0 mm K-1 d. SNLc/d. Ts ~ 3. 5± 1. 5 Wm-2 K-1 CMIP 3 non-volcanic r. p. allan@reading. ac. uk © University of Reading 2007 Reanalyses/ Obs CMIP 3 volcanic www. nerc-essc. ac. uk/~rpa AMIP 3

Clear-sky outgoing longwave radiation (Wm-2) ERA 40 NCEP-1 ERBS/Sca. Ra. B/CERES r. p. allan@reading. ac. uk AMIP ensemble GISS_E_R volcanic ensemble © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Clear-sky outgoing longwave radiation (Wm-2) ERA 40 NCEP-1 ERBS/Sca. Ra. B/CERES r. p. allan@reading. ac. uk AMIP ensemble GISS_E_R volcanic ensemble © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Radiative cooling/Latent heating ─ OBS ─ ERA 40 --- NCEP Clear-sky atmospheric longwave cooling ─ SSM/I r. p. allan@reading. ac. uk AMIP 3 GISSvolc © University of Reading 2007 Precipitation www. nerc-essc. ac. uk/~rpa

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 r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Extra slides… r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

But water vapour is rising at a faster rate (~7%/K) Convective rainfall draws in moisture from surroundings r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Increase in clear-sky longwave radiative cooling to the surface ∆SNLc (Wm-2) CMIP 3 volcanic NCEP ~ +0. 7 Wm-2 decade-1 r. p. allan@reading. ac. uk © University of Reading 2007 ERA 40 SSM/I-derived www. nerc-essc. ac. uk/~rpa

Links to precipitation r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

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 r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

Tropical Subsidence regions d. P/dt ~ -0. 1 mm day-1 decade-1 OCEAN AMIP r. p. allan@reading. ac. uk LAND SSM/I GPCP © University of Reading 2007 CMAP www. nerc-essc. ac. uk/~rpa

Are the results sensitive to the reanalysis data? • Changes in the reanalyses cannot explain the bulk of the trends in precipitation r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

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) r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa

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) r. p. allan@reading. ac. uk © University of Reading 2007 www. nerc-essc. ac. uk/~rpa