r.p.allan@reading.ac.uk© University of Reading 2009EUMETSAT 2009 1

Monitoring changes in precipitation and radiative energy using satellite data and

climate modelsRichard P. Allan - University of Reading

Thanks to Viju John and Brian Soden

r.p.allan@reading.ac.uk© University of Reading 2009EUMETSAT 2009 2

• Increased Precipitation• More Intense Rainfall• More droughts• Wet regions get wetter,

dry regions get drier

Precipitation Change (%)

CLIMATE MODEL PROJECTIONS IPCC WGI

Precipitation Intensity

Dry Days

r.p.allan@reading.ac.uk© University of Reading 2009EUMETSAT 2009 3

Low-level water vapour rises with temperature at ~7%/K in models & observations

John et al. (2009) GRL

models

Wat

er V

apou

r (m

m)

r.p.allan@reading.ac.uk© University of Reading 2009EUMETSAT 2009 4

1979-2002

For a given precipitation event, more moisture would suggest more intense rainfall

Can realism of model projections be assessed?

r.p.allan@reading.ac.uk© University of Reading 2009EUMETSAT 2009 5

Frequency of rainfall intensities vary with

SST in models and obs

• Frequency of intense rainfall increases with warming in models and satellite data

• Model scaling close to 7%/K expected from Clausius Clapeyron

• SSM/I satellite data suggest a greater response of intense rainfall to warming

dP/dSST=7%/K

Allan and Soden (2008) Science

r.p.allan@reading.ac.uk© University of Reading 2009EUMETSAT 2009 6 Trenberth et al. (2009) BAMS

r.p.allan@reading.ac.uk© University of Reading 2009EUMETSAT 2009 7

Models simulate robust response of clear-sky radiation to warming (~2-3 Wm-2K-1) and a resulting

increase in precipitation to balance (~3 %K-1) e.g. Allen and Ingram (2002) Nature, Lambert & Webb (2008) GRL

Rad

iativ

e co

olin

g, c

lear

(W

m-2K

-1)

Allan (2009) J Clim

r.p.allan@reading.ac.uk© University of Reading 2009EUMETSAT 2009 8

Contrasting precipitation response expected

Pre

cipi

tatio

n Heavy rain follows moisture (~7%/K)

Mean Precipitation linked to

radiation balance (~3%/K)

Light Precipitation (-?%/K)

Temperature

e.g.Held & Soden (2006) J. Clim; Trenberth et al. (2003) BAMS; Allen & Ingram (2002) Nature

r.p.allan@reading.ac.uk© University of Reading 2009EUMETSAT 2009 9

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

GPCP/NCEP Models

ascent

descent

Allan and Soden (2007) GRL

Pre

cipi

tatio

n ch

ange

(m

m/d

ay)

Allan and Soden (2007) GRL; John et al. (2009) GRL

r.p.allan@reading.ac.uk© University of Reading 2009EUMETSAT 2009 10

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

GPCP/NCEP Models

ascent

descent

Allan and Soden (2007) GRL

Pre

cipi

tatio

n ch

ange

(m

m/d

ay)

Land descent

Allan and Soden (2007) GRL; John et al. (2009) GRL

Updated analysis

r.p.allan@reading.ac.uk© University of Reading 2009EUMETSAT 2009 11

Estimated variability in longwave radiative cooling of the atmosphere and precipitation

John et al. (2009) GRL

r.p.allan@reading.ac.uk© University of Reading 2009EUMETSAT 2009 12

Allan et al. (2007) QJRMS

2008

Are the cloud feedback and water/energy cycles issues linked?

Combining GERB, CALIPSO and SSM/I

r.p.allan@reading.ac.uk© University of Reading 2009EUMETSAT 2009 13

Conclusions• Low level moisture responses robust

– Around 7%/K in models & SSM/I

• Explains enhanced frequency of intense rainfall with warming in models and obs– Differences between individual models/obs

• Mean and regional precipitation response: a tug of war – Slow rises in radiative cooling (~2-3 Wm-2K-1): robust in the models but

difficult to monitor using satellite data (CERES, GERB, SRB)– Rises in low-level moisture (~7%/K) faster than precipitation (~3%/K) – Reduced frequency? Rich get richer?– Who cares about drought/flooding over the ocean?

• 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 water/energy cycles?– New NERC project PREPARE with Met Office/ETH Zurich starting soon

r.p.allan@reading.ac.uk© University of Reading 2009EUMETSAT 2009 14

Daily Satellite Microwave Observations over tropical ocean appear to confirm warmer months are associated with more frequent intense rainfall

Allan and Soden (2008) ScienceMore frequentLess frequent