Changes in Water Vapour, Clear-sky Radiative Cooling and Precipitation

Richard P. Allan

Environmental Systems Science Centre, University of Reading, UK

Thanks to Brian Soden

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

Climate Impacts

Changing character of precipitation

• Convective rainfall draws in moisture from surroundings

• 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

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)

- Changes in P with warming estimated to be ~3%K-1

- Consistent with model estimates (~2%K-1)

Held and Soden (2006) J. Clim

∆P

(%

)

7 % K

-1

∆T (K)

Precipitation 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

dSNLc/dCWV ~ 1 ─ 1.5 W kg-1

SNLc = clear-sky surface net down longwave radiation

CWV = column integrated water vapour

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

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 in subsidence regions (less light rainfall?)

• Locally, mixed signal from the above• Method: Analyse separately precipitation over

the ascending and descending branches of the tropical circulation

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

Allan and Soden, 2007, GRL

Projected changes in Tropical Precipitation

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

Extra slides…

Outline• Clear-sky radiative cooling:

– radiative convective balance– atmospheric circulation

• Earth’s radiation budget– Understand clear-sky budget to

understand cloud radiative effect

• Method:– analyse relationship between water vapour,

clear-sky radiative cooling and precipitation– Satellite observations, reanalyses, climate

models (atmosphere-only/fully coupled)

Models reproduce observed increases in total column water

vapour

Tropical Oceans

1980 1985 1990 1995 2000 2005

Ts

CWV

LWc

SFC

ERA40

NCEP

SRB

HadISST

SMMR, SSM/I

Derived:SMMR, SSM/I, Prata)

Allan (2006) JGR 111, D22105

Clear-sky OLR with surface temperature: + ERBS, ScaRaB, CERES; SRB

Calibration or sampling?

Tropical Oceans

Surface Net LWc

Clear-sky OLR

Clear-sky Atmos LW cooling

QLWc

ERBS, ScaRaB, CERES

Derived

ERA40

NCEP

SRB

HadISST

Allan (2006) JGR 111, D22105

Linear least squares fit

• Tropical ocean: descending regime

• Dataset dQLWc/dTs Slope

• ERA-40 3.7±0.5 Wm-2K-1

• NCEP 4.2±0.3 Wm-2K-1

• SRB 3.6±0.5 Wm-2K-1

• OBS 4.6±0.5 Wm-2K-1

ERA40 NCEP

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