[email protected]© University of Reading 2007rpa Monitoring satellite observations and model simulations of changes in the atmospheric

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

Monitoring satellite observations and model simulations of changes in the

atmospheric hydrological cycle since 1979

Richard P. Allan

Environmental Systems Science Centre, University of Reading, UK

Brian Soden

RSMAS, University of Miami, USA

Viju John

Mat Office, UK


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

Motivation


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)


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


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


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) GRL

Tropical Precipitation ResponseAllan and Soden, 2007, GRL


Projected changes in Tropical Precipitation

Allan and Soden, 2007, GRL


Are observed trends sensitive to instrument/ algorithm?

Tropical ocean ascent


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

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


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…


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)


AMIP3

CMIP3 non-volcanic

CMIP3 volcanic

Reanalyses/ Observations

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


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)

Documents

[email protected]© University of Reading 2007rpa Monitoring satellite observations and model simulations of changes in the atmospheric