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

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

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

University of Reading 2007www.nerc-essc.ac.uk/~rpa 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 ~ W kg -1 dCWV (mm)

University of Reading 2007www.nerc-essc.ac.uk/~rpa Linear fit dSNLc/dTs ~ 3.5±1.5 Wm -2 K -1 dCWV/dTs ~ 3.0±1.0 mm K -1 CMIP3 non-volcanicCMIP3 volcanic Reanalyses/ ObsAMIP3 Models, reanalyses and observations show increased surface net downward longwave with warming due to increased water vapour

University of Reading 2007www.nerc-essc.ac.uk/~rpa ERA40 NCEP-1AMIP ensemble ERBS/ScaRaB/CERESGISS_E_R volcanic ensemble Clear-sky outgoing longwave radiation (Wm -2 )

University of Reading 2007www.nerc-essc.ac.uk/~rpa ERA40 NCEP-1AMIP ensemble ERBS/ScaRaB/CERESGISS_E_R volcanic ensemble Clear-sky outgoing longwave radiation (Wm -2 )

University of Reading 2007www.nerc-essc.ac.uk/~rpa Clear-sky atmospheric longwave cooling Precipitation SSM/I AMIP3 GISSvolc OBS ERA NCEP Radiative cooling/Latent heating

University of Reading 2007www.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

University of Reading 2007www.nerc-essc.ac.uk/~rpa 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) Nature Trenberth and Dai (2007) GRL Tropical Precipitation Response Allan and Soden, 2007, GRL

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

University of Reading 2007www.nerc-essc.ac.uk/~rpa Are observed trends sensitive to instrument/ algorithm? Tropical ocean ascent

University of Reading 2007www.nerc-essc.ac.uk/~rpa Histograms of the frequency of precipitation in bins of intensity (e.g. 0-10%, %, %, etc). Test model precipitation response to ENSO (+B.Soden) Changes in tropical precipitation frequency

University of Reading 2007www.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

University of Reading 2007www.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

University of Reading 2007www.nerc-essc.ac.uk/~rpa Extra slides…

University of Reading 2007www.nerc-essc.ac.uk/~rpa 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 )

University of Reading 2007www.nerc-essc.ac.uk/~rpa AMIP3 CMIP3 non- volcanic CMIP3 volcanic Reanalyses/ Observations Increase in atmospheric cooling over tropical ocean descent ~4 Wm -2 K -1

University of Reading 2007www.nerc-essc.ac.uk/~rpa Links to precipitation

University of Reading 2007www.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

University of Reading 2007www.nerc-essc.ac.uk/~rpa Tropical Subsidence regions dP/dt ~ -0.1 mm day -1 decade -1 OCEANLAND AMIP SSM/IGPCPCMAP

University of Reading 2007www.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

University of Reading 2007www.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)

University of Reading 2007www.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 GRL)