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University of Reading 2007www.nerc-essc.ac.uk/~rpa Observed and simulated changes in water vapour, precipitation and the clear-sky.

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Presentation on theme: "University of Reading 2007www.nerc-essc.ac.uk/~rpa Observed and simulated changes in water vapour, precipitation and the clear-sky."— Presentation transcript:

1 University of Reading 2007www.nerc-essc.ac.uk/~rpa Observed and simulated changes in water vapour, precipitation and the clear-sky longwave radiation budget of the surface and atmosphere Richard P. Allan Environmental Systems Science Centre, University of Reading, UK

2 University of Reading 2007www.nerc-essc.ac.uk/~rpa Earths energy balance Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p.94

3 University of Reading 2007www.nerc-essc.ac.uk/~rpa Earths energy balance Kiehl and Trenberth, 1997; Also IPCC 2007 tech. summary, p.94 Precip: +78 Wm -2 SW heating +67 Wm -2 LW cooling -169 Wm -2

4 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)

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

6 University of Reading 2007www.nerc-essc.ac.uk/~rpa Increases in water vapour enhance clear- sky longwave radiative cooling of atmosphere to the surface This is offset by enhanced absorption of shortwave radiation by water vapour Changes in greenhouse gases, aerosol and cloud alter this relationship… Tropical oceans

7 University of Reading 2007www.nerc-essc.ac.uk/~rpa Sensitivity test: tropical oceans Clear-sky Longwave shortwave TOA SFC ATM ATM 1K increase in tropospheric T, constant RH Greenhouse gas changes from 1980 to 2000 assuming different rates of warming

8 University of Reading 2007www.nerc-essc.ac.uk/~rpa Increase in atmospheric LW cooling over tropical ocean descent ~4 5 Wm -2 K -1 AMIP3 CMIP3 non- volcanic CMIP3 volcanic Reanalyses/ Observations

9 University of Reading 2007www.nerc-essc.ac.uk/~rpa Increased moisture (~7%/K) – increased convective precipitation Increased radiative cooling – smaller mean rise in precipitation (~3%/K) Implies reduced precipitation away from convective regimes (less light rainfall?) Locally, mixed signal from the above

10 University of Reading 2007www.nerc-essc.ac.uk/~rpa Method: Analyse separately precipitation over the ascending and 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

11 University of Reading 2007www.nerc-essc.ac.uk/~rpa GPCP CMAPAMIP3 Tropical Precipitation Response Allan and Soden, 2007, GRL Model precipitation response smaller than the satellite observations (see also Wentz et al Science; Chou et al GRL, etc)

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

13 University of Reading 2007www.nerc-essc.ac.uk/~rpa Could changes in aerosol and their indirect effect on cloud be driving changes in the tropical hydrological cycle through the surface radiation budget? Mishchenko et al. (2007) Science; Wild et al. (2005) Science

14 University of Reading 2007www.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

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

16 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

17 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

18 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

19 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)

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


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