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Changes in Water Vapour, Clear-sky Radiative Cooling and Precipitation Richard P. Allan Environmental Systems Science Centre, University of Reading, UK.

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Presentation on theme: "Changes in Water Vapour, Clear-sky Radiative Cooling and Precipitation Richard P. Allan Environmental Systems Science Centre, University of Reading, UK."— Presentation transcript:

1 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

2 How the hydrological cycle responds to a radiative imbalance is crucial to society (e.g. water supply, agriculture, severe weather) Climate Impacts

3 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 BAMS)

4 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) - 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)

5 Precipitation linked to clear-sky longwave radiative cooling of the atmosphere

6 Increased moisture enhances atmospheric radiative cooling to surface ERA40 NCEP Allan (2006) JGR 111, D22105 dSNLc/dCWV ~ W kg -1 SNLc = clear-sky surface net down longwave radiation CWV = column integrated water vapour

7 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 )

8 Tropical Oceans dCWV/dTs ~2 4 mm K -1 dSNLc/dTs ~3 5 Wm -2 K -1 AMIP3 CMIP3 non- volcanic CMIP3 volcanic Reanalyses/ Observations

9 AMIP3 CMIP3 non- volcanic CMIP3 volcanic Reanalyses/ Observations Increase in atmospheric cooling over tropical ocean descent ~4 Wm -2 K -1

10 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

11 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

12 Tropical Subsidence regions dP/dt ~ -0.1 mm day -1 decade -1 ) OCEANLAND AMIP SSM/IGPCPCMAP Allan and Soden, 2007, GRL

13 Projected changes in Tropical Precipitation Allan and Soden, 2007, GRL

14 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 -2 K -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 GRL) Observing systems: capturing decadal variability problematic

15 Extra slides…

16 Outline Clear-sky radiative cooling: –radiative convective balance –atmospheric circulation Earths 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)

17 Models reproduce observed increases in total column water vapour

18 Tropical Oceans Ts CWV LWc SFC ERA40 NCEP SRB HadISST SMMR, SSM/I Derived: SMMR, SSM/I, Prata) Allan (2006) JGR 111, D22105

19 Clear-sky OLR with surface temperature: + ERBS, ScaRaB, CERES; SRB Calibration or sampling?

20 Tropical Oceans Surface Net LWc Clear-sky OLR Clear-sky Atmos LW cooling Q LWc ERBS, ScaRaB, CERES Derived ERA40 NCEP SRB HadISST Allan (2006) JGR 111, D22105

21 Linear least squares fit Tropical ocean: descending regime DatasetdQ LWc /dTs Slope ERA-403.7±0.5Wm -2 K -1 NCEP4.2±0.3Wm -2 K -1 SRB3.6±0.5Wm -2 K -1 OBS4.6±0.5Wm -2 K -1 ERA40 NCEP

22 Implications for tropical precipitation (GPCP)? ERA40 Q LWc GPCP P OBS Q LWc Pinatubo?

23 Comparison of AMIP3 models, reanalyses and observations over the tropical coeans

24 Also considering coupled model experiments including greenhouse gas and natural forcings

25 Clear-sky vs resolution

26 Sensitivity study Based on GERB- SEVIRI OLR and cloud products over ocean: dOLRc/dRes ~0.2 Wm -2 km -0.5 Suggest CERES should be biased low by ~0.5 Wm -2 relative to ERBS

27 Links to precipitation


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