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

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

Changing character of precipitation 1979-2002 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)

Changes in Q expected to be ~3 Wm-2K-1 (e.g. Allen and Ingram, 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) 7 % K-1 - Changes in P with warming estimated to be ~3%K-1 - Consistent with model estimates (~2%K-1) ∆P (%) ∆T (K) Held and Soden (2006) J. Clim

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

Increased moisture enhances atmospheric radiative cooling to surface ERA40 NCEP dSNLc/dCWV ~ 1 ─ 1.5 W kg-1 SNLc = clear-sky surface net down longwave radiation CWV = column integrated water vapour Allan (2006) JGR 111, D22105

Increase in clear-sky longwave radiative cooling to the surface ∆SNLc (Wm-2) CMIP3 CMIP3 volcanic NCEP ERA40 SSM/I-derived ~ +1 Wm-2 per decade

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

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

Increased moisture (~7%/K) Increased radiative cooling  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

Tropical Precipitation Response Model precipitation response smaller than the satellite observations see also Wentz et al. (2007) Science GPCP CMAP AMIP3 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: Earth’s radiation budget Method: 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 Atmosphere-only and fully coupled climate models are able to reproduce the observed variations in column integrated water vapour and its dependence on surface temperature over the oceans. Integrated water vapour is a key variable since it affects: Precipitation over convective regions Radiative cooling of the atmosphere to the surface Global precipitation through a combination of the above Strongly positive water vapour feedback With regards to the last point, column integrated water vapour is sensitive to moisture in the lower troposphere while changes in moisture throughout the middle and upper troposphere are more important for water vapour feedback and it is important to ensure that models accurately capture changes in water vapour at these levels.

Tropical Oceans Ts CWV LWc SFC 1980 1985 1990 1995 2000 2005 ERA40 NCEP SRB HadISST Ts CWV LWc SFC SMMR, SSM/I Derived:SMMR, SSM/I, Prata) 1980 1985 1990 1995 2000 2005 Allan (2006) JGR 111, D22105

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

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

Linear least squares fit ERA40 NCEP 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

Implications for tropical precipitation (GPCP)? GPCP P ERA40 QLWc 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