Changes in Water Vapour Clearsky Radiative Cooling and

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 • 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) 1979 -2002

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) -1 K ∆P (%) 7% ∆T (K) Held and Soden (2006) J. Clim - Changes in P with warming estimated to be ~3%K-1 - Consistent with model estimates (~2%K-1)

Precipitation linked to clearsky longwave radiative cooling of the atmosphere

Increased moisture enhances atmospheric radiative cooling to surface ERA 40 NCEP d. SNLc/d. CWV ~ 1 ─ 1. 5 W kg-1 SNLc = clear-sky surface net down longwave radiation Allan (2006) JGR 111, D 22105 CWV = column integrated water vapour

Increase in clear-sky longwave radiative cooling to the surface ∆SNLc (Wm-2) CMIP 3 volcanic NCEP ERA 40 SSM/I-derived ~ +1 Wm-2 per decade

Tropical Oceans d. CWV/d. Ts ~2 ─ 4 mm K-1 d. SNLc/d. Ts ~3 ─ 5 Wm-2 K-1 AMIP 3 CMIP 3 nonvolcanic CMIP 3 volcanic Reanalyses/ Observations

Increase in atmospheric cooling over tropical ocean descent ~4 Wm-2 K-1 AMIP 3 CMIP 3 volcanic CMIP 3 nonvolcanic Reanalyses/ Observations

• 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

Tropical Precipitation Response • Model precipitation response smaller than the satellite observations – see also Wentz et al. (2007) Science Allan and Soden, 2007, GRL GPCP AMIP 3 CMAP

Tropical Subsidence regions d. P/dt ~ -0. 1 mm day-1 decade-1) OCEAN AMIP LAND 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-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. 2007 GRL) • Observing systems: capturing decadal variability problematic

Extra slides…

Outline • Clear-sky radiative cooling: – 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

ERA 40 NCEP SRB Tropical Oceans Had. ISST Ts SMMR, SSM/I CWV Derived: SMMR, SSM/I, Prata) LWc SFC 1980 1985 1990 1995 2000 2005 Allan (2006) JGR 111, D 22105

Clear-sky OLR with surface temperature: + ERBS, Sca. Ra. B, CERES; SRB Calibration or sampling?

ERA 40 NCEP Tropical Oceans SRB Had. ISST ERBS, Sca. Ra. B, CERES Derived Surface Net LWc Clear-sky OLR Clear-sky Atmos LW cooling QLWc Allan (2006) JGR 111, D 22105

ERA 40 NCEP Linear least squares fit • Tropical ocean: descending regime • • • Dataset ERA-40 NCEP SRB OBS d. QLWc/d. Ts Slope 3. 7± 0. 5 Wm-2 K-1 4. 2± 0. 3 Wm-2 K-1 3. 6± 0. 5 Wm-2 K-1 4. 6± 0. 5 Wm-2 K-1

Implications for tropical precipitation (GPCP)? GPCP P ERA 40 QLWc OBS QLWc Pinatubo?

Comparison of AMIP 3 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 GERBSEVIRI OLR and cloud products over ocean: • d. OLRc/d. Res ~0. 2 Wm-2 km-0. 5 • Suggest CERES should be biased low by ~0. 5 Wm-2 relative to ERBS

Links to precipitation