1. Links Between Clear-sky Radiation, Water Vapour and the Hydrological Cycle Richard P. Allan 1, Viju O. John 2 1 Environmental Systems Science Centre, University of Reading, UK 2 Hadley Centre, Met Office, Exeter, UK Email: r.p.allan@reading.ac.ukr.p.allan@reading.ac.uk R.Allan supported by NERC grant NE/C51785X/1 Surface longwave radiation Downward Upward PRECIPITATION CHANGES There is mounting evidence that models do not capture current changes in the hydrological cycle Does this discrepancy relate to cloud or aerosol effects or limitations in the models or satellite data? Changes in observed precipitation (P) are sensitive to the time-period and the dataset used SENSITIVITY Models and observations show similar sensitivities of clear-sky radiation and water vapour to warming This implies a consistent impact on the water cycle through changes in the atmospheric and surface radiation balance VARIABILITY There are coherent responses of clear-sky radiation, water vapour and precipitation to temperature changes associated with the El Niño Southern Oscillation and a warming trend Discrepancies exist between reanalyses, observations and models. Also evident is a trend in clear-sky radiation due to moistening INTRODUCTION There is a balance between the net atmospheric radiative cooling and latent/sensible heating Changes in clear-sky radiation exert a strong constraint on changes in hydrological cycle It is important to establish that models adequately represent changes in clear-sky radiation In this analysis, models and satellite datasets are used to examine relationships between clear-sky radiation and aspects of the atmospheric hydrological cycle Clear-sky radiative cooling (Wm -2 ) SURFACE RADIATIVE COOLING For low-latitude regions, much of the longwave radiative spectrum is saturated by water vapour Water vapour continuum absorption strongly constrains the surface radiation balance SSM/I NCEP Models, reanalyses and observations all produce a robust, rapid rise in clear-sky atmospheric longwave radiative cooling to the surface with increased moisture (~1 Wm -2 mm -1 ) Sensitivity experiments for the period 1980- 2000 show the influence of warming, assuming constant relative humidity, on radiation balance Clear-sky Surface Net Longwave Radiation (Wm -2 ) Column water vapour (mm) TRMM Surface net downward Longwave radiation at the surface Surface Temperature Column water vapour Surface net downward clear-sky longwave radiation Clear-sky OLR Clear-sky atmospheric longwave radiative cooling Precipitation ALL ASCENT DESCENT Deseasonalized monthly anomalies: models, reanalyses and observations Rising atmospheric longwave radiative cooling is partly offset by changes in greenhouse gases and shortwave absorption by water vapour Moisture changes in the free troposphere and near the surface determine the global water cycle response to anthropogenic warming TOA (LW) SFC (LW) ATM (LW) ATM (SW) Absorbed clear-sky Shortwave radiation in the atmosphere Water vapour with TemperatureClear-sky LW with water vapour Clear-sky LW with TemperatureClear-sky OLR with T