links between clear-sky radiation, water vapour and the hydrological cycle richard p. allan 1, viju...

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: [email protected] 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 Temperatur e 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 Temperature Clear-sky LW with water vapour Clear-sky LW with Temperature Clear-sky OLR with T

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Links Between Clear-sky Radiation, Water Vapour and the Hydrological Cycle Richard P. Allan1, Viju O. John2

1Environmental Systems Science Centre, University of Reading, UK2Hadley Centre, Met Office, Exeter, UK

Email: [email protected]

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 cycleClear-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-2mm-1)

• Sensitivity experiments for the period 1980-2000 show the influence of warming, assuming constant relative humidity, on radiation balance C

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-sky

Sur

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Net

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Rad

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m-2)