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: r.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 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

lear

-sky

Sur

face

Net

Lo

ngw

ave

Rad

iatio

n (W

m-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 Temperature Clear-sky LW with water vapour

Clear-sky LW with Temperature Clear-sky OLR with T