3) Empirical estimate of surface longwave radiation Use an empirical estimate of the clear-sky surface downward longwave radiation (SDLc) to estimate the SNLc (Prata 1996; Allan et al. 2004) Satellite microwave measurements of column integrated water vapour from the SMMR and SSM/I instruments are used as input to the formula The ocean surface temperature (Ts) is prescribed from HadISST data The near-surface atmospheric temperature (T 0 ) is estimated using the monthly da Silva climatology of surface minus near surface temperature SDLc = { 1 – ( CWV)exp[-( CWV) 0.5 ] } σT 0 4 SNLc=ε s (SDLc- σT s 4 ) 1) INTRODUCTION Surface clear-sky net downward longwave radiation (SNLc) is critical for the surface energy balance and the radiative cooling of the atmosphere, thereby representing a crucial parameter determining the global water cycle SNLc is strongly dependent on the column integrated water vapour (CWV) To obtain an accurate estimate of the decadal variability in SNLc over the ocean we utilise a well calibrated record of CWV from satellite microwave instruments and use these as input to an empirical model 6) CONCLUSIONS Variability in the surface net clear-sky radiation balance at the surface (SNLc) is dominated by fluctuations in the total column integrated water vapour (CWV) An empirical estimate of SNLc based on satellite microwave observations of CWV shows excellent agreement with the interannual variability from the NCEP reanalysis The sensitivity, dSNLc/dTs ~ 3 Wm -2 K -1, suggests a rapid increase in atmospheric cooling to the surface (surface heating) with warming over the tropical ocean References: Allan, R. P., M.A. Ringer, J.A. Pamment and A. Slingo (2004) J. Geophys. Res., 109, D18107; Henderson, P.W. (2006) PhD thesis, University of Reading; Prata, A.J. (1996) Q. J. R. Meteorol. Soc., 122, p ) Future Plans / Improvements Use surface radiation observations from the Atmospheric Radiation Measurement (ARM) and Baseline Surface Radiation Network (BSRN) to calibrate the Prata (1996) formula (left). Use surface observations of low- altitude cloud to extend the formula to cloudy conditions (right) Combine empirical calculation of SNLc with satellite observations of the top of atmosphere outgoing longwave radiation to provide an estimate of the clear-sky longwave radiative cooling of the atmosphere and its variability over the tropical oceans Prata formula (Box 3) Prata fit to ARM data (modified coefficients) Figure 4: Atmospheric clear-sky effective emissivity (ε 0 ) as a function of column integrated water vapour (CWV) for four ARM sites and corresponding simulations from a model. Also shown are fits to the observations using standard and modified forms of the Prata (1996) formula (Henderson 2006). Figure 5: Observations of low-cloud from EECRA ship observations (a), surface cloud longwave radiative effect from SRB (b), and their scatter (c) for multi-annual mean climatology 4) Decadal Variability of CWV and SNLc over Tropical Oceans Calculate deseasonalised anomalies of CWV and SNLc over tropical ocean There is a strong correspondence between CWV and SNLc anomalies ERA40 data displays spurious variability in water vapour and this affects the accuracy of SNLc. However, ERA40 provides the most realistic spatial climatology of water vapour and clear-sky radiation (Allan et al. 2004) The Empirical estimate of SNLc variability is in excellent agreement with the NCEP reanalysis and SRB data. A sensitivity, dSNLc/dTs ~ 3 Wm -2 K -1 is calculated using empirical estimate; thus the surface is less able to cool radiatively as temperatures warm. Figure 3: Deseasonalised monthly anomalies of CWV and SNLc over tropical oceans for reanalyses products, observations and the Prata (1996) formula using observational input Empirical formula using SMMR and SSM/I 2) Net longwave radiation at the surface and water vapour Over the tropical oceans, large portions of the longwave spectrum are saturated at low levels due to water vapour absorption (left). The net longwave flux is dependent on the window region of the spectrum The window spectral region is dominated by water vapour continuum absorption which scales strongly with column integrated water vapour (CWV) Figure 1: Surface upward and downward longwave irradiance calculated using a standard tropical profile as input to a narrow band radiative transfer scheme Figure 2: Interannual changes in global monthly- mean column integrated water vapour (CWV) and surface clear-sky net downward longwave radiation (SNLc) from reanalysis data (ERA40 and NCEP-I). LW up LW down Surface Irradiance, Tropical Profile ARM siteARMModel Barrow Lamont ++ Darwin Manus ** CWV (cm) Empirical estimate of variability in clear-sky surface longwave radiation over the ocean Richard P. Allan and Peter W. Henderson Environmental Systems Science Centre, University of Reading, UK R.Allan supported by NERC grant NE/C51785X/1