3) Empirical estimate of surface longwave radiation

1. Empirical estimate of variability in clear-sky surface longwave radiation over the ocean Richard P. Allan and Peter W. HendersonEnvironmental Systems Science Centre, University of Reading, UK Email: rpa@mail.nerc-essc.ac.uk R.Allan supported by NERC grant NE/C51785X/1 Surface Irradiance, Tropical Profile • 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) • 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 (T0) is estimated using the monthly da Silva climatology of surface minus near surface temperature LW up LW down SDLc = {1 – (1 + 0.1CWV)exp[-(1.2+0.3CWV)0.5] }σT04 SNLc=εs(SDLc- σTs4) 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). Figure 1: Surface upward and downward longwave irradiance calculated using a standard tropical profile as input to a narrow band radiative transfer scheme • 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-2K-1 is calculated using empirical estimate; thus the surface is less able to cool radiatively as temperatures warm. • 5) 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 (ε0) 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). □ Empirical formula using SMMR and SSM/I Figure 5:Observations of low-cloud from EECRA ship observations (a), surface cloud longwave radiative effect from SRB (b), and their scatter (c) for 1983-94 multi-annual mean climatology Figure 3: Deseasonalised monthly anomalies of CWV and SNLc over tropical oceans for reanalyses products, observations and the Prata (1996) formula using observational input • 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 CWV (cm) • 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-2K-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. 1127