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Richard P. Allan, A. Slingo Met Office, Hadley Centre for Climate Prediction and Research, U.K.

Simultaneous surface and top of atmosphere radiative flux measurement and applicability to the evaluation of climate models. Richard P. Allan, A. Slingo Met Office, Hadley Centre for Climate Prediction and Research, U.K. Thanks to Nicolas Clerbaux (RMIB). Introduction.

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Richard P. Allan, A. Slingo Met Office, Hadley Centre for Climate Prediction and Research, U.K.

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  1. Simultaneous surface and top of atmosphere radiative flux measurement and applicability to the evaluation of climate models Richard P. Allan, A. Slingo Met Office, Hadley Centre for Climate Prediction and Research, U.K. Thanks to Nicolas Clerbaux (RMIB)

  2. Introduction • Radiative heating - driver of atmospheric circulation • Radiative feedbacks and climate change. • Can climate models simulate radiation budget and its variability? i.e. physical processes • Diurnal variability, “anomalous” solar absorption, etc. • Sampling: clear-sky/dynamic regime • Exploratory study combining radiative fluxes derived from Meteosat top of atmosphere radiance with collocated surface BSRN measurements

  3. Problems for climate model evaluation • Point measure vs Grid Box simulation • Sampling: Temporal, Spatial, Clear-sky • Surface albedo: inhomogeneity, Cos(zenith)/spectral dependence • Diurnal sampling bias/Angular dependence models • Error in atmospheric composition/radiative properties or radiation theory/parametrization

  4. Atmospheric solar absorption • “Anomalous” atmospheric absorption • Do cloudy atmospheres absorb more solar radiation than clear-sky? • Pathlength effects • Sampling inconsistency • Aerosol uncertainty Above: Pathlength effects

  5. Clear-sky sampling • Is the atmosphere ever really clear?! • Limited cloud “measurements” • Aerosol uncertainty - effects on sampling • Use of radiative fluxes to sample clear-sky • Dutton (1993), J. Atmos. Ocean. Tech, 10, p.326 [LW surface rad] • Allan (2000), J. Climate, 13, p.1951 [LW and SW surface rad] • Long & Ackerman (2000), J. Geophys. Res.,105, p.15 609 [1-min SW obs (direct, diffuse) and time variation] • Marty & Philipona (2000), GRL,27, p.264 • Simulated clear-sky (Type II) sampling inconsistent with observations [e.g. Cess & Potter (1987)]

  6. Simple Clear-sky sampling [Allan (2000), J. Clim, 13, p.1951-1964] Right: Surface longwave fluxes Below: Incoming SW flux

  7. Evaluation of surface clear -sky downward LW radiation at Barrow (below) and Tropical West Pacific site (right) Clear-sky sampling for (a) LW and (b) SW methods • Simulated SDL overestimated for warmer, moister profiles • Simulated SDL underestimated for colder, drier profiles

  8. Carpentras BSRN/Meteosat data: Monthly processing • Use method similar to ERBE satellite radiation budget • Mean monthly 30 minute res. diurnal cycle (clear-sky/all-sky) • Surface Albedo, Potter at al. (1988): s=0(1+d)/(1+2d) • Adjust SW for: • changing insolation during month • point/grid-box insolation disparity • Interpolation applied to monthly-mean diurnal cycle

  9. Verification of LW clear-sky sampling • Surface LW emission, E=Ts4 • K1=OLR/E; K2=SDL/E • Clear if K1>0.6; K2<0.8 • Verify during daytime: • Meteosat cloud amount • SW radiation • Additional observations?

  10. Sensitivity to clear-sky sampling method • Significant diurnal cycle of LW radiation • Small sensitivity to clear-sky filtering method • Some significant difference to Meteosat cloud amount only filtering

  11. Mean diurnal cycle for BSRN data + Meteosat TOA fluxes for April 1999 • Differences in clear-sky/all-sky near-surface temperature/humidity • Significant diurnal cycle of top of atmosphere LW radiation (OLR) • Apply same sampling to reanalysis and climate model data. • ...Apply methodology on GERB data: evaluate climate model

  12. Calculate monthly atmospheric solar absorption • Evidence of “anomalous” cloudy absorption - sensitive to surface albedo • Insensitive to clear- sky sampling type

  13. ERA-40 preliminary data comparison with BSRN/Meteosat dataRed: surface downwelling, Blue: top of atmosphere outgoing

  14. Conclusions • Methodology for consistent all-sky/clear-sky sampling of surface and satellite radiation budget data • Column atmospheric solar absorption sensitive to surface albedo and cloud position independent of in-cloud “anomalous” absorption issues • Techniques applicable to future GERB-data (right: Meteosat GERB- test OLR): use to verify diurnal cycle of radiation in climate models

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