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Cloud screening and aerosol retrievals from MAX-DOAS measurements at Ukkel

BIRA contribution to WP3:. Cloud screening and aerosol retrievals from MAX-DOAS measurements at Ukkel. Clio Gielen Michel Van Roozendael, Francois Hendrick , Caroline Fayt, Christian Hermans, Gaia Pinardi, Didier Gilllotay, and Tim Vlemmix.

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Cloud screening and aerosol retrievals from MAX-DOAS measurements at Ukkel

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  1. BIRA contribution to WP3: Cloud screening and aerosol retrievals from MAX-DOAS measurements at Ukkel Clio Gielen Michel Van Roozendael, Francois Hendrick, Caroline Fayt, Christian Hermans, Gaia Pinardi, Didier Gilllotay, and Tim Vlemmix + Hugo De Backer, Veerle De Bock, and Quentin Laffineur from RMI

  2. Goals • Qualify sky and cloud conditions • from MAX-DOAS data • Apply to Ukkel station to improve aerosol retrievals • Tasks 3.1 (algorithm improvements) and 3.2 (operating a MAX-DOAS instrument at Ukkel)

  3. Sky conditions: the colour index • Trace gas and aerosol retrievals affected by clouds • → introduce: additional photon absorption • multiple scattering • strong temporal variation • Degenerate problem: thin/thick clouds, broken clouds, fog, haze, white clouds, grey clouds, high/low aerosol load, .... • Use observed colour of the sky • for characterisation • → Colour Index = ratio of intensities

  4. Colour Index characteristics • Investigation of the colour index shows that: • - the CI = high for clear skies • - the CI < in the presence of clouds, aerosols, fog,... • - the CI = a constant value over time in the presence of • optically thick clouds • - the CI = highly variable in the presence of • broken/scattered clouds

  5. Cloud-Screening flags • Using the colour index we define two flags: • - a flag to describe the general sky condition (clear/polluted/full-cloud cover) • → Colour-Index flag • - a flag to note the presence of scattered clouds • → Broken-Cloud flag

  6. Broken-Cloud flag • Temporal variation to determine presence of scattered clouds • → model observations with double-sine function • and detect outliers • → no information on presence of full-cloud cover!

  7. Colour-Index measurements: Ukkel

  8. Colour-Index flag: sky condition By combining measurements and simulations we define 3 sky-condition regions Good= clear sky Med= thin clouds/ low aerosols Bad= thick clouds/ high aerosols Good Mediocre Bad

  9. Impact on aerosol retrievals ◊ AOD measurements (Aeronet+ Brewer) + AOD retrievals (MAX-DOAS + bePROprofiling code) + no cloud screening + CI-flag=good/med + CI-flag=good/med and no broken clouds

  10. Impact on aerosol retrievals + no cloud screening + CI-flag= good/med + CI-flag= good/med and no broken clouds

  11. Cloud-cover from thermic infrared data

  12. Interpretation of aerosol data at Ukkel • Exploiting the ensemble of data at BIRA-RMI (task 3.3): • BIRA: • Aerosol AOD and extinction profiles (MAX-DOAS) • Aerosol AOD (CIMEL) • Cloud screening (MAX-DOAS) • Cloud-cover fraction (thermic infrared) • RMI: • Aerosol AOD (Brewer) • Backscatter profiles/mixing-layer height (lidar) • Cloud screening (lidar/Brewer) • + Aethalometer in the future

  13. Vertical Profiles: BIRA-RMI Aerosol AOD (BIRA/RMI) Backscatter/ cloud/MLH (RMI) Aerosol vert.prof+ cloudscreening(BIRA) Aerosol vert.prof (BIRA)

  14. Status • - Operational cloud-screening method for MAX-DOAS measurements • - Aerosol retrievals for full Ukkel dataset (2011-2013): AOD and extinction profiles • - Applying cloud screening strongly improves aerosol retrievals • - Good agreement between BIRA cloud screening and RMI data • - Good agreement between MAX-DOAS cloud screening and thermal infrared cloud-cover data

  15. Future tasks for the last part of the project • - Further improvement of cloud-screening method: • Include O4 DSCDs → distinguish clouds/aerosols/fog... + cloud bottom height • - Detailed study of aerosol retrievals at Ukkel (BIRA + RMI): • Trends (seasonal/daily/weekly) • Correlation with meteorology • - Estimation of the radiative forcing effect at Ukkel(task 3.3)

  16. Aerosol load Cloud optical depth Colour-Index simulations Simulations performed by Tim Vlemmix, with DAK plan-parallel model

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