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Radar and lidar calibration

Radar and lidar calibration. Ewan O’Connor, Robin Hogan, Anthony Illingworth, Nicolas Gaussiat, Dominique Bouniol, Darcy Ladd, Henk Klein Baltink. Overview. Radar calibration Typical methods Comparison with other wavelengths Calibration in rain Inter-calibration

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Radar and lidar calibration

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  1. Radar and lidar calibration Ewan O’Connor, Robin Hogan, Anthony Illingworth, Nicolas Gaussiat, Dominique Bouniol, Darcy Ladd, Henk Klein Baltink

  2. Overview • Radar calibration • Typical methods • Comparison with other wavelengths • Calibration in rain • Inter-calibration • Consistency between cloud radars • Lidar calibration • Molecular calibration • Calibration using liquid water layers

  3. Radar calibration – by comparison • Use calibration target of known backscatter cross-section • Link budget calculation • Compare to calibrated radar • At Chilbolton use 3GHz polarimetric scanning radar • 3GHz calibrated using Z, ZDR, KDP, redundancy in heavy rain (Goddard et al., 1994) • Correct for gaseous attenuation at 35/94GHz • Check for Mie scattering, attenuation at 35/94GHz

  4. Radar calibration – in rain

  5. Radar calibration – in rain • Compare observations with theory Problem: apparent bias in observations

  6. Radar calibration – in rain Why bias in observations? • Additional problem in rain - Radome gets wet • Causes severe attenuation 9 – 13 dB

  7. Radar calibration – in rain • Solution • Keep radome dry in rain (use cover) • Observations now agree with theory • Estimate radome attenuation

  8. Radar calibration – in rain Compare independent methods – good agreement

  9. Inter-calibration Cabauw,The Netherlands SIRTA, Palaiseau (Paris), France Chilbolton, UK

  10. Inter-calibration French radar 94 GHz RASTA is mobile • Solution: drive to each site and place radars next to one another

  11. Inter-calibration – results • Add 6dB to RASTA • 13dB radome attenuation (c.f. 11dB for Galileo) Agreement between all 3 radars ~1dB

  12. Lidar calibration – molecular

  13. Lidar calibration – molecular

  14. Lidar calibration – liquid layers Optically thick liquid layers appear to have constant integrated backscatter (B) - except when precipitating

  15. Lidar calibration – liquid layers Calculate theoretical lidar ratio, S At 905 nm S is constant for the range of liquid cloud droplet sizes Theory says B = 1/2S Calibration method: scale observed B (from liquid water clouds) until equal to theoretical 1/2S However, additional complication due to multiple scattering

  16. Lidar calibration – liquid layers Multiple scattering depends on lidar design, range, and droplet size Use factor η to describe effect B = 1/2ηS η variation with range and lidar design can be calculated (Eloranta 1998) Calibration within 7%

  17. Dual wavelength microwave radiometer • Brightness temperatures -> Liquid water path • Improved technique – Nicolas Gaussiat • Use lidar to determine whether clear sky or not • Adjust coefficients to account for instrument drift • Removes offset for low LWP LWP - initial LWP - lidar corrected

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