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Cloud Layer Overlap and the Influence of Vertical and Temporal Resolution of Radar Data Oliver Sievers Markus Quante GKSS Research Center Institute for Coastal Research D-21502 Geesthacht Outline Cloud Layer Overlap Cloud Layer Overlap


  1. Cloud Layer Overlap and the Influence of Vertical and Temporal Resolution of Radar Data Oliver Sievers Markus Quante GKSS Research Center Institute for Coastal Research D-21502 Geesthacht

  2. Outline Cloud Layer Overlap • Cloud Layer Overlap • Influence of Data Resolution • Radar / Ceilometer Comparison • Conclusions & Outlook Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  3. Cloud Layer Overlap: Introduction Cloud Layer Overlap = + − Current Models: C c c c c rand a b a b • Random Overlap • Maximum Overlap = C max( c a c , ) max b -> True Overlap? Combined Cloud Cover as function of level separation for all overlap models (mean values over 30mins / 60mins) Vert. cont. / non-cont. clouds Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  4. Combined Cloud Cover Cloud Layer Overlap Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  5. Combined Cloud Cover Cloud Layer Overlap Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  6. Combined Cloud Cover Cloud Layer Overlap Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  7. Combined Cloud Cover Cloud Layer Overlap Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  8. Overlap Parameter α Cloud Layer Overlap Combined Cloud Cover not useable to describe cloud overlap α =-1 Needed: Parametrisation of true overlap as function of overlap models α =0 => Overlap Parameter α α =+1 = α + − α C C ( 1 ) C true max rand Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  9. Overlap Parameter α Cloud Layer Overlap Full Resolution (82.5 m / 5secs) 30 mins mean CC Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  10. Overlap Parameter α Cloud Layer Overlap Full Resolution (82.5 m / 5secs) 30 mins mean CC 60 mins mean CC Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  11. Earlier Publication Cloud Layer Overlap From: Hogan and Illingworth, 2000, QJR Meteorol. Soc., 126, 2903-2909 Used resolution: rough cloudmask about 2 mins / 360 m / 60 mins average BBC: no convergence at random overlap for large level separation, exponentiell fitting is „dared“ - but full resolution! Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  12. Overlap Parameter α Cloud Layer Overlap 60 mins mean CC Res: 82.5 m / 5 sec Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  13. Overlap Parameter α Cloud Layer Overlap 60 mins mean CC Res: 82.5 m / 5 sec Res: 330 m / 2 min Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  14. Overlap Parameter α Cloud Layer Overlap 60 mins mean CC Res: 82.5 m / 5 sec Res: 330 m / 2 min Res: Res: 330 m / 5 sec Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  15. Overlap Parameter α Cloud Layer Overlap 60 mins mean CC Res: 82.5 m / 5 sec Res: 330 m / 2 min Res: 82.5 m /2 min Res: Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  16. Radar and Ceilometer Disagreement Cloud Layer Overlap Ceilometer n/a Cloud-Free Cloudy Cloud-Free 0.8 % 18.9 % 5.4 % Radar Cloudy 1.3 % 25.2 % 48.4 % Values relative to Radar profiles (259.370) Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  17. Radar and Ceilometer Disagreement Cloud Layer Overlap Ceilometer n/a Cloud-Free Cloudy Cloud-Free 0.8 % 18.9 % 5.4 % Radar Cloudy 1.3 % 25.2 % 48.4 % Values relative to Radar profiles (259.370) • Thin clouds? • Very low clouds? Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  18. Example for thin layer Cloud Layer Overlap Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  19. Ceilometer cloud base distribution w/o radar cloud Cloud Layer Overlap Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  20. Radar and Ceilometer Disagreement Cloud Layer Overlap Ceilometer n/a Cloud-Free Cloudy Cloud-Free 0.8 % 18.9 % 5.4 % Radar Cloudy 1.3 % 25.2 % 48.4 % Values relative to Radar profiles (259.370) • Very high (=Ice-) clouds? • Thin clouds? • Insects? • Very low clouds? • ??? • ??? Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  21. Difficult cloud for ceilometer Cloud Layer Overlap Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  22. Radar cloud base vs. thickness (w/o ceilometer) Cloud Layer Overlap Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  23. Summary Cloud Layer Overlap • High clouds • Thin clouds • Insects (?) • Overlapping of layers Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  24. Conclusions Cloud Layer Overlap • Cloud overlap depends on used resolution • For vert. cont. clouds: • Overlap parameter decreases with level separation • Using full resolution, decrease is appr. linear • Reducing data to mean 2-minutes-value increases α • Reducing vert. res. to 330 m gives a exponentiell decrease for increasing level sparation & converging at random overlap • Vertical non-cont. clouds are appr. random overlapped Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

  25. Conclusions (cont.) & Outlook Cloud Layer Overlap • Radar fails to see thin clouds • In boundary layer many radar signals remain (in 20% of all cases w/o ceilometer cloud, isolated pixels in lowest range gate exist) Outlook: • Modify Cloudmask • Check Ceilometer Accuracy vs. Temperature • Cloud Classification Oliver Sievers, CLIWA-NET-Workshop, 16.-19.12.2002, Madrid

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