2014-05-20: Evaluation of SCIAMACHY Oxygen A band cloud heights using Cloudnet measurements

Two SCIAMACHY O2 A band cloud height products (FRESCO v6 and ESA Level 2 v5.02) are evaluated using ground-based radar/lidar measurements between January 2003 and December 2011. The radar/lidar profiles are obtained at the Cloudnet sites of Cabauw and Lindenberg, and are averaged for 1 h centered at the SCIAMACHY overpass time. In total we have 217 cases of single-layer clouds and 204 cases of multilayer clouds. We find that the ESA L2 cloud top height has a better agreement with the Cloudnet cloud top height than the Cloudnet cloud middle height.
The ESA L2 cloud top height is on average 0.4 km higher than the Cloudnet cloud top height, with a standard deviation of 3.1 km. The FRESCO cloud height is closer to the Cloudnet cloud middle height than the Cloudnet cloud top height.

The mean difference between the FRESCO cloud height and the Cloudnet cloud middle height is ?0.1 km with a standard deviation of 1.9 km. The ESA L2 cloud top height is higher than the FRESCO cloud height. The differences between the SCIAMACHY cloud (top) height and the Cloudnet cloud top height are linked to cloud optical thickness. The SCIAMACHY cloud height products are further compared to the Cloudnet cloud top height and the Cloudnet cloud middle height in 1 km bins. For single-layer clouds, the difference between the ESA L2 cloud top height and the Cloudnet cloud top height is less than 1 km for each cloud bin at 37 km. The difference between the FRESCO cloud height and the Cloudnet cloud middle height is less than 1 km for each cloud bin at 06 km. The results are similar for multilayer clouds, but the percentage of cases having a bias within 1 km is smaller than for single-layer clouds. We may conclude that the FRESCO cloud height is accurate for low and middle level clouds, whereas the ESA L2 cloud top height is more accurate for middle level clouds. Both products are less accurate for high clouds.

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Comparison between SCIAMACHY ESA L2 cloud height, FRESCO cloud height, and Cloudnet cloud top and cloud base height for single-layer clouds. The cases are sorted according to the Cloudnet cloud top height. The grey bar indicates the altitude range of the cloud according to the Cloudnet product. Here the cases with FRESCO effective cloud fraction below 0.1 are excluded.