2016-12-22: Improvements to the OMI O2–O2 operational cloud algorithm

The OMI (Ozone Monitoring Instrument on board of NASA’s EOS Aura satellite) OMCLDO2 cloud product supports trace gas retrievals of for example ozone and nitrogen dioxide. A new version of the OMI OMCLDO2 cloud product is presented that contains several improvements, of which the introduction of a temperature correction on the O2?O2 slant columns and the updated look-up-tables have the largest impact. The cloud pressure retrievals of the improved algorithm are compared with ground-based Radar-Lidar observations for three sites in the mid-latitudes, including the CESAR Cabauw site in The Netherlands.

The OMI (Ozone Monitoring Instrument on board of NASA’s EOS Aura satellite) OMCLDO2 cloud product supports trace gas retrievals of for example ozone and nitrogen dioxide. The OMCLDO2 algorithm derives the effective cloud fraction and effective cloud pressure using a DOAS fit of the O2?O2 absorption feature around 477 nm. A new version of the OMI OMCLDO2 cloud product is presented that contains several improvements, of which the introduction of a temperature correction on the O2?O2 slant columns and the updated look-up-tables have the largest impact. Whereas the differences in the effective cloud fraction are on average limited to 0.01, the differences of the effective cloud pressure can be up to 200 hPa, especially at cloud fractions below 0.3. As expected, the temperature correction depends on latitude and season. The updated look-up tables have a systematic effect on the cloud pressure at low cloud fractions. The improvements at low cloud fractions are very important for the retrieval of trace gases in the lower troposphere, for example for nitrogen dioxide and formaldehyde. The cloud pressure retrievals of the improved algorithm are compared with ground-based Radar-Lidar observations for three sites in the mid-latitudes, including the CESAR Cabauw site in The Netherlands. For low clouds that have a limited vertical extent the comparison yields good agreement. For higher clouds, which are vertically extensive and often contain several layers, the satellite retrievals give a lower cloud-height. For high clouds, mixed results are obtained.

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Results from the OMCLDO2 version 2 algorithm for 14 May 2005. a) effective cloud fraction, b) effective cloud pressure, c) difference of the effective cloud fraction (version 1.2.3 minus version 2), d) difference of the effective cloud pressure (version 1.2.3 minus version 2), e) scene albedo, f) scene pressure, g) temperature correction factor ?, and h) number of wavelengths used in the DOAS fit.