| Popis: |
The OMI (Ozone Monitoring Instrument) 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 cloud fraction are limited to approximately 0.1, the differences of the 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. For low clouds that have a limited vertical extent the comparison is favorable. 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. |
