Validation of the OMI Cloud Pressures Derived from Rotational Raman Scattering: a Study of the Effects of Two-Layer Clouds and Absorbing Aerosols
Abstract
The OMI cloud pressure product is necessary to correct the mission-critical total ozone product for ozone amount unseen by the instrument beneath clouds. Two cloud products are currently created from OMI data. One product is based on O2-O2 absorption. The other product derives cloud pressure from the high frequency structure of the top-of-atmosphere (TOA) reflectance in UV caused by rotational Raman scattering (RRS). RRS results in filling-in of Fraunhofer lines in the TOA spectrum. Clouds reduce the amount of filling-in of Fraunhofer lines, with the amount of filling-in related to cloud pressure. Comparisons of OMI cloud pressures derived from RRS with Aqua MODIS cloud-top pressures show noticeable differences between them in some geophysical situations such as two-layer cloud and the presence of absorbing aerosol. In the presence of multiple cloud decks, the RRS algorithm retrieves higher pressures than the top of the upper deck that MODIS is sensitive to. Absorbing aerosol can cause the RRS algorithm to retrieve cloud pressures lower than MODIS pressures. Radiative transfer (RT) calculations were performed to simulate the effects of those geophysical conditions on the cloud pressure retrievals. RT simulations were also used for quantifying possible errors related to using the Mixed Lambert-Equivalent Reflectivity (MLER) concept in the OMI cloud pressure algorithm. The RT simulation results show that (1) MLER is adequate for optically thick clouds; (2) For a model of cirrus overlapping optically thick water clouds, the RRS algorithm retrieves cloud pressure that is up to 200 hPa higher than for lower layer cloud-top pressure. That increase in the retrieved cloud pressure is attributed to additional scattering between two cloud layers that enhances RRS; (3) Absorbing aerosol with absorption optical depth of 0.1 placed above the cloud reduces the retrieved cloud pressure by 100 200 hPa depending on cloud optical depth.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.A51E0141V
- Keywords:
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- 0319 Cloud optics;
- 0360 Radiation: transmission and scattering;
- 3360 Remote sensing