Validation of the OMI/Aura O2-O2 Cloud Properties With MODIS/Aqua

Cloud correction is an important step in the retrieval process of trace gases from OMI spectra. There are two OMI cloud algorithms: one is based on the O_2--O_2 absorption band near 477 nm and the other is based on the filling-in of Fraunhofer lines by Raman scattering. Both algorithms have been developed specifically for OMI to retrieve an effective cloud fraction and cloud pressure. Verification and validation of these cloud products are relevant to the validation of the trace gas retrievals from OMI, as clouds are one of the main sources of inaccuracy in the retrieval of the total ozone column outside polar regions, and a main source of error for the retrieval of tropospheric pollutants. In this presentation a quantitative comparison between the OMI O_2--O_2 cloud product and MODIS in the A-train will be shown. Both retrievals are complementary since OMI uses reflected UV/VIS radiation while the MODIS cloud pressure retrieval operates in the thermal infra-red channels. To investigate the observed differences in cloud properties obtained with these instruments a comparison with model studies is shown as well. The effective cloud fraction retrieved from OMI measurements compares well with an effective cloud fraction obtained from MODIS data; a correlation between the two data sets of 0.92 is found, with regression coefficients of 0.034 and 0.93 for offset and slope (OMI as a function of MODIS), respectively. Large differences between OMI and MODIS retrieved cloud pressures of up to 250 hPa are found. This difference depends strongly on the cloud fraction, and it seems that the MODIS retrieval deviates for certain common atmospheric conditions: MODIS finds a higher cloud top pressure than OMI for broken low altitude clouds, while the reverse is expected from the properties of the OMI retrieval algorithm. In the paper these differences will be discussed.