Aerosol direct radiative effect over clouds from a synergy of Ozone Monitoring Instrument (OMI) and Moderate Resolution Imaging Spectroradiometer (MODIS) reflectances

Abstract

Abstract. The retrieval of geophysical parameters is increasingly\ndependent on synergistic use of satellite instruments. More\nsophisticated parameters can be retrieved and the accuracy of retrievals\ncan be increased when more information is combined. In this paper,\na synergistic application of Ozone Monitoring Instrument (OMI), on the Aura platform, and Moderate Resolution Imaging Spectroradiometer (MODIS), on the Aqua platform, Level 1B reflectances is described, enabling the retrieval of the aerosol direct\nradiative effect (DRE) over clouds using the differential aerosol\nabsorption (DAA) technique. This technique was first developed for\nreflectances from the SCanning Imaging Absorption spectroMeter for\nAtmospheric CHartographY (SCIAMACHY) on the Environmental Satellite (Envisat), which had the unique\ncapability of measuring contiguous radiances from the ultraviolet (UV)\nat 240 to 1750\u2009nm in the shortwave-infrared (SWIR), at a moderate\nspectral resolution of 0.2 to 1.5\u2009nm. However, the spatial resolution\nand global coverage of SCIAMACHY was limited, and Envisat stopped\ndelivering data in 2012. In order to continue the DRE data retrieval,\nreflectances from OMI and MODIS, flying in formation, were combined from\nthe UV to the SWIR. This resulted in reflectances at a limited but\nsufficient spectral resolution, available at the OMI pixel grid, which\nhave a much higher spatial resolution and coverage than SCIAMACHY. The\ncombined reflectance spectra allow the retrieval of cloud microphysical\nparameters in the SWIR, and the subsequent retrieval of aerosol DRE over\ncloud scenes using the DAA technique. For liquid cloud scenes in the\nsouth-east Atlantic region with cloud fraction (CF) >0.3 , the area-averaged instantaneous aerosol DRE over clouds in June to August 2006 was 25 \u2009Wm −2 with a\nstandard deviation of 30\u2009Wm −2 . The maximum area-averaged\ninstantaneous DRE from OMI–MODIS in August 2006 was 75.6±13 \u2009Wm −2 . The new aerosol DRE over-cloud dataset from OMI–MODIS is\ncompared to the SCIAMACHY dataset for the period 2006 to 2009, showing a\nvery high correlation. The OMI–MODIS DRE dataset over the Atlantic Ocean\nis highly correlated to above-cloud AOT measurements from OMI and MODIS.\nIt is related to AOT measurements over Ascension Island in 2016, showing\nthe transport of smoke all the way from its source region in Africa over\nthe Atlantic to Ascension and beyond.