Abstract
Abstract. This study presents an approach for the quantification of cloud–aerosol transition-zone broadband longwave radiative effects at the top of the atmosphere (TOA) during daytime over the ocean, based on satellite observations and radiative transfer simulation. Specifically, we used several products from MODIS (MODerate Resolution Imaging Spectroradiometer) and CERES (Clouds and the Earth's Radiant Energy System) sensors for the identification and selection of CERES footprints with a horizontally homogeneous transition-zone and clear-sky conditions. For the selected transition-zone footprints, radiative effect was calculated as the difference between the instantaneous CERES TOA upwelling broadband longwave radiance observations and corresponding clear-sky radiance simulations. The clear-sky radiances were simulated using the Santa Barbara DISORT (DIScrete Ordinates Radiative Transfer program for a multi-Layered plane-parallel medium) Atmospheric Radiative Transfer model fed by the hourly ERA5 reanalysis (fifth generation ECMWF ReAnalysis) atmospheric and surface data. The CERES radiance observations corresponding to the clear-sky footprints detected were also used for validating the simulated clear-sky radiances. We tested this approach using the radiative measurements made by the MODIS and CERES instruments on board the Aqua platform over the southeastern Atlantic Ocean during August 2010. For the studied period and domain, transition-zone radiative effect (given in flux units) is on average equal to 8.0 ± 3.7 W m−2 (heating effect; median: 5.4 W m−2), although cases with radiative effects as large as 50 W m−2 were found.
Highlights
Cloud and aerosol are particular names for two specific particle suspensions in the atmosphere which have been widely studied but continue to contribute the largest uncertainty to estimates and interpretations of the Earth’s changing energy budget (Boucher et al, 2013)
From this figure it can be seen that the criteria explained in section 2.1 for the selection of transition-zone footprints allow up to 10 % contamination by “Non-lost” classes, the fraction of these classes combined in the transition-zone footprints analyzed is on average about 5 %
Was smaller than what it would have been if no suspension was present
Summary
Cloud and aerosol are particular names for two specific particle suspensions in the atmosphere which have been widely studied but continue to contribute the largest uncertainty to estimates and interpretations of the Earth’s changing energy budget (Boucher et al, 2013). As the information available about the transition zone and its interactions with radiation (in both longwave and shortwave bands) is very limited, the area corresponding to the transition zone in climatic, meteorological, and atmospheric studies and models is usually considered as an area containing either aerosols or optically thin clouds. The transition zone is frequently neglected in cloud–aerosol-related studies, the above numbers and the vast area that potentially may contain the transition-zone state give importance to the necessity of further exploring it For this reason, within the frame of the study, a method for the quantification of the broadband longwave radiative effects of the transition zone at TOA over the ocean on the basis of instantaneous satellite observations and radiative transfer calculations is presented. This method is applied over the southeastern Atlantic Ocean, where cloudy conditions are frequent and transition-zone conditions are expected to be frequently observed
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