Abstract
The Clouds and Earth’s Radiant Energy System (CERES) has been monitoring clouds and radiation since 2000 using algorithms developed before 2002 for CERES Edition 2 (Ed2) products. To improve cloud amount accuracy, CERES Edition 4 (Ed4) applies revised algorithms and input data to Terra and Aqua MODerate-resolution Imaging Spectroradiometer (MODIS) radiances. The Ed4 cloud mask uses 5–7 additional channels, new models for clear-sky ocean and snow/ice-surface radiances, and revised Terra MODIS calibrations. Mean Ed4 daytime and nighttime cloud amounts exceed their Ed2 counterparts by 0.035 and 0.068. Excellent consistency between average Aqua and Terra cloud fraction is found over nonpolar regions. Differences over polar regions are likely due to unresolved calibration discrepancies. Relative to Ed2, Ed4 cloud amounts agree better with those from the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). CALIPSO comparisons indicate that Ed4 cloud amounts are more than or as accurate as other available cloud mask systems. The Ed4 mask correctly identifies cloudy or clear areas 90%–96% of the time during daytime over nonpolar areas depending on the CALIPSO–MODIS averaging criteria. At night, the range is 88%–95%. Accuracy decreases over land. The polar day and night accuracy ranges are 90%–91% and 80%–81%, respectively. The mean Ed4 cloud fractions slightly exceed the average for seven other imager cloud masks. Remaining biases and uncertainties are mainly attributed to errors in Ed4 predicted clear-sky radiances. The resulting cloud fractions should help CERES produce a more accurate radiation budget and serve as part of a cloud property climate data record.
Highlights
T HE NASA Clouds and Earth’s Radiant Energy System (CERES) combines broadband radiances measured by a scanning radiometer with high-resolution narrowband imager data to determine the components of a given broadband field of view and retrieve an accurate flux from the radiance [1]
Since the creation of MODerate-resolution Imaging Spectroradiometer (MODIS) C5 radiance datasets stopped in March 2017, the CERES project switched to the MODIS Collection 6.1 radiances
One of the more notable changes is evident in boreal polar regions where day and night CFs are more consistent for Edition 4 (Ed4) [Fig. 19(a) and (b)] than for Edition 2 (Ed2) [Fig. 19(c) and (d)]
Summary
T HE NASA Clouds and Earth’s Radiant Energy System (CERES) combines broadband radiances measured by a scanning radiometer with high-resolution narrowband imager data to determine the components of a given broadband field of view and retrieve an accurate flux from the radiance [1]. The nature of the cloudy and clear areas within a given CERES broadband scanner radiance must be determined in order to unfilter the broadband measurements [2], select the anisotropic directional models (ADMs) used to convert radiance to flux [3], and provide input for computations of surface and atmospheric irradiances [4], [5]. The first step in that process is the cloud mask or scene identification (ID) process, which decides if each individual imager pixel within the CERES scanner footprint is cloudy or clear. The accuracy of the initial scene ID is essential to ensure the reliability of the CERES data record
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