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Atmospheric Measurement Techniques An interactive open-access journal of the European Geosciences Union

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doi:10.5194/amt-2017-75
© Author(s) 2017. This work is distributed
under the Creative Commons Attribution 3.0 License.
Research article
13 Apr 2017
Review status
This discussion paper is under review for the journal Atmospheric Measurement Techniques (AMT).
The effects of different footprint sizes and cloud algorithms on the top-of-atmosphere radiative flux calculation from CERES instrument on Suomi-NPP
Wenying Su1, Lusheng Liang2, Walter F. Miller2, and Victor E. Sothcott2 1Science Directorate, NASA Langley Research Center, Hampton, Virginia
2Science Systems & Applications, Inc., Hampton, Virginia
Abstract. Only one CERES instrument is onboard the Suomi-NPP and it has been placed in cross-track mode since launch, it is thus not possible to construct a set of angular distribution models (ADMs) specific for CERES on NPP. Edition 4 Aqua ADMs were used for flux inversions for CERES-NPP measurements. However, the footprint size of CERES-NPP is larger than that of CERES-Aqua, as the altitude of the NPP orbit is higher than that of the Aqua orbit. Furthermore, cloud retrievals from VIIRS and MODIS, the imagers fly alongside CERES-NPP and CERES-Aqua, are also different. To quantify the flux uncertainties due to the footprint size difference between CERES-Aqua and CERES-NPP, and due to both the footprint size difference and cloud property difference, a simulation is designed using the MODIS pixel level data which are convolved with the CERES-Aqua and CERES-NPP spectral response functions into their respective footprints. The simulation is designed to isolate the effects of footprint size difference and cloud property difference on flux uncertainty from calibration difference between CERES-NPP and CERES-Aqua. The footprint size difference between CERES-Aqua and CERES-NPP introduces instantaneous flux uncertainties in monthly gridded CERES-NPP of less than 4.0 Wm−2 for SW, and less than 1.0 Wm−2 for both daytime and nighttime LW. The global monthly mean instantaneous SW flux from simulated CERES-NPP has a low bias of 0.4 Wm−2 and an uncertainty of 0.8 Wm−2, the LW has a bias of about 0.1 Wm−2 and an uncertainty of 0.2 Wm−2. These uncertainties are within the uncertainties of CERES ADMs. When both footprint size and cloud property differences are considered, the uncertainties of monthly gridded CERES-NPP SW flux can be up to 20 Wm−2 in the Arctic regions where cloud optical depth retrievals from VIIRS differ significantly from MODIS. The global monthly mean instantaneous SW flux from simulated CERES-NPP has a high bias of 1.1 Wm−2 and an uncertainty of 2.4 Wm−2. LW flux shows less sensitivity to cloud property differences than SW flux, with the uncertainties of about 2 Wm−2 in monthly gridded LW flux and about 0.3 Wm−2 for global area-weighted monthly mean LW flux. These results highlight the importance of consistent cloud retrieval algorithms to maintain the accuracy and stability of the CERES climate data record.

Citation: Su, W., Liang, L., Miller, W. F., and Sothcott, V. E.: The effects of different footprint sizes and cloud algorithms on the top-of-atmosphere radiative flux calculation from CERES instrument on Suomi-NPP, Atmos. Meas. Tech. Discuss., doi:10.5194/amt-2017-75, in review, 2017.
Wenying Su et al.
Wenying Su et al.
Wenying Su et al.

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Short summary
The footprint size of CERES-NPP is larger than that of CERES-Aqua, because the altitude of the NPP orbit is higher than that of the Aqua orbit. Additionally, the cloud retrievals from VIIRS and MODIS, the imagers fly alongside CERES-NPP and CERES-Aqua, are also different. This paper addresses the impact of these differences on the CERES-NPP fluxes inverted using the CERES-Aqua angular distribution models.
The footprint size of CERES-NPP is larger than that of CERES-Aqua, because the altitude of the...
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