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The effects of different footprint sizes and cloud algorithms on the top-of-atmosphere radiative flux calculation from the Clouds and Earth's Radiant Energy System (CERES) instrument on Suomi National Polar-orbiting Partnership (NPP)
Only one Clouds and Earth's Radiant Energy System (CERES) instrument is onboard the Suomi National Polar-orbiting Partnership (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 are used for flux inversions for NPP CERES measurements.
However, the footprint size of NPP CERES is greater than that of Aqua CERES,
as the altitude of the NPP orbit is higher than that of the Aqua orbit.
Furthermore, cloud retrievals from the Visible Infrared Imaging Radiometer
Suite (VIIRS) and the Moderate Resolution Imaging Spectroradiometer
(MODIS), which are the imagers sharing the spacecraft with NPP CERES and Aqua CERES,
are also different. To quantify the flux uncertainties due to the footprint
size difference between Aqua CERES and NPP CERES, 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
Aqua CERES and NPP CERES point spread functions (PSFs) into their respective
footprints. The simulation is designed to isolate the effects of footprint
size and cloud property differences on flux uncertainty from calibration and
orbital differences between NPP CERES and Aqua CERES. The footprint size
difference between Aqua CERES and NPP CERES introduces instantaneous flux
uncertainties in monthly gridded NPP CERES measurements of less than 4.0 W m−2
for SW (shortwave) and less than 1.0 W m−2 for both daytime and nighttime
LW (longwave). The global monthly mean instantaneous SW flux from simulated NPP CERES
has a low bias of 0.4 W m−2 when compared to simulated
Aqua CERES, and the root-mean-square (RMS) error is 2.2 W m−2
between them; the biases of daytime and nighttime LW flux are close to zero
with RMS errors of 0.8 and 0.2 W m−2. These uncertainties are
within the uncertainties of CERES ADMs. When both footprint size and cloud
property (cloud fraction and optical depth) differences are considered, the
uncertainties of monthly gridded NPP CERES SW flux can be up to
20 W m−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 NPP CERES has a high bias
of 1.1 W m−2 and the RMS error increases to
5.2 W m−2. LW flux shows less sensitivity to cloud property
differences than SW flux, with uncertainties of about 2 W m−2
in the monthly gridded LW flux, and the RMS errors of global monthly mean daytime
and nighttime fluxes increase only slightly. These results highlight the
importance of consistent cloud retrieval algorithms to maintain the accuracy
and stability of the CERES climate data record
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