Uncertainties in CERES Top-of-Atmosphere Fluxes Caused by Changes in Accompanying Imager

Wenying Su,Hailan Wang,Zachary A Eitzen,Lusheng Liang

doi:10.3390/rs12122040

Abstract

The Clouds and the Earth’s Radiant Energy System (CERES) project provides observations of Earth’s radiation budget using measurements from CERES instruments on board the Terra, Aqua, Suomi National Polar-orbiting Partnership (S-NPP), and NOAA-20 satellites. The CERES top-of-atmosphere (TOA) fluxes are produced by converting radiance measurements using empirical angular distribution models, which are functions of cloud properties that are retrieved from imagers flying with the CERES instruments. As the objective is to create a long-term climate data record, not only calibration consistency of the six CERES instruments needs to be maintained for the entire time period, it is also important to maintain the consistency of other input data sets used to produce this climate data record. In this paper, we address aspects that could potentially affect the CERES TOA flux data quality. Discontinuities in imager calibration can affect cloud retrieval which can lead to erroneous flux trends. When imposing an artificial 0.6 per decade decreasing trend to cloud optical depth, which is similar to the trend difference between CERES Edition 2 and Edition 4 cloud retrievals, the decadal SW flux trend changed from − 0.3 5 ± 0.18 Wm − 2 to 0.61 ± 0.18 Wm − 2 . This indicates that a 13% change in cloud optical depth results in about 1% change in the SW flux. Furthermore, different CERES instruments provide valid fluxes at different viewing zenith angle ranges, and including fluxes derived at the most oblique angels unique to S-NPP (>66 ∘ ) can lead to differences of 0.8 Wm − 2 and 0.3 Wm − 2 in global monthly mean instantaneous SW flux and LW flux. To ensure continuity, the viewing zenith angle ranges common to all CERES instruments (<66 ∘ ) are used to produce the long-term Earth’s radiation budget climate data record. The consistency of cloud properties retrieved from different imagers also needs to be maintained to ensure the TOA flux consistency.

Highlights

Earth’s radiation budget (ERB) at the top-of-atmosphere (TOA) is one of the most critical variables that shows the combined effects of many processes within the Earth-atmosphere system
Climate data records that are decades long generally involve multiple satellites and consistency of the measurements across multiple platforms is key to maintain the stability of the measurements
Clouds and the Earth’s Radiant Energy System (CERES) TOA flux data set is produced by using the radiances measured by the CERES instruments, which are converted to fluxes by using the scene-type dependent angular distribution models (ADMs)

Summary

Introduction

Earth’s radiation budget (ERB) at the top-of-atmosphere (TOA) is one of the most critical variables that shows the combined effects of many processes within the Earth-atmosphere system. For FM1-FM5, LW radiance is derived as the difference between total and SW channels; for FM6, the LW channel measurement offers an independent check to the LW radiance derived by differencing total and SW channels These measured radiances (I) at a given sun–Earth–satellite geometry are converted to outgoing reflected solar and emitted thermal TOA radiative fluxes (F) as: F(θ0). The CERES measured radiances are converted to fluxes using scene-type dependent anisotropic factors (Equation (1)). These anisotropic factors are determined using the CERES rotating azimuth plane data collected at the beginning of the Terra and Aqua missions and the cloud properties retrieved from MODIS spectral measurements.

CERES ADMs

CERES TOA SW

Findings

Discussion and Conclusions