Abstract
The records of the Advanced Very High Resolution Radiometer (AVHRR) instrument observations can resolve the current lack of a long global climate data record of Reflected Solar Flux (RSF), by transforming these measurements into broadband flux at the top-of-atmosphere. This paper presents a methodology for obtaining daily mean RSF (Wm−2) from AVHRR. First, the narrowband reflectances are converted to broadband reflectance using empirical regressions with the Clouds and the Earth’s Radiant Energy System (CERES) observations. Second, the anisotropy is corrected by applying Angular Distribution Models (ADMs), which convert directional reflectance into a hemispherical albedo. Third, the instantaneous albedos are temporally interpolated by a flexible diurnal cycle model, capable of ingesting any number of observations at any time of day, making it suitable for any orbital configuration of NOAA and MetOp satellites. Finally, the twilight conditions prevailing near sunrise and sunset are simulated with an empirical model. The entire day is then integrated into a single daily mean RSF. This paper furthermore demonstrates the methodology by validating a full year (2008) of RSF daily means with the CERES SYN1deg data record, both on daily and subdaily scale. Several configurations are tested, each excluding particular satellites from the constellation in order to mimic orbital changes (e.g., orbital drift), and to assess their relative importance to the daily mean RSF. The best performance is obtained by the combination of at least one mid-morning (NOAA-17 or MetOp-A) and one early afternoon (NOAA-18) orbit. In this case, the RMS difference with CERES is about 7 Wm−2. Removing NOAA-18 degrades the performance to an RMS difference of 12 Wm−2, thereby providing an estimate of the impact of NOAA-19’s orbital drift between 2016 and 2020. Very early or late observations (NOAA-15, NOAA-16) provide little added value, and both mid-morning orbits turn out to be almost interchangeable given their close temporal proximity.
Highlights
Broadband top-of-atmosphere (TOA) Reflected Solar Flux (RSF) is an essential climate variable of which a high-quality data record of satellite measurements with sufficient length (“Climate Data Record” or CDR) is needed by, among others, the climate modeling and climate monitoring communities, preferably spanning several decades [1].To this end, various approaches have been proposed and implemented
Measurements and the broadband measurements from Clouds and the Earth’s Radiant Energy System (CERES). Using this Advanced Very High Resolution Radiometer (AVHRR)-derived broadband reflectance, the current paper describes the retrieval of the daily mean RSF (Wm−2 ), which could be aggregated to monthly means to constitute a long-term, broadband energy balance dataset that would fit the needs of the climate modeling and monitoring communities
The year 2008 is characterized by an AVHRR-carrying constellation of five satellites (Figure 2a), each with a specific semi-constant local equatorial crossing time: NOAA-15 (~5 h,~17 h), MetOp-A (~9 h30), NOAA-17 (~10 h), NOAA-18 (~13 h30), and NOAA-16 (~4 h30,~16 h30)
Summary
Broadband top-of-atmosphere (TOA) Reflected Solar Flux (RSF) is an essential climate variable of which a high-quality data record of satellite measurements with sufficient length (“Climate Data Record” or CDR) is needed by, among others, the climate modeling and climate monitoring communities, preferably spanning several decades [1].To this end, various approaches have been proposed and implemented (a review is available in Dewitte and Clerbaux [2]). Energy System (CERES) [4], the Geostationary Earth Radiation Budget (GERB) [5] or the Scanner for Radiation Budget (ScaRaB) [6], the narrow-FOV approach has been selected because the wide-FOV provided limited scientific outcomes due to the enormous size of the FOV and the difficulty of relating the fluxes with particular scene types. ERB missions rely on broadband (BB) radiometers that provide integrated observations of the radiation over large parts of the electromagnetic spectrum: “shortwave” (0.3–4 μm) and “longwave” (4–50 μm) Such instruments are not widely deployed in space because the usefulness of their data is mostly limited to the particular ERB community. This is in contrast to other multi-spectral instruments, such as MODIS, which are useful for a wide range of different geophysical variables
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