Climate Quality Broadband and Narrowband Solar Reflected Radiance Calibration Between Sensors in Orbit

Abstract

As the potential impacts of global climate change become more clear [1], the need to determine the accuracy of climate prediction over decade-to-century time scales has become an urgent and critical challenge. The most critical tests of climate model predictions will occur using observations of decadal changes in climate forcing, response, and feedback variables. Many of these key climate variables are observed by remotely sensing the global distribution of reflected solar spectral and broadband radiance. These "reflected solar" variables include aerosols, clouds, radiative fluxes, snow, ice, vegetation, ocean color, and land cover. Achieving sufficient satellite instrument accuracy, stability, and overlap to rigorously observe decadal change signals has proven very difficult in most cases and has not yet been achieved in others [2].One of the earliest efforts to make climate quality observations was for Earth Radiation Budget: Nimbus 6/7 in the late 1970s, ERBE in the 1980s/90s, and CERES in 2000s are examples of the most complete global records. The recent CERES data products have carried out the most extensive intercomparisons because if the need to merge data from up to 11 instruments (CERES, MODIS, geostationary imagers) on 7 spacecraft (Terra, Aqua, and 5 geostationary) for any given month. In order to achieve climate calibration for cloud feedbacks, the radiative effect of clear-sky, all-sky, and cloud radiative effect must all be made with very high stability and accuracy. For shortwave solar reflected flux, even the 1% CERES broadband absolute accuracy (1-σ confidence bound) is not sufficient to allow gaps in the radiation record for decadal climate change. Typical absolute accuracy for the best narrowband sensors like SeaWiFS, MISR, and MODIS range from 2 to 4% (1-σ). IPCC greenhouse gas radiative forcing is ~ 0.6 Wm -2 per decade or 0.6% of the global mean shortwave reflected flux, so that a 50% cloud feedback would change the global reflected flux by ~ 0.3 Wm -2 or 0.3% per decade in broadband SW calibration change. Recent results comparing CERES reflected flux changes with MODIS, MISR, and SeaWiFS narrowband changes concluded that only SeaWiFS and CERES were approaching sufficient stability in calibration for decadal climate change [3].Results using deep convective clouds in the optically thick limit as a stability target may prove very effective for improving past data sets like ISCCP. Results for intercalibration of geostationary imagers to CERES using an entire month of regional nearly coincident data demonstrates new approaches to constraining the calibration of current geostationary imagers.The new Decadal Survey Mission CLARREO is examining future approaches to a "NIST-in-Orbit" approach of very high absolute accuracy reference radiometers that cover the full solar and infrared spectrum at high spectral resolution but at low spatial resolution. Sampling studies have shown that a precessing CLARREO mission could calibrate other geo and leo reflected solar radiation and thermal infrared ...

CLARREO Approach for Reference Intercalibration of Reflected Solar Sensors: On-Orbit Data Matching and Sampling

“…Instruments that operate in this manner include (but are not limited to) CERES on Terra, Aqua, Suomi National Polar-orbiting Partnership (Suomi NPP), and JPSS platforms and MODIS, VIIRS, and AVHRR imagers on Terra, Aqua, NPP, JPSS, and MetOp satellites. In addition, intercalibration requires that the two measurements to be compared are obtained a short time apart, as reported by Wielicki et al [25]. The results reported here are obtained by assigning this temporal constraint a value of 5 min.…”

“…Thus, the net intercalibration transfer uncertainty is limited to about √ 2 × 0.3% (k = 2). A study was carried out to establish the dependence of onorbit time, space, and angle data matching noise on matching accuracy [25]. The authors used data from the AVHRR instruments on NOAA-17 and NOAA-18 satellites, when their orbits crossed.…”

Geostationary Visible Imager Calibration for the CERES SYN1deg Edition 4 Product

“…Pixel-level GEO and Terra-MODIS C5 band 1 radiances are averaged on a 0.5 • latitude by 0.5 • longitude grid. The large calibration footprint mitigates any navigation uncertainty, parallax errors, and influence of cloud advection when matching two satellite images [28]. The gridded averages must be coincident with 15 min and angle (ray) matched within 15 • in both view and azimuthal angle.…”

Opportunities to Intercalibrate Radiometric Sensors from International Space Station

“…Current methods of measuring these two variables from spacecraft are not sufficiently accurate; an order of magnitude improvement is required in the case of reflected solar radiation (Wielicki et al 2013). Two proposed complementary missions address the need for greater accuracy by employing instruments with onboard calibration that permits traceability to international metrological standards.…”

“…Two proposed complementary missions address the need for greater accuracy by employing instruments with onboard calibration that permits traceability to international metrological standards. The first of these, Climate Absolute Radiance and Refractivity Observatory (CLARREO) (Wielicki et al 2013), is one of the four toppriority missions recommended in the 2007 decadal survey report by the National Research Council (2007). The second, Traceable Radiometry Underpinning Terrestrialand Helio-Studies (TRUTHS) (Fox et al 2003(Fox et al , 2011, is proposed by the U.K. National Physical Laboratory.…”