Grant Matthews

Multi-Instrument Comparison of Top-of-Atmosphere Reflected Solar Radiation

Abstract Observations from the Clouds and the Earth’s Radiant Energy System (CERES), Moderate Resolution Imaging Spectroradiometer (MODIS), Multiangle Imaging Spectroradiometer (MISR), and Sea-Viewing Wide-Field-of-View Sensor (SeaWiFS) between 2000 and 2005 are analyzed in order to determine if these data are meeting climate accuracy goals recently established by the climate community. The focus is primarily on top-of-atmosphere (TOA) reflected solar radiances and radiative fluxes. Direct comparisons of nadir radiances from CERES, MODIS, and MISR aboard the Terra satellite reveal that the measurements from these instruments exhibit a year-to-year relative stability of better than 1%, with no systematic change with time. By comparison, the climate requirement for the stability of visible radiometer measurements is 1% decade−1. When tropical ocean monthly anomalies in shortwave (SW) TOA radiative fluxes from CERES on Terra are compared with anomalies in Photosynthetically Active Radiation (PAR) from SeaWiF...

Celestial body irradiance determination from an underfilled satellite radiometer: application to albedo and thermal emission measurements of the Moon using CERES

The Clouds and the Earths Radiant Energy System (CERES) is a program that measures the Earth radiation budget (ERB) from two polar orbiting satellite platforms. CERES radiometers are designed to make stable broadband measurements of scattered solar and emitted thermal radiative flux leaving Earth with an accuracy of 1% or better. Using versatile and programmable scan modes, it is also possible for every CERES instrument to view the Moon on each orbit. However, until now, it has not been possible to derive absolute measurements of lunar irradiance using CERES because the Moons disk fills only 10% of the telescope field of view. This work presents a method of integrating CERES raster-scan data in order to obtain a measurement of the average scattered solar and emitted thermal radiance from the entire lunar disk. The technique results in excellent agreement between CERES instruments on different satellites as to lunar albedo and emitted thermal flux. The average broadband Moon albedo is measured by CERES at a value of 0.1362 (+/-2-3%) when normalized to a static lunar phase angle of 7 degrees using the U.S. Geological Survey lunar irradiance Robotic Lunar Observatory model. The method for the first time also yields very accurate measurements of the thermal irradiance emitted from the Moon. These suggest an average long-wave flux of 977 Wm(-2) (+/-2-3% at 7 degrees phase), implying an approximate mean surface temperature of around 92 degrees C. Statistical analysis on available data suggests that a CERES instrument performing monthly lunar measurements could utilize the Moon as a stability target and reduce calibration drifts to 0.3% per decade or less within an instruments lifetime. Given the success of the technique, a solar calibration system is proposed that will allow precise tracking of an ERB instruments optical degradation using the Sun.

Validation protocol for climate quality CERES measurements

The CERES Flight Model-1 and -2 instruments flew aboard the Terra into orbit in December 1999 and the FM-3 and -4 instruments flew on the Aqua spacecraft in May 2002. To date these instruments have provided seven years of measurements on Terra and five years on Aqua. The accuracy requirement for CERES is 0.5% for longwave radiances and 1.0% for shortwave. Achieving this objective is possible by using experience from the ERBE instrument to evolve the CERES design and the methods for analyzing the data. In order to achieve and maintain this accuracy, an internal calibration system and an attenuated view of the Sun are used. Subsequently, to validate that this accuracy has been achieved, a number of techniques have been developed which cover a range of temporal and spatial scales. This ensemble of methods provides a protocol which assures that the CERES measurements are of climate quality. In addition to retrieving fluxes at the top of the atmosphere, the CERES program uses data from other instruments aboard the spacecraft to compute the radiation balance at the surface and at levels through the atmosphere. Finally, the CERES data products are upgraded as higher-level data products show the need for revisions. The calibration stability is better than 0.2% and traceability from ground to in-flight calibration is 0.25%

Coloration Determination of Spectral Darkening Occurring on a Broadband Earth Observing Radiometer: Application to Clouds and the Earth's Radiant Energy System (CERES)

It is estimated that in order to best detect real changes in the Earths climate system, space based instrumentation measuring the Earth Radiation Budget (ERB) must remain calibrated with a stability of 0.3% per decade. Such stability is beyond the specified accuracy of existing ERB programs such as the Clouds and the Earths Radiant Energy System (CERES, using three broadband radiometric scanning channels: the shortwave 0.3 - 5μm, total 0.3- > 100μm, and window 8 - 12μm). It has been shown that when in low earth orbit, optical response to blue/UV radiance can be reduced significantly due to UV hardened contaminants deposited on the surface of the optics. Since typical onboard calibration lamps do not emit sufficient energy in the blue/UV region, this darkening is not directly measurable using standard internal calibration techniques. This paper describes a study using a model of contaminant deposition and darkening, in conjunction with in-flight vicarious calibration techniques, to derive the spectral shape of darkening to which a broadband instrument is subjected. Ultimately the model uses the reflectivity of Deep Convective Clouds as a stability metric. The results of the model when applied to the CERES instruments on board the EOS Terra satellite are shown. Given comprehensive validation of the model, these results will allow the CERES spectral responses to be updated accordingly prior to any forthcoming data release in an attempt to reach the optimum stability target that the climate community requires.

Radiometric performance of the CERES broadband radiometers on the Terra and Aqua spacecraft

The Clouds and the Earths Radiant Energy System (CERES) is the only program currently measuring the global Earth Radiation Budget (ERB) from space. Two CERES units are located on the EOS Terra platform and two more are placed on the EOS Aqua satellite. Each of the four operational CERES instruments uses three broadband radiometric scanning telescopes: the shortwave (SW 0.3 - 5μ), total (0.3 - >100μ), and window (8 - 12μ) channels. Rigorous pre-launch ground calibration and in-flight calibration is performed on each CERES unit to achieve an accuracy goal of 1% for SW flux and 0.5% for outgoing LW flux.

Transfer of radiometric standards between multiple low earth orbit climate observing broadband radiometers: application to CERES

The Clouds and the Earths Radiant Energy System (CERES) is the only project currently measuring the global Earth Radiation Budget (ERB) from space. Two CERES instruments are located on the EOS Terra platform and two more are placed on the EOS Aqua satellite. One more CERES unit provided 8 months of ERB data in 1998 from the TRMM platform. Each of the CERES devices uses three broadband radiometric scanning telescopes: the shortwave (SW 0.3 → 5μm), Total (0.3 → 100μm), and window (8 → 12μm) channels. Rigorous pre-launch ground calibration is performed on each CERES unit to achieve an accuracy goal of 1% for Short Wave (SW) and 0.5% for outgoing Long Wave (LW) radiance. Any ground to flight or in-flight changes in radiometer response is monitored using onboard calibration sources. For the total and window channels these take the form of concentric groove blackbodies, while the SW channels use stable tungsten lamps. Recent studies have shown that the SW response of space based broadband radiometers can change dramatically due to optical contamination. With these changes having most impact on optical response to blue-UV radiance, where tungsten lamps are largely devoid of output, such changes are hard to monitor accurately using existing on-board sources. This study details an attempt to use the vicarious stability metric of deep convective clouds (DCC), nighttime LW scenes and a newly developed SW optical darkening model to place all CERES instrument measurements on the same radiometric scale. The results show that scene dependant dispersion in nadir comparisons between instruments on the same satellite are significantly reduced. Also the suggestion is that the pre-flight contamination of the CERES instruments may require an increase in Terra and Aqua measured SW flux. A larger necessary increase in Aqua SW flux is believed to be due to greater pre-flight contamination of the CERES Aqua optics.

Spectral balancing of a broadband Earth observing radiometer with co-aligned Short Wave channel to ensure accuracy and stability of broadband daytime Outgoing Long-Wave Radiance measurements: Application to CERES

In order to best detect real changes in the Earths climate system, it is estimated that in space based instrumentation measuring the Earth Radiation Budget (ERB) must remain calibrated with a stability of 0.3Wm−2 per decade and reach an absolute accuracy of 1Wm−2. Such stability is beyond that specified by existing ERB programs such as the Clouds and the Earths Radiant Energy System (CERES, using three broadband radiometric scanning channels: the shortwave (SW 0.3−5um), Total (0.3− > 100um), and window (8−12um)). The CERES measurement of daytime outgoing longwave radiance (OLR) is obtained using subtraction of the SW channel signal from that of the co-aligned Total channel telescope. This requires precise balancing of the estimated response of the Total channel optics with those of the SW only channel when viewing daytime Earth scenes. Any post ground calibration contamination of Total channel optics that reduces its response to SW radiance can therefore upset this balancing process, introducing biases and trends in measurements of daytime LW radiance. This paper presents a new methodology used for balancing Total and SW channel spectral responses for all daytime Earth scenes using a model of contaminant spectral darkening. The results of the technique when applied to both CERES units on Terra are shown to remove significant trends and biases in measurements of daytime LW radiance.

Analysis of clouds and the Earth's radiant energy system (CERES) lunar measurements

Clouds and the Earths Radiant Energy System (CERES) instruments were designed to measure the reflected shortwave and emitted longwave radiances of the Earths radiation budget and to investigate the cloud interactions with global radiances for the long-term monitoring of Earths climate. The CERES instrument with the three scanning thermistor bolometers measure broadband radiances in the shortwave (0.3 to 5.0 micrometer), total (0.3 to >100 micrometer) and 8 - 12 micrometer water vapor window regions. The four CERES instruments (Flight Models 1 through 4) aboard Earth Observing System (EOS) Terra and Aqua platforms were instrumental in conducting lunar radiance measurement on a regular basis. Moon-reflected solar radiances were measured with the shortwave sensor while both moon-reflected solar and moon-emitted longwave radiances were measured using the total sensor. The CERES sensors performed lunar measurements at various phase angles ranging from four to ten degrees before and after each full moon phase. Additional measurements were also conducted during the lunar eclipse events. The resulting filtered radiances were normalized to the mean sun-moon distance and the mean earth-moon distance. The lunar radiances measured by the sensors from all CERES instruments for a period of January 2001 to June 2007 were analyzed and compared. The CERES total sensor results showed a variation of about +/- 0.5 percent, while a +/- 2.0 percent variation was seen in shortwave sensor results.

Calculation of the static in-flight telescope-detector response by deconvolution applied to point-spread function for the Geostationary Earth Radiation Budget experiment

The Geostationary Earth Radiation Budget (GERB) experiment is a broadband satellite radiometer instrument program intended to resolve remaining uncertainties surrounding the effect of cloud radiative feedback on future climate change. By use of a custom-designed diffraction-aberration telescope model, the GERB detector spatial response is recovered by deconvolution applied to the ground calibration point-spread function (PSF) measurements. An ensemble of randomly generated white-noise test scenes, combined with the measured telescope transfer function results in the effect of noise on the deconvolution being significantly reduced. With the recovered detector response as a base, the same model is applied in construction of the predicted in-flight field-of-view response of each GERB pixel to both short- and long-wave Earth radiance. The results of this study can now be used to simulate and investigate the instantaneous sampling errors incurred by GERB. Also, the developed deconvolution method may be highly applicable in enhancing images or PSF data for any telescope system for which a wave-front error measurement is available.

Determination of wavelength-dependent spectral darkening occurring on a broadband Earth observing radiometer: application to clouds and the Earth's radiant energy system (CERES)

It is estimated that in order to best detect real changes in the Earths climate system, space based instrumentation measuring the Earth Radiation Budget (ERB) needs to remain calibrated with a stability of 0.3% per decade. This stability is beyond the specification of existing ERB programs such as the Clouds and the Earths Radiant Energy System (CERES, using three broadband radiometric scanning channels: the shortwave 0.3 - 5μm, total 0.3- > 100μm, and window 8 - 12μm). It is known that when in low earth orbit, optical response to blue/UV radiance can be reduced significantly due to UV hardened contaminants deposited on the surface of the optics. Typical onboard calibration lamps do not emit sufficient energy in the blue/UV region, hence this darkening is not directly measurable using standard internal calibration techniques. This paper details a study using a model of contaminant deposition and darkening, in conjunction with in-flight vicarious calibration techniques, to derive the spectral shape of darkening to which a broadband instrument is subjected. The model ultimately uses the reflectivity of Deep Convective Clouds as a stability metric. The results of the model when applied to the CERES instruments on board the EOS Terra satellite are shown. Given comprehensive validation of the model, these results will allow the CERES spectral responses to be updated accordingly prior to any forthcoming data release in an attempt to reach the optimum stability target that the climate community requires.