Daniel L. Morstad

Geostationary Enhanced Temporal Interpolation for CERES Flux Products

AbstractThe Clouds and the Earth’s Radiant Energy System (CERES) instruments on board the Terra and Aqua spacecraft continue to provide an unprecedented global climate record of the earth’s top-of-atmosphere (TOA) energy budget since March 2000. A critical step in determining accurate daily averaged flux involves estimating the flux between CERES Terra or Aqua overpass times. CERES employs the CERES-only (CO) and the CERES geostationary (CG) temporal interpolation methods. The CO method assumes that the cloud properties at the time of the CERES observation remain constant and that it only accounts for changes in albedo with solar zenith angle and diurnal land heating, by assuming a shape for unresolved changes in the diurnal cycle. The CG method enhances the CERES data by explicitly accounting for changes in cloud and radiation between CERES observation times using 3-hourly imager data from five geostationary (GEO) satellites. To maintain calibration traceability, GEO radiances are calibrated against Mode...

Characterization of Terra and Aqua MODIS VIS, NIR, and SWIR Spectral Bands' Calibration Stability

The Moderate Resolution Imaging Spectroradiometer (MODIS) has successfully operated onboard the Terra spacecraft for more than 12 years and the Aqua spacecraft for more than ten years. It has 20 reflective solar bands covering the visible (VIS), near infrared (NIR), and short-wave infrared (SWIR) spectral regions. They are calibrated on orbit using regularly scheduled solar diffuser measurements and lunar observations. In recent years, observations over selected ground targets are also used to monitor detector responses at different angles of incidence. This paper provides a brief description of MODIS on-orbit calibration and characterization methodologies and examines the calibration stability of the VIS, NIR, and SWIR spectral bands over the entire missions of both instruments. Results obtained from four different vicarious approaches (deserts, Dome Concordia, deep convective cloud, and simultaneous nadir overpass) show that Terra MODIS VIS and NIR spectral bands have a wavelength-dependent drift in reflectance with a drop up to 8% in the shortest wavelength region. All four approaches have a relative agreement to within 2.0% with an uncertainty of less than 1.5% for most bands. It is anticipated that the improvements made in the MODIS Collection 6, with additional corrections based on the desert reflectance trending results, will significantly reduce, if not completely remove, some of the trending drifts identified in the Collection-5 data product.

The Characterization of Deep Convective Clouds as an Invariant Calibration Target and as a Visible Calibration Technique

Deep convective clouds (DCCs) are ideal visible calibration targets because they are bright nearly isotropic solar reflectors located over the tropics and they can be easily identified using a simple infrared threshold. Because all satellites view DCCs, DCCs provide the opportunity to uniformly monitor the stability of all operational sensors, both historical and present. A collective DCC anisotropically corrected radiance calibration approach is used to construct monthly probability distribution functions (PDFs) to monitor sensor stability. The DCC calibration targets were stable to within 0.5% and 0.3 % per decade when the selection criteria were optimized based on Aqua MODerate Resolution Imaging Spectroradiometer 0.65-μm -band radiances. The Tropical Western Pacific (TWP), African, and South American regions were identified as the dominant DCC domains. For the 0.65-μm band, the PDF mode statistic is preferable, providing 0.3% regional consistency and 1% temporal uncertainty over land regions. It was found that the DCC within the TWP had the lowest radiometric response and DCC over land did not necessarily have the highest radiometric response. For wavelengths greater than 1 μm, the mean statistic is preferred, and land regions provided a regional variability of 0.7 % with a temporal uncertainty of 1.1 % where the DCC land response was higher than the response over ocean. Unlike stratus and cirrus clouds, the DCC spectra were not affected by water vapor absorption.

Optimized identification of worldwide radiometric pseudo-invariant calibration sites

Long-term radiometric stability monitoring of visible and near-infrared Earth observing sensors has been enhanced over the past decade through the use of pseudo-invariant calibration sites on the Earths surface. Significant work has been done to characterize sites primarily in the Sahara and Middle East desert regions, with some additional work at other locations throughout the world. The work described in this paper attempts to locate those sites that can be considered optimal from a temporal stability measure. To accomplish this, an invariant site identification algorithm was developed that can locate a statistically optimal region in an automated fashion. Results generated from studying virtually all previously identified pseudo-invariant sites indicate that there are six sites in the Sahara and Middle East that can achieve variabilities as low as 2% in the visible and near infrared (VNIR) and 2%–3% in the shortwave infrared (SWIR). The Sonoran Desert site was identified in North America and produces 2%–3% variabilities in the VNIR and 4%–5% in the SWIR. In addition, it has a large amount of historical data available for calibration of historic sensors. Additional sites in Dunhuang, China, and Barreal Blanco, Argentina, show excellent potential for long-term monitoring using high-altitude desert sites as well as potential for improved monitoring in the SWIR region. Lastly, regular acquisition of these sites by current and future Earth observing sensors is strongly recommended for additional reductions in uncertainties associated with these sites, for cost-effective monitoring of sensor stability, and for sensor cross-calibration.

The Radiometric Stability and Scaling of Collection 6 Terra- and Aqua-MODIS VIS, NIR, and SWIR Spectral Bands

The Moderate Resolution Imaging Spectroradiometer (MODIS) Calibration Team has recently released the Collection 6 (C6) radiances, which offer broad improvements over Collection 5 (C5). The recharacterization of the solar diffuser, lunar measurements, and scan mirror angle corrections removed most of the visible channel calibration drifts. The visible band calibration stability was validated over the Libyan Desert, Dome-C, and deep convective cloud (DCC) invariant Earth targets, for wavelengths less than 1 μm. The lifetime stability of Terra and Aqua C6 is both within 1%, whereas the Terra C5 degradation exceeded 2% for most visible bands. The MODIS lifetime radiance trends over the invariant targets are mostly within 1%; however, the band-specific target fluctuations are inconsistent, which suggests that the stability limits of the invariant targets have been reached. Based on Terra- and Aqua-MODIS nearly simultaneous nadir overpass (NSNO) radiance comparisons, the Terra and Aqua C6 calibration shows agreement within 1.2%, whereas the C5 calibration exceeds 2%. Because the MODIS instruments are alike, the same NSNOs are used to radiometrically scale the Terra radiances to Aqua. For most visible bands, the Terra-scaled and Aqua C6 radiances are consistent to within 0.5% over Dome-C, DCC, and for geostationary visible imagers having similar spectral response functions, which are used as transfer radiometers. For bands greater than 1 μm, only minor calibration adjustments were made, and the C6 calibration is stable within 1% based on Libya-4.

Desert-Based Absolute Calibration of Successive Geostationary Visible Sensors Using a Daily Exoatmospheric Radiance Model

A desert daily exoatmospheric radiance model (DERM) based on a well-calibrated (reference) geostationary Earth orbit (GEO) satellite visible sensor can be used to transfer the calibration to a (target) GEO sensor located at the same equatorial longitude location. The DERM is based on the reference GEO daily radiances observed over a single pseudoinvariant calibration site (PICS) being that the daily angular conditions are repeated annually for any historical or successive colocated GEO. The GEO-specific PICSs used in the study are first inspected using the well-calibrated Aqua-MODerate Resolution Imaging Spectroradiometer (MODIS) exoatmospheric reflectances for stability. The Libyan Desert site was found to be stable within 1 % over ten years. The average clear-sky daily local-noon interannual variability based on Meteosat-9 0.65- μm top-of-atmosphere radiances over the Libyan Desert is 0.74 %, which implies that the combined surface and atmospheric column is invariant. A spectral band adjustment factor, based on Scanning Imaging Absorption Spectrometer for Atmospheric Cartography spectral radiances, is used to account for sensor spectral response function (SRF) differences between the reference and target GEO. The GEO reference calibration was based on the GEO/Aqua-MODIS ray-matched radiance intercalibration transfer technique. The reference Meteosat-9 DERM and ray-matched calibration consistency was within 0.4 % and 1.9 % for Meteosat-8 and Meteosat-7, respectively. Similarly, GOES-10 and GOES-15 were calibrated based on the GOES-11 DERM using the Sonoran Desert and were found to have a consistency within 1 % and 3 %, respectively.

The Intercalibration of Geostationary Visible Imagers Using Operational Hyperspectral SCIAMACHY Radiances

Spectral band differences between sensors can complicate the process of intercalibration of a visible sensor against a reference sensor. This can be best addressed by using a hyperspectral reference sensor whenever possible because they can be used to accurately mitigate the band differences. This paper demonstrates the feasibility of using operational Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY) large-footprint hyperspectral radiances to calibrate geostationary Earth-observing (GEO) sensors. Near simultaneous nadir overpass measurements were used to compare the temporal calibration of SCIAMACHY with Aqua Moderate Resolution Imaging Spectroradiometer band radiances, which were found to be consistent to within 0.44% over seven years. An operational SCIAMACHY/GEO ray-matching technique was presented, along with enhancements to improve radiance pair sampling. These enhancements did not bias the underlying intercalibration and provided enough sampling to allow up to monthly monitoring of the GEO sensor degradation. The results of the SCIAMACHY/GEO intercalibration were compared with other operational four-year Meteosat-9 0.65-μm calibration coefficients and were found to be within 1% of the gain, and more importantly, it had one of the lowest temporal standard errors of all the methods. This is more than likely that the GEO spectral response function could be directly applied to the SCIAMACHY radiances, whereas the other operational methods inferred a spectral correction factor. This method allows the validation of the spectral corrections required by other methods.

Use of Pseudo-Invariant Sites for Long-Term Sensor Calibration

Over the past several years, pseudo-invariant sites for trending the radiometric gain of multispectral satellite imaging systems has proven to be very useful and accurate. For example, the lifetime calibration of the Landsat 5 Thematic Mapper (TM) was recently updated based in large part upon this approach. This effort reduced the error in the gain estimate from roughly 13 percent to less than 5 percent and integrated a variety of absolute calibration estimates for the sensor. Typically, pseudo-invariant sites are located in arid regions where very little temporal and spatial change occurs over long periods of time and significant extent. Sites in the Sahara desert are especially attractive. However, drawbacks of this approach are that the sites are relatively unacceptable and, for some instruments, only limited numbers of data sets are available. As a result, it would be helpful to use sites that are observed often, such as locations in North America, so that the technique could be applied to more sensors both historically and in the future. The drawback to these sites is that they are typically of much smaller extent and, therefore, are more difficult to use from a geometric perspective. Recent work has shown that small pseudo-invariant sites, if carefully chosen, can also be used for accurate trending of satellite instruments. This paper will illustrate an approach that suggests the calibration site commonly known as Sonoran Desert can be used to observe the characteristic calibration curve of Landsat 5 TM. Because of the significant number of observations of this site, and other similar sites in North America, there is good potential to extend this technique to additional multispectral instruments for enhanced long term monitoring of radiometric gain.

Desert based absolute calibration of visible sensors

Post-launch calibration and characterization of Geostationary Earth Observing (GEO) satellite sensors, which lack on-board visible calibration, is a keenly felt need in many applications for studying long-term global climate changes. This paper proposes a vicarious technique of calibrating GEO visible sensors for the Clouds and the Earths Radiant Energy System (CERES) project, using a kernel-based bidirectional reflectance distribution function (BRDF) model derived over an invariant desert site. The technique is illustrated with two Meteosat-9 visible channels, 0.65um and 0.86um, whose radiometric gains have been computed using a BRDF model of Libya-4 desert site. The Libya-4 BRDF model is TOA based and is derived from ten years of clear-sky Aqua observations over this site. The calibration results are validated by a direct comparison with the corresponding gains derived from the GEO-to-MODIS ray-matching technique.