A31B-0304
Traceability in Ocean Color: the MOBY Example
Developing and maintaining accurate climate data records on regional and global scales are key objectives of the NOAA Climate Mission Goal. Current climate-relevant satellite ocean remote sensing parameters include sea- surface height, sea-surface temperature, sea-ice, ocean surface winds, and ocean color water-leaving spectral radiance. Many ocean color remote sensing missions, including the Sea-viewing Wide Field-of-View (SeaWiFS) sensor and the Terra and Aqua Moderate Resolution Imaging Spectroradiometer (MODIS) sensors, have developed a set of protocols for on-orbit sensor characterization and calibration that includes in situ vicarious calibration using the Marine Optical BuoY (MOBY), the primary U.S. facility for vicarious calibration of ocean color satellite sensors. Having just celebrated 10 years of continuous operation, over 8400 MOBY data sets have been acquired and used in the vicarious calibration of these ocean color satellite sensors. In this talk, we present the efforts undertaken by the MOBY team to ensure measurement traceability to the International System of Units (SI). The term traceability in this context has a specific meaning, requiring a full analysis and description of uncertainties and an unbroken chain of calibrations back to an internationally recognized primary standard maintained by a national metrology institute (NMI), in this case the U.S. National Institute of Standards and Technology.
A31B-0305
AIRS Pre-flight Calibration and In Orbit Validation
The Atmospheric Infrared Sounder (AIRS) on the EOS Aqua Spacecraft was launched on May 4, 2002. AIRS acquires hyperspectral infrared radiances in the 3.7-15.4 um spectral region with spectral resolution of better than 1200. The AIRS was designed to measure small changes in the hydrological cycle and has demonstrated in-flight exceptional radiometric and spectral stability and accuracy. This accuracy is achieved in orbit by transferring the calibration from a Large Area Blackbody to the on-board blackbody (LABB). The LABB theoretical emissivity is in excess of 0.9999 and temperature uncertainty is less than 50 mK. The LABB emitted radiance is NIST traceable through thermistors located on the internal surfaces. The radiometric accuracy predictions for AIRS based on the On-Board Calibrator, LABB, and pre-flight measurements give an accuracy of 0.2K – 3 sigma. Spectral calibration is achieved through pre-flight testing and in-orbit observation of atmospheric lines. http://airs.jpl.nasa.gov
A31B-0306
Cross-Calibration Approach Improves Accuracy for Hyperspectral Imaging
Hyperspectral imaging of the Earth's surface in the visible and near infrared from the CLARREO mission requires high levels of radiometric accuracy with stability maintained on-orbit. Traditional approaches rely on ground- based calibrations for accuracy and on-board sources for tracking post-launch changes in instrument sensitivity. The proposed on-orbit cross-calibration approach improves radiometric accuracy and stability by transferring the highly accurate CLARREO Total Solar Irradiance Sensor's (TSIS's) spectral irradiance measurements to the Earth-viewing hyperspectral imager, supplementing traditional instrument calibrations by maintaining on-orbit traceability of radiometric accuracy as well as long-term sensitivity tracking. With intermittent solar viewing using signal attenuation of large and well-known magnitude, this hyperspectral imager can acquire solar spectral measurements simultaneously with the on-board TSIS, transferring the TSIS's higher absolute accuracy levels to the hyperspectral imager. We present details of this cross-calibration concept and a CLARREO hyperspectral imager instrument design with the desired solar viewing capability.
A31B-0307
NIST Thermal Emission Metrology in Support of Climate Benchmarks Traceability
Creation of reliable long term climate records requires traceability to international standards such as those maintained in the U.S. by NIST. We will provide an update on recent progress in the area of Thermal Emission Metrology Standards, and discuss the techniques and instrumentation which may help to achieve "climate benchmark" data quality for missions such as CLARREO. NIST's Fourier Infrared Spectrophotometric Laboratory (FTIS) and Advanced Infrared Radiometry and Imaging (AIRI) Facility, among other functions, are providing primary national level support for measurements of absolute spectral emissivity and spectral radiance of materials, cavities and blackbody sources at near ambient temperatures. Scale realization is performed in a systematic manner, employing first principle realization methods, applying the best known complementary and independent techniques, and conducting internal and international comparisons for uncertainty validation. The established measurement capabilities enable pre-flight calibration of space radiometric instruments, as well as in-flight monitoring to maintain and validate their traceability. A program for absolute spectrally resolved radiance traceability of the self-calibrating spaceborne blackbody for the CLARREO mission could serve as an example of such a comprehensive support approach. It integrates pre-flight emissivity and temperature calibration at the coupon, component and system levels, as well as evaluation of tools used for the on-board monitoring and validation of calibrations. The ongoing joint FTIS/AIRI project aimed at the construction of the Controlled Background Emissometer (CoBE) has great synergy with spaceborne radiometry support requirements and would substantially enhance our capabilities to calibrate transfer standard targets and blackbodies in the spectral range of 1 micron to 50 microns and over the temperature range of 200 K to 1000 K in vacuum or strictly controlled environment conditions.
A31B-0308
Climate Data Records of Sea Surface Temperatures
Sea-surface temperature (SST) is a geophysical variable intimately involved in the climate system. It provides the bottom boundary condition for much of the atmosphere, being the primary source of heat for atmospheric energy, and is a potential "global thermometer" that can reveal evidence of a changing climate. It is also one of the few variables that can be derived on global scales using satellite remote sensing with uncertainty characteristics that can be well defined. Given the need for long-term climate records "of high accuracy, tested for systematic errors on-orbit, and tied to irrefutable standards such as those maintained in the U.S. by NIST" the validation of satellite-derived SSTs using shipboard M-AERIs (Marine-Atmospheric Emitted Radiance Interferometers) is being undertaken. The measurements of the M-AERIs can be traced to NIST standards. To achieve an adequate description of the error characteristics the validation data sets should encompass the full climatological range of not only the retrieved variables, but also those that introduce uncertainties. Thus, for example, the validation of SSTs should cover not only the full range of SSTs, but also span the range of atmospheric water vapor distribution. The presentation will describe the approach of establishing and maintaining Climate Data Records of SST involving a long-term campaign of ship-based infrared radiometry.
A31B-0309
SI Traceable Laboratory Determinations of O2 A-Band Line Parameters for Space-Based Measurements of CO2 and Global Surface Pressure
Presently, raw radiance data generated by remote-sensing measurements of the atmosphere must be converted into calibrated radiances using known molecular properties of the atmospheric column. To ensure the integrity and long-term continuity of such retrievals, these calibrations need to be grounded in high-quality intrinsic molecular data with well-defined traceability to the International System of Units (SI). In this context, because the molar fraction of O2 in the atmosphere is accurately known and essentially time invariant, space-based observations of the O2 A-band spectrum are critical to many remote-sensing applications. Consequently accurate characterization of this spectrum constitutes a well-defined path for SI traceability in both space- and ground-based atmospheric retrievals of many atmospheric species. An important example includes retrievals of CO2 columns by orbiting spacecraft, [e.g., NASA's Orbiting Carbon Observatory, (OCO)] in which photon path lengths are to be determined in terms of co-observed O2 A-band spectra. Similarly, accurate measurements of the O2 A-band in support of meteorological observations and weather modeling have the potential to determine global surface pressure. These and other applications require low relative uncertainty (≪ 0.5%) and SI traceability in O2 line parameters such as intensity, transition frequency, pressure-induced frequency shifts and broadening, and other quantities related to line shape and line mixing effects. Here, we present the status of O2 A-band spectral models and reference line parameters data, and we discuss limitations that motivate additional theoretical and experimental research.
A31B-0310
Low-Uncertainty Measurements of O2 A-Band Line Positions
We measured frequencies for 32 transitions of the O2 A-band (b1Σ+g ← X3Σ-u) using the frequency-stabilized cavity ring-down spectrometer located at NIST, Gaithersburg, MD. Absolute spectral positions were calibrated in terms of the hyperfine components of the D1 (12985.19 cm-1) and D2 (13042.90 cm-1) atomic transitions of 39K. A continuous- wave external cavity diode laser synchronously probed the ring-down cavity and 39K absorption cell. The latter system was arranged in a double-pass configuration and yielded Doppler-free saturation spectra of 39K with a resolution of 300 kHz. Using this calibration, the ring-down cavity's free-spectral range (used for interpolation and extrapolation from the 39K transition reference points) was determined to within 80 Hz. The combined uncertainty in the O2 A-band transition frequencies was < 1 MHz, which is more than 40 times lower than previously reported uncertainties for the O2 A-band. Excited state molecular constants were calculated from these results and compared with previously determined values. These new transition frequencies should serve as a convenient secondary calibration standard in the 760 nm to 770 nm wavelength region.
A31B-0311
NIST TXR Validation of S-HIS radiances and a UW-SSEC Blackbody
The ability to accurately validate infrared spectral radiances measured from space by direct comparison with airborne spectrometer radiances was first demonstrated using the Scanning High-resolution Interferometer Sounder (S-HIS) aircraft instrument flown under the AIRS on the NASA Aqua spacecraft in 2002 with subsequent successful comparisons in 2004 and 2006. The comparisons span a range of conditions, including arctic and tropical atmospheres, daytime and nighttime, and ocean and land surfaces. Similar comprehensive and successful comparisons have also been conducted with S-HIS for the MODIS sensors, the Tropospheric Emission Spectrometer (TES), and most recently the MetOp Infrared Atmospheric Sounding Interferometer (IASI). These comparisons are part of a larger picture that already shows great progress toward transforming our ability to make, and verify, highly accurate spectral radiance observations from space. A key challenge, especially for climate, is to carefully define the absolute accuracy of satellite radiances. Our vision of the near-term future of spectrally resolved infrared radiance observation includes a new space-borne mission that provides benchmark observations of the emission spectrum for climate. This concept, referred to as the CLimate Absolute Radiance and REfractivity Observatory (CLARREO) in the recent NRC Decadal Survey provides more complete spectral and time-of-day coverage and would fly basic physical standards to eliminate the need to assume on-board reference stability. Therefore, the spectral radiances from this mission will also serve as benchmarks to propagate a highly accurate calibration to other space-borne IR instruments. For the current approach of calibrating infrared flight sensors, in which thermal vacuum tests are conducted before launch and stability is assumed after launch, in-flight calibration validation is essential for highly accurate applications. At present, airborne observations provide the only source of direct radiance validation with resulting traceable uncertainties approaching the level required for remote sensing and climate applications (0.1 K 3- sigma). For the calibration validation process to be accurate, repeatable, and meaningful, the reference instrument must be extremely well characterized and understood, carefully maintained, and accurately calibrated, with the calibration accuracy of the reference instrument tied to absolute standards. Tests of the S-HIS absolute calibration have been conducted using the NIST transfer radiometer (TXR). The TXR provides a more direct connection to the Blackbody reference sources maintained by NIST than the normal traceability of blackbody temperature scales and paint emissivity measurements. Two basic tests were conducted: (1) comparison of radiances measured by the S-HIS to those from the TXR, and (2) measuring the reflectivity of a UW-SSEC blackbody by using the TXR as a stable detector. Preliminary results from both tests are very promising for confirming and refining the expected absolute accuracy of the S-HIS.
A31B-0312
Constraining Climate Sensitivity Using SI Traceable Satellite Data
Profiles of microwave refractivity obtained by radio occultation using the Global Navigation Satellite System (GNSS) provide a benchmark of climate that is on-orbit traceable to the international definition of the second. We have shown that profiles of microwave refractivity provide a high signal-to-noise ratio measurement of the poleward expansion of the Hadley circulation, a sensitive measure of climate change. This measure of Hadley expansion is obtained through the application of standard methods of fingerprinting. By accounting for the uncertainty in the fingerprint of future climate change, these standard fingerprinting methods may be transformed to provide a powerful means of detecting regional climate change, especially those changes pertaining to bulk atmospheric structure and surface air temperature. In this presentation, we examine the prospect of constraining the transient sensitivity of the climate system with timeseries of GNSS occultation. It is expected that a timeseries of GNSS occultation will serve to estimate the response of the climate to forcing, but timeseries of spectrally resolved infrared radiance will also be necessary to estimate the radiative forcing by anthropogenic greenhouse gases. Together, the transient sensitivity of the climate can be observationally determined. The multi-model ensemble of the Coupled Model Intercomparison Project phase 3 (CMIP3) provides a convenient ensemble of opportunity to evaluate the certainty of the physics that relates trends in atmospheric microwave refractivity to trends in surface air temperature. The physical relationship is strong, and hence the utilization of the refractivity profiles provides a strong suppression of natural variability that decreases the amount of time necessary to estimate the transient sensitivity of the climate. We explore the application of this method to the combined timeseries obtained from the CHAMP and COSMIC missions.
A31B-0313
An investigation of the capability of CLARREO to calibrate operational sounders with a focus on both spatial and temporal sampling uncertainties
In addition to the primary goals of CLARREO of producing climate products and providing rigorous tests of climate forecasts, a secondary goal is to provide an in-orbit standard for the inter-calibration of other infrared space-borne sensors. The goal is to use CLARREO to improve the absolute calibration and consistency of other infrared observations from space. This study considers the case where three CLARREO satellites in 90-degree polar orbits with 120-degree orbital separation are used to transfer the absolute calibration from the CLARREO instruments to an operational advanced IR sounder in a sun-synchronous orbit, e.g. IASI on MetOp or CrIS on NPOESS. This particular inter-calibration goal addresses the needs of the operational weather community for improved absolute accuracy while opening the possibility of using the weather observations to contribute to the long-term climate record. To quantify the radiometric calibration of operational sounders using CLARREO requires that the uncertainties resulting from the inter-calibration process be well characterized and smaller than the required accuracy of the calibration. It is the goal of this study to investigate the accuracy to which CLARREO can be used to inter-calibrate operational sounders (IASI and CrIS) for climate studies and in the process determine the necessary noise characteristics of CLARREO to meet the inter-calibration requirements. The primary sources of uncertainty in the inter-calibration process are from instrument noise and the spatial and temporal sampling differences of the instruments. The impact of random sensor noise is relatively straightforward to assess. The uncertainties resulting from the spatial and temporal sampling differences, which are dynamic, and function of the natural scene variability are more difficult to characterize and the focus of this work. Spatial and temporal collocation errors are analyzed using one year of Aqua MODIS data, along with knowledge of the spatial and temporal sampling characteristics of the sounders (IASI and CrIS) and CLARREO. We find that the inter-calibration between CLARREO and the operational sounders can be performed with an accuracy of less than 0.1 K 3-sigma for MODIS band 32 (11 microns). This assumes monthly averages, nadir CLARREO sampling with 100 km diameter FOVs, and CLARREO single FOV random noise of less than 0.6 K NEDT
A31B-0314
Global Climatologies Based on Radio Occultation Data: A Potential Source for Climate Benchmarks
Radio Occultation (RO) data using Global Positioning System (GPS) signals have the potential to deliver climate benchmark measurements, since they can be traced, at least in principle, to the international standard for the second. The special climate utility of RO data arises from their accuracy and long-term stability due to self- calibration. We developed a retrieval scheme at the Wegener Center, which is especially focused on minimizing potential bias of atmospheric parameters and on using background information (which is needed for high altitude initialization) in a transparent way. The German research satellite CHAMP (CHAllenging Minisatellite Payload for geoscientific research) provided the first opportunity to create RO based climatologies over more than 6 years. Overlap with data from the Taiwan/U.S. FORMOSAT-3/COSMIC (Formosa Satellite Mission 3/Constellation Observing System for Meteorology, Ionosphere and Climate, F3C) mission allows testing the consistency of climatologies derived from different satellites. We show initial results for monthly and seasonal zonal mean dry temperature climatologies. Our results indicate excellent agreement between RO climatologies from different F3C satellites as well as between data from different RO missions. After subtraction of the estimated respective sampling errors, altitude- and latitude-resolved seasonal dry temperature climatologies derived from two different F3C satellites in closely adjacent orbits agree to within < 0.1 K almost everywhere in the considered domain between 8 km and 30 km altitude. While there is still the possibility of common systematic errors, the results obtained so far show that RO data from different satellites can be indeed combined without need for inter- calibration.
A31B-0315
Phase Change Material Development and Space Qualification for Long Term IR Sensor Calibration Stability
The Global Earth Observation System of Systems (GEOSS), which has the goal of providing information support for steady development of the entire world climate system and the mitigation of effects of catastrophic phenomena, provides a calibration challenge to the satellite sensor operators of the world. Because of the climate community's need for a time series of data extending over decades, the most stringent calibration requirements are for the long-term stability and accuracy of optical measurements. Specific climate record accuracy requirements have been debated and published in recent workshops in the USA*. To meet sensor accuracy requirements over the full life of a sensor and through a series of sensors, on-board calibration systems must be improved. A potential solution for IR sensors observing top of the atmosphere radiances that require a calibration target in the 273 to 310 K range is the development and space qualification of phase transition phenomenon temperature calibration devices. Including a reliable, SI traceable temperature calibration verification system on on-board calibration devices can help produce the required high-quality long-term (extending over decades) records. While gallium (302.9146 K) and water (273.15 K) are commonly used references in terrestrial applications, their temperatures are not optimal for earth viewing calibration. We have been developing melting temperature curves for the bimetallic eutectic alloys Ga-In (288.5 K), Ga-Sn (293.5 K), Ga-Zn (298.5 K), and Ga-Al (300.2 K) in small-size cells suitable for space application. Our results show that Ga and some Ga-based eutectic alloys in small cells can be used as stable, SI-traceable melting fixed points. The repeatability of melting transition temperatures of Ga, Ga-In, Ga-Sn, and Ga-Zn fixed points is presented. Plans to validate the behavior of these cells in a microgravity environment are also discussed. * Ohring, G., B. Wielicki, R. Spencer, W.J. Emery, and R. Datla, 2005: Calibration for measuring global climate change: Report of a workshop. Bull. Am. Met. Soc., 86, 1303–1313. *Achieving Satellite Instrument Calibration for Climate Change (ASIC3) Workshop, Lansdowne, VA, May 16-18, 2006
A31B-0316
CLARREO Mission Requirements, Technological Readiness, and Calibration/Validation Concepts
NASA has selected a new climate mission named CLARREO in the recent Decadal Survey from the National Research Council (NRC) as a promising new start in 2008. CLARREO stands for Climate Absolute Radiance and Refractivity Observatory, conveying that it will measure spectrally resolved radiance from the earth and atmospheric bending of GPS signals related to atmospheric structure (refractivity) to detect climate change. The CLARREO mission is based on some new paradigms for making climate benchmark observations. First, when defining the right radiation measurements to choose for a climate record, the goal should be to maximize the information content about atmospheric and surface properties, rather than to monitor the total radiative energy budget, the conventional calorimeter experiment. The idea is to use spectrally resolved radiances to gain sensitivity, because the spectrally integrated total energy budget can miss significant changes that cancel each other out, and at the same time to characterize the changes. For CLARREO, regional averages of nadir-viewing radiance spectra will reveal signatures of changes in climate forcing and response that can be related to changes in temperature and water vapor structure, atmospheric stability, cloudiness or aerosols, surface properties, and trace gases. The far infrared region of the spectrum, out to 200 wavenumber (50 microns), is required for sensitivity to thick ice clouds and upper level water vapor. Radiative signatures from climate models will be used for interpretation. Second, to reduce the time to unequivocally resolve climate trends, IR radiance spectra and GPS refractivity were selected as quantities with high information content that can be measured with high calibration accuracy referenced to international standards provided on orbit (SI measurements). For the infrared radiance spectra, a brightness temperature accuracy of 0.1 K confirmed on orbit is practical (with a 99% confidence that the limit is not exceeded). The accuracy of GPS refractivity depends on time measurements that can be made extremely accurately, with a corresponding accuracy for upper level temperature that can also be better than 0.1 K. Establishing SI measurements in space alleviates the need to overlap subsequent generations of satellites to establish a climate record. Third, CLARREO will make use of a new set of orbits that give coverage of the whole globe and all times of day to minimize sampling biases. Sampling biases have equal importance to measurement errors (the RSS of both contributions gives the overall accuracy). This new sampling approach uses three, equally-spaced, truly polar obits (90º inclination) that do not precess in inertial space. These orbits will cover all latitudes and longitudes, and give equal sampling for all times of day every two months. Recent simulations using MODIS data and NPP PEATE processing capabilities show that these orbits will also allow CLARREO to be used for highly accurate cross-calibration to the high spectral resolution sounders in sun-synchronous orbit (AIRS on NASA Aqua, IASI on EUMETSAT MetOp, and CrIS on NPOESS).
A31B-0317
Testing Climate Model Fidelity Using Lag Autocovariance of Infrared Radiances
A number of studies have shown that climate model sensitivity to changes in radiative forcing can in some cases be diagnosed using statistics based on lag autocorrelations of model variables. This offers the promise of ranking models by their fidelity to real world data using a metric directly relevant to the models' ability to predict climate sensitivty. Here we examine the ability of these statistics to determine model climate sensitivity using infrared brightness temperature at a number of frequencies, and examine how the error properties of realistic simulated satellite data sets propagate to the error in determining climate sensitivity and climate model fidelity.
A31B-0318
New Instrumentation for Characterizing the Moon as a Radiometric Standard for Space-based Radiometry
The need to understand and monitor climate change has led to proposed radiometric accuracy requirements for space-based remote-sensing instruments that are very stringent. However, many of these requirements are unmet by the current fleet of earth orbiting instruments. A major problem is quantifying the changes that instruments undergo during the launch and throughout the mission. While on-orbit calibrators and monitors have been developed, they too can suffer changes from the launch and harsh space environment. One potential solution is to use the moon as a calibration reference source. Already the stability of the moon has been used to remove drift and to cross-calibrate different instruments. But, at present, the uncertainty of the absolute lunar spectral irradiance is too high for absolute on-orbit calibration of climate monitoring instruments. To enable use of the moon as an absolute calibration standard, we present in this paper an Earth-based instrument to measure the lunar spectral irradiance to an uncertainty of 1 % (k=1) over the spectral range from 320 nm to 2500 nm with a spectral resolution of approximately 0.3 %. The instrument would be flown on high altitude balloons and deployed at high elevation astronomical observatories in order to mitigate the effects of the Earth's atmosphere on the lunar observations. Periodic calibrations using advanced instrumentation and techniques available from NIST would ensure SI traceability and low radiometric uncertainties for the lunar irradiance measurements.
A31B-0319
Ultra-Fine-Grained Atmospheric Aerosol and Trace Gas Measurements with Future Space- based Tunable Laser LIDAR: The STARCaL Project
We propose a tunable laser-based satellite-mounted LIDAR system for a very wide variety of applications in atmospheric and climate studies, ground- and space-based astronomical telescope calibration, as well as national defense and security. This broad range of applications is joined by a common need for beam pointability, wavelength tunability from visible through the near-infrared, and precision radiometry, and thus could be accomodated via a single satellite platform. Wavelength tunability from 300 to 2400 um is provided via well-tested optical parametric oscillator technology, and the beam is precisely sampled using an electrical substitution radiometer. Upcoming astrophysical projects investigating the recently- discovered accelerated expansion of the universe (from the so-named "dark energy") require significant improvements in both spectrophotometric and absolute flux calibration of telescopes, which this project provides. Simultaneously, STARCaL provides wavelength-tunable, pointable LIDAR data analogous to CALIPSO, with ultra- precise beam radiometry, for precision measurements of aerosols and trace gases in all parts of the atmosphere. The technology for this project is now well-tested in space and on the ground. http://www.starcal.org