Winter 2008
JAIVEx: A collaboration and validation success
Nick Ciganovich and Denny Hackel integrate the S-HIS into the WB-57.
In October 2006 the Europeans launched their first operational polar-orbiting satellite, MetOp-A. Among the new instruments flying on this platform, the Infrared Atmospheric Sounding Interferometer (IASI) has helped to strengthen an international effort to monitor and better understand our planet’s environmental systems. To assess the accuracy of the data from this important instrument, American and European agencies successfully collaborated in the Joint Airborne IASI Validation Experiment (JAIVEx). Scientists and engineers from SSEC made significant contributions to JAIVEx during the Texas-based field campaign from 14 April to 7 May 2007 and to the data analysis that followed.
IASI was designed to gather operational meteorological sounding data with a very high level of accuracy, as part of the Global Earth Observation System of Systems (GEOSS). Begun in 2005 the GEOSS project seeks to utilize existing and new hardware and software to supply data and information at no cost to the global science community. In addition to improving medium-range weather forecasts, IASI data will complement the measurements from the U.S. advanced sounder, the Cross-track Infrared Sounder (CrIS), that is scheduled for launch in 2009 on the NPOESS Preparatory Platform (NPP).
Conducted shortly before MetOp-A went operational, JAIVEx was centered around three primary objectives. First, participants sought to validate and characterize the radiometric performance of IASI. The second objective was to validate the performance of different algorithms designed to retrieve temperature, humidity, ozone and carbon monoxide profiles from IASI spectral radiance measurements, over land and ocean, and under cloudy as well as clear sky conditions. Finally, the field program allowed participants to gather a diverse set of IASI spectra with co-located airborne and in-situ observations. This data set will further the development of innovative techniques to assimilate IASI data into numerical weather prediction models, utilizing as many channels as possible.
The IASI sensor design has roots in a concept developed at SSEC in the late 1980s. The concept also inspired two similar aircraft-based, high spectral resolution infrared interferometer sounders that measure upwelling terrestrial and atmospheric emitted radiance: SSEC’s Scanning High Resolution Interferometer Sounder (S-HIS) and the NPOESS Airborne Sounder Testbed-Interferometer (NAST-I). S-HIS and NAST-I have participated in numerous field campaigns where the data have been applied to the development and validation of radiative transfer models, atmospheric sounding algorithms, surface and cloud properties, and sensor trade studies. Primarily funded through the U.S. Integrated Program Office (IPO), both S-HIS and NAST-I contributed to the JAIVEx campaign as a part of the high altitude NASA WB-57 instrument suite.
Figure 1. IASI 900 cm-1 window brightness temperature swath on 19 April 2007 (background image). ARM site enlargement shows imager data with individual IASI FOVs (large circles) and WB57 flight track with S-HIS and NAST-I nadir view FOVs. Spectral data averaged within the red box is provided in Figure 2.
JAIVEx also included a second aircraft: a modified BAe 146 from the United Kingdom. The Facility for Airborne Atmospheric Measurements (FAAM) BAe 146, carried the Airborne Research Interferometer Evaluation System (ARIES). Much like S-HIS and NAST-I, ARIES is similar in design to IASI. The FAAM also carried a number of in-situ aircraft probes, and had the ability to release dropsondes to further assess the atmospheric state of the troposphere.
Both aircraft flew out of Ellington Field in Houston, Texas, the home base of the NASA WB-57. This location provided access to uniform scene conditions for flights over the Gulf of Mexico and a wealth of ground-based measurements from the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) program central facility located in north central Oklahoma.
For most of the field campaign the WB-57 flew at a nominal altitude of 59,000 ft, while the BAe 146 sampled the atmosphere below the WB-57 at altitudes between 100 and 35,000 ft. High-altitude observations coincident with the satellite overpasses provide NIST (National Institute for Standards and Technology) traceable validation of the on-orbit satellite observations. Flight durations of five to six hours facilitated comparisons of the MetOp-A operational measurement capability with observations from NASA’s A-Train research satellites, primarily from the Atmospheric Infrared Sounder (AIRS) aboard the EOS Aqua platform. Because both MetOp-A and the A-Train are in ascending polar orbits with a four-hour time gap, a five- to six-hour aircraft sortie ensured that the aircraft sensor data could be used as a calibration transfer reference for each of the satellite systems. The data analysis team is focusing on results from one case, a clear sky IASI, S-HIS, and NAST-I measurement intercomparison which occurred over the ARM site on 19 April 2007.
IASI Radiometric Validation Study
The 19 April 2007 flight offered a prime opportunity to compare IASI to both S-HIS and NAST-I measurements during clear sky conditions above the ARM site. The IASI ground swath was nearly centered over the ARM site, which provided coincident nadir views with the aircraft instruments within minutes of the MetOp-A satellite overpass. The background image in Figure 1 shows the IASI 900 cm-1 window brightness temperature measurement swath over the continental United States for this case. These data yield a quick, rough estimate of the effective surface temperature for each point (e.g., a synopsis of clear versus cloudy conditions). Ground-based lidar measurements at the ARM site confirmed that the sky was clear. The foreground image in Figure 1 is an enlargement of the original image that shows select IASI, S-HIS, and NAST-I FOVs superimposed over IASI imager data. Given the large variation in FOV size between the satellite and aircraft instruments, the data within the red box were averaged for each instrument to allow more accurate comparisons.
Figure 2. IASI (black), NAST-I (blue), and S-HIS (red) measured brightness temperature, apodized to match S-HIS spectral resolution, for the respective instrument FOVs represented within the red box in Figure 1. Discrepancies are due to atmospheric contributions between the satellite (IASI) and WB57 aircraft (NAST-I and S-HIS).
Figure 2 shows the mean brightness temperature spectra for IASI (black), S-HIS (red), and
NAST-I (blue) measurements within the red box in Figure 1. Because each instrument has a
different spectral resolution, IASI and NAST-I data have been mathematically resampled to
match the S-HIS spectral resolution. Spectral regions that provide atmospheric contributions
(e.g., 2200-2400 cm-1) between the satellite and aircraft altitude (about 59,000 ft) have
noticeable differences resulting from the atmospheric emission that occurred above the aircraft.
However, focusing on select spectral regions where the emission contributions are limited
to sub-aircraft altitude, shows that measurements from all three instruments agree to within
0.2 K. This first look at clear sky results from the flight on 19 April 2007 demonstrates
that IASI performance is outstanding, with data suggesting that radiometric calibration
is on the 0.1 K level. Such accuracy is necessary to provide improved temperature, water
vapor and trace gas soundings, and to initialize numerical weather prediction models.
Overall, the JAIVEx flights included four ARM flights and three Gulf of Mexico flights—including
five joint MetOp-A and Aqua missions. Results shown here are only a subset of the data gathered
in this campaign.
JAIVEx has been heralded as not only a rewarding calibration/validation field program but also a successful US-European collaboration. The latter is likely the most important--a precursor to the future of operational remote sensing measurements within the GEOSS objectives—as our planet depends on successful global collaboration to overcome the fiscal responsibilities required to place state-of-the-art satellite systems in orbit; and to provide the human resources necessary to analyze, comprehend, and validate an otherwise overwhelming amount of data.
