Radiometric validation of the Atmospheric Infrared Sounder over the Antarctic Plateau

Validation of infrared satellite instruments, after they have been launched into orbit, has traditionally relied on views of the relatively warm sea surface. The Antarctic Plateau provides a complementary validation target from space because it is also homogeneous over large areas, yet it is relatively cold. During summer, cloud-free conditions occur often and the atmospheric humidity is very low with values of total column water vapor around 1 mm. Radiance validation experiments were conducted at Dome C, Antarctica (75 degrees S, 123 degrees E; 3280 m), in January 2003 and again in December 2003/January 2004 for the Atmospheric Infrared Sounder ( AIRS). The Polar Atmospheric Emitted Radiance Interferometer (PAERI) measured upwelling and downwelling infrared radiance at the surface during overpasses of NASA's Earth Observing System Aqua satellite. Radiosondes were launched coincidentally with many of these overpasses. These data are used to calculate the outgoing spectral radiance at the top of the atmosphere (TOA), which is then compared with spectral radiance measurements from AIRS in selected frequency bands. At the 95% confidence level, the brightness temperature differences are -0.12 +/- 0.24 K (M-07), -0.15 +/- 0.23 K (M-08), and -0.13 +/- 0.24 K (M-09). These results validate the AIRS radiances at low temperatures ( 230 to 250 K) relative to the TOA calculations between 788 and 973 cm(-1). The cold bias may be partially explained by subvisible or undetected clouds. Ground-based validation experiments over the Antarctic Plateau can yield valuable information about the low-radiance performance of infrared satellite instruments. However, one must use ground-based instruments with a large enough field of view to average over the significant radiance variations that exist across individual snow dunes. Validation of AIRS at low radiance is important because many of the geophysical parameters that are of scientific interest are actually retrieved from low radiance values, including upper tropospheric humidity and properties of high clouds.