Several MODIS cloud product algorithms are being developed at the University of Wisconsin for the generation of day-1 products after the launch of MODIS. MODIS Airborne Simulator (MAS) radiometric data collected form NASA's ER-2 platform is being used to simulate MODIS spectral bands for testing and refinement of the cloud product algorithms. Spectral characterization is an important component of the MAS calibration. MAS LWIR bands are spectrally characterized in ambient conditions using a monochromator and are corrected for source spectral shape and atmospheric attenuation. An atmospheric correction based on LBLRTM forward model transmittances demonstrates that strong spectral absorption features, such as Q-branch CO2 absorption near 13.9 micrometers , are effectively removed from the spectral measurements with the aid of a small spectral position correction. Comparisons of MAS in-flight data to well- calibrated HIS instrument data indicate that MAS LWIR spectral calibration drift over time is less than 5 percent of FWHM. The MODIS CO2 cloud top height retrieval shows small dependence on the spectral characterization, with retrieved cloud top height changing by less than 0.5 km in response to a 5 percent spectral position change. This is within the tolerance of other error sources in the cloud top properties algorithm.
KEYWORDS: Black bodies, Calibration, Long wavelength infrared, Temperature metrology, MODIS, Short wave infrared radiation, Carbon dioxide, Absorption, Clouds, Spectral resolution
The impact of non-unit calibration blackbody emissivity on MODIS airborne simulator (MAS) absolute thermal calibration accuracy is investigated. Estimates of blackbody effective emissivity were produced for MAS infrared channels using laboratory observations of a thermally controlled external source in a stable ambient environment. Results are consistent for spectrally close atmospheric window channels. SWIR channels show an effective emissivity of about 0.98; LWIR channels show an effective emissivity of about 0.94. Using non-unit blackbody effective emissivity reduces MAS warm scene brightness temperatures by about 1 degree Celsius and increases cold scene brightness temperatures by more than 5 degrees Celsius as compared to those inferred from assuming a unit emissivity blackbody. To test the MAS non- unit effective emissivity calibration, MAS and high- resolution interferometer sounder (HIS) LWIR data from a January 1995 ER-2 flight over the Gulf of Mexico were compared. Results show that including MAS blackbody effective emissivity decreases LWIR absolute calibration biases between the instruments to less than 0.5 degrees Celsius for all scene temperatures, and removes scene temperature dependence from the bias.
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