Tracking long-term stability of Aqua MODIS and AIRS at different scan angles

Abstract

The near-simultaneous observations from Moderate Resolution Imaging Spectroradiometer (MODIS) and Atmospheric Infrared Sounder (AIRS) sensors on-board the Aqua spacecraft provide a good opportunity to track the relative calibration stability of both sensors over their entire mission. The AIRS is a hyper-spectral sensor with a spectral resolution of around 5 nm that covers the spectral range of most MODIS thermal emissive bands (TEB) except for a few small gaps. The simulated MODIS radiances can be derived by convolving AIRS spectral measurements with MODIS relative spectral response functions. Using spatially collocated pixels, the differences between MODIS observed and AIRS simulated brightness temperatures are computed for most MODIS TEB at various scan angles within ±49.5 degree of the nadir position. Two regions of different scene temperatures are selected: the central Atlantic Ocean and the Dome C at Antarctica. The trending of the MODIS - AIRS brightness temperature differences (BTD) is derived for years 2003- 2008. Results show that the magnitudes of the BTD are spectral band dependent. The values of BTD are generally less than ± 0.5K for most TEB with a few exceptions. For band 27, the BTD are about -1.2 K over the Dome Concordia and about -3.0 K over the northern Atlantic Ocean. Bands 35 and 36 have BTD about +1.0 K over the Atlantic Ocean. The trending results show that the BTD changes over a six-year period are less than 0.3 K for all calculable MODIS bands, and are slightly larger at large scan angles than those near nadir. Our results are consistent with previous studies.