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Redemann, J, Schmid, B, Eilers, J A, Kahn, R, Levy, R C, Russell, P B, Livingston, J M, Hobbs, P V, Smith, W L, Holben, B N. (2003). Suborbital measurements of spectral aerosol optical depth and its variability at sub-satellite-grid scales in support of CLAMS, 2001. Eos Trans. AGU, 84(46), Fall Meet. Suppl. 2003, Abstract # A32C-06

As part of the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) experiment, July 10 - August 2, 2001, the 14-channel NASA Ames Airborne Tracking Sunphotometer (AATS-14) was operated aboard the University of Washington CV-580 during 10 research flights (\~45 flight hours). Among others, CLAMS research goals included validation of satellite-based retrievals of aerosol properties. AATS-14 measures the direct solar beam transmission at 14 discrete wavelengths (354-1558 nm), yielding aerosol optical depth spectra and columnar water vapor for such validation purposes. Comparisons of AOD between the AERONET Cimel instrument at COVE and airborne measurements by AATS-14 in its vicinity showed good agreement with largest r-square correlation coefficients at wavelengths of 380 and 500nm ($>$0.99). Coordinated low-level flight tracks of the CV-580 during Terra overpass times permitted validation of over-ocean MODIS level 2 (MOD04\_L2) multi-wavelength AOD data (10x10km, nadir) in 16 cases on 3 separate days. While the correlation between AATS-14 and MODIS-derived AOD was poor with an r-square of 0.55, almost 75% of all MODIS AOD measurements were still in the desired uncertainty range ($\pm$0.03$\pm$0.05*AOD). This may be due to the small AODs (generally less than 0.1 at 500nm) encountered in these comparison cases. An analogous coordination exercise resulted in 7 exact over-ocean match-ups between AATS-14 and MISR measurements. The comparison between AATS-14 and the MISR standard algorithm regional mean AODs showed a much stronger correlation with an r-square of 0.94. However, MISR AODs were systematically higher than the corresponding AATS values, with an rms difference of 0.06. AATS data collected during nine extended low-level CV-580 flight tracks were used to assess spatial variability in AOD at horizontal scales up to 100 km. At UV and mid-visible wavelengths, the largest absolute gradients in AOD were 0.1-0.2 per 50 km horizontal distance. In the near IR, analogous gradients rarely measured 0.05. On any given day, the relative gradients in AOD were remarkably similar for all wavelengths, with maximum values of 70% per 50 km and more typical values of 25%. We will discuss the implications of these unique measurements of AOD spatial variability for common validation practices of satellite data products and for comparisons to large-scale aerosol models.

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Updated: 14-Jan-2005