|
Levy, R. C., L. A. Remer, J. V. Martins, Y. J. Kaufman, A. Plana-Fattori, J. Redemann, P. B. Russell, and B. Wenny, 2005:
Evaluation of the MODIS aerosol retrievals over ocean and land during CLAMS.
J. Atmos. Sci., 62, 974-992.
The Chesapeake Lighthouse Aircraft Measurements for Satellites (CLAMS) took place from July 10 to August 2, 2001, in a combined ocean/land region that included the Chesapeake Lighthouse (COVE) and the Wallops Flight Facility (WFF), both along coastal Virginia, U.S.A. This experiment was designed mainly for validating instruments and algorithms aboard the Terra satellite platform, including the MODerate resolution Imaging Spectrometer (MODIS). Over the ocean, MODIS retrieved optical depths (AOD) at seven wavelengths and an estimate of the aerosol size distribution. Over the land, MODIS retrieved AOD at three wavelengths plus qualitative estimates of the aerosol size. Temporally coincident measurements of aerosol properties were made with a variety of sun-photometers from ground sites, and airborne just above the surface. The set of sun-photometers provided unprecedented spectral coverage from visible (VIS) to the solar near infrared (NIR) and infrared (IR) wavelengths. In this study, we compare AOD and aerosol size retrieved from MODIS with similar measurements from the sun-photometers. Over the nearby ocean, the MODIS AOD in the VIS and NIR correlated well with sun-photometer measurements, nearly fitting a one to one line on a scatterplot. As we move from ocean to land, there is a pronounced discontinuity of the MODIS AOD, where MODIS compares poorly to the sun-photometer measurements. Especially in the blue wavelength, MODIS AOD is too high in clean aerosol conditions and too low under larger aerosol loadings. Using the 6-S radiative code to perform atmospheric correction, we find inconsistency in the surface albedo assumptions used by the MODIS lookup tables. We demonstrate how we can correct the high bias at low aerosol loadings. By using updated urban/industrial aerosol climatology for the MODIS lookup table over land, we show that we can correct the low bias for larger aerosol loadings. Understanding and improving MODIS retrievals over the East Coast may point to strategies for correction in other locations, thus improving the global quality of MODIS. Improvements in regional aerosol detection could also lead to the use
of MODIS for monitoring air pollution.
Full text (pdf)
Back to top
|
