High-Resolution Image

AERONET is a NASA sponsored ground based monitoring network that consists of identical multi-channel Cimel radiometers for assessing aerosol optical properties. In addition to aerosol, the network is now beginning to monitor cloud optical properties using AERONET "idle time" inappropriate for aerosol study. When the Sun is blocked by clouds, the radiometers are being programmed so that instead of "going to sleep" they will look straight up and measure zenith radiance. We call this new feature an AERONET "cloud mode." Several Cimel radiometers have now been equipped with this mode.

There have been two major problems with inferring cloud optical depth from measurements of zenith radiance: (i) lack of a unique relationship to determine cloud optical depth, even for "plane-parallel" clouds; and (ii) a strong influence of 3D cloud structure on measured radiance. To solve these problems, a new method was developed that exploits the large brightness of vegetation-covered surfaces at near-infrared wavelengths, when compared to the brightness at visible wavelengths. For example, in the RED spectral region near 670 nm wavelength, chlorophyll in each green leaf absorbs 90-95% of solar radiation, so the brightness of vegetated surface is quite low. But the same leaf absorbs only 10% of incident solar radiation at NIR wavelengths near 870 nm, so the vegetation in NIR reflects much more radiation than in RED. The physical basis for the retrieval is based on the fact that the contrast between NIR and RED radiances tells us how thick are clouds above us. Since green vegetation reflects more of the incoming radiation in NIR than in RED, ground measurements under thin clouds have little contrast between RED and NIR, while thick clouds reflect much more of the surface-reflected radiation in the NIR than in RED, making the contrast in zenith radiances much larger.

If AERONET measurements are combined with surface reflectance properties (estimated from an overflying satellite instrument, MODIS in this case) then zenith radiances at RED and NIR wavelengths can be used to determine the overlying cloud properties. This is illustrated in the figure, using surface data from Terra's MODIS. The left panel shows a calculated set of curves for one particular choice of RED and NIR surface reflectances, and for a range of possible cloud optical depths τ, and cloud fractions A_c. Also plotted are three groups of 10 data-points measured by an AERONET sunphotometer on July 28, 2002 at the DOE/ARM site in Oklahoma at three different times -- 13:45, 13:58, and 14:11 UT. The plot shows that the 3 clusters of AERONET measurements correspond to three different pairs of cloud properties (A_c=0.9; τ=28), (A_c=0.8; τ=22), and (A_c=0.4; τ=12). The right panel shows an image from DOE/ARM's "all sky camera" at 14:00 UT, a time close to that of the middle cluster of points in the left panel. Note that A_c = 0.8 is not a horizontal cloud fraction that would be seen in the surface camera, or the satellite image, but a "radiatively effective" fraction that accounts for the horizontal inhomogeneity of the clouds.

For details see the recent paper "The "RED versus NIR" plane to retrieve broken-cloud optical depth from ground-based measurements" by A. Marshak (NASA GSFC), Y. Knyazikhin (Boston University), K. Evans (UMBC JCET), and W. Wiscombe (NASA GSFC) just published in the August issue of Journal of the Atmospheric Sciences.