TSI: almucantar examples

An almucantar scan is a measurement of irradiances from the sky at the same elevation as the sun (see wikipedia). Irradiances from such a scan as a function of the angle distance from the sun (derived with 3D vector geometry) give the average scatter function of the aerosol in the atmosphere. The almucantar scan is routinely used in the AERONET global sunphotometer network to derive aerosol properties. Almucatar scans shown here are derived from the TSI images. The camera is not calibrated thus regard the plots here as qualitative. Almucantar data from the AERONET instrument at JOYCE can be found here.

Below are shown on the right side plots of the scatter fnctions derived from the tsi images. Colors indicate the respective channel of the image (red, green, blue), the (Angstroem) interpolated value for 550nm (yellow) and the Angstroem exponent (gray). Solid lines represent the hemisphere west (to the left of the sun in the image) and dashed lines the hemisphere east of the sun (to the right of the sun in the image).

Very clear air (no horizon brightening) with a strong slope in the scatter functions for the red, green and blue channel (red, green, blue lines) but a rather low spectral slope (angstroem exponent as gray line). Minimum of the scatter function is not reached at 90deg indicating the presence of aerosol (pure Rayleigh scattering by air molecules would have a minimum at 90deg). Note the only small differences in the scatter functions between east and west of the sun. The blue channel is saturated up to scattering angles of about 35 degrees.


Higher aerosol content per volume but low boundary layer (note the narrow band of horizon brightening). Absolutum of angstroem exponent is rather large. The blue channel is saturated up to 55deg scattering angle but as the irradiances are not calibrated this means not necessarily a broader forward scattering peak.


Even thin clouds are clearly visible as strong deviations in the scatter functions and the angstroem exponent.


Nearly homogenous stratus cloud with nearly constant scatter functions and constant angstroem exponent.