Hello, another Q to experts out there. Can someone point out reference to measurements of global and diffuse NIR (700-1500 nm) down-welling measurements in clear sky conditions. We are having a hard time explaining as to why at-sensor NIR readings (1.5 km above ground) increase with increasing solar elevation in low vegetation, asphalt, sand etc, but not to the same extent in trees, and especially some deciduous tree species behave really oddly as we see no increase in signals at all! (at view-zenith angles of 0-20 degrees, while solar elevation goes up from 30 to 40 degrees and irradiance should increase 25%). Visible range signals go up with solar illumination as can be expected. If this is not an artifact it has certainly been noted by someone. Anyone familiar with reference or textbook? Our vegetation is very dark at 400-700 nm (rho 0.02-0.04), while rho are 0.2-0.5 in green vegetation, incl. trees).

ilkka