Montecarlo computations of solar reflectance from ice and water clouds at near IR frequencies.

The 1.6 and 3.9 BRDF have been investigated for ice/water omogeneous/inhomogeneous clouds with different sun-sensor geometries. Our findings confirm that NIR reflectances at different wavelengths have variable sensitivity to re at different levels inside a cloud layer with higher frequencies channels sounding deeper in the clouds. When high-re profiles within homogeneous clouds are considered, the center of gravity of scattering m is located 3-4 four timeweighting functions at 1.6 m. On the other hand when smalldeeper than at 3.9 droplets/ice crystals are considered the penetration capabilities of the two wavelengths is much closer. In inhomogeneous clouds, the weighting functions depend on the vertical variability of both re and liquid water content. In this case general conclusions cannot be drawn but case by case studies are needed. This topic has to be deepened by considering a greater number of realistic re/wc profiles derived from in situ measurements (including multi-layered clouds). Finally for ice clouds, our preliminary study reveals that, even at high optical thickness details of the phase function seem to be relevant for a correct evaluation of m.the bidirectional reflection function, even at 3.9 More simulations are necessary to precise this sentence, by using different shapes and sizes for the ice particles. The relevance of our results is connected to the fact that the outputs of these simulations can drive bispectral m totecniques that use the combination of 1.6 and 3.9 retrieve vertical profiles of effective radii.