Identification of small ice cloud particles using passive radiometric observations

There is currently significant uncertainty about the extent to which cirrus clouds are composed of ”small” ice crystals smaller than about 20 μm effective radius. This is due in part to concerns that in-situ measurements from aircraft are plagued by ice particle shattering on instrument inlets, artificially biasing effective radii low. Here, we apply space-based measurements to the problem. We find that a space-based infrared split-window technique is less sensitive but more accurate than a visible-near-infrared technique for confident assessment of whether thin cirrus clouds have small effective radii, independent of a normal range of retrieval assumptions. Due to the sensitivities of the infrared split-window technique, however, our method can only accurately determine the presence of small particles for ice clouds with optical depths between roughly 0.5 and 3.0. Applied to MODIS data, we find that a very conservative minimum of 15-20% of such thin cirrus globally are composed of small ice crystals, but that the actual value could be as high as 40%, and even higher for cold clouds or those in the tropics. We find find that retrievals are in good agreement with airborne probe measurements from the CRYSTAL-FACE field campaign, implying that, for the cases examined, the impact of inlet shattering on measurements must have been limited.