Infrared remote sensing of ambient small particles observed during ARM SPARTICUS

Description

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 as a constraint on these in situ estimates. Based upon signal-to-noise considerations, a retrieval scheme based upon the infrared split-window technique was designed to confidently identify the presence of small ice particles, regardless of potential inversion assumptions. The more commonly used operational retrieval approach using visible and near-infrared measurements was shown to suffer from potential uncertainties in inversion assumptions such as surface albedo and crystal habit that may make retrieved effective radii highly dubious. Airborne probe estimates of particle size from the ARM SPARTICUS field campaign were compared to infrared observations from Aqua MODIS instrument for co-incident A-Train overpasses. For these cases, agreement between retrieval results and in situ estimates were good in general, implying that the impact of inlet shattering on measurements must have been limited.