A cloud conundrum: optically thick yet invisible ice clouds observed at visible, infrared, and radar wavelengths

A recently discovered and peculiar form of ice cloud occupies the upper troposphere and lowermost stratosphere. These clouds can have a nearly synoptic-scale (e.g. <1000 km) horizontal dimension and have the common characteristic that they are optically opaque at visible and thermal IR wavelengths yet are transparent to 94 gHz cloud radar. These clouds have been observed as the byproduct of pyrocumulonimbus storms, volcanic eruptions, and dust-entrained synoptic scale extratropical cyclones. Striking for their visible and IR optical depth, they are hypothesized to be comprised of an extraordinary cloud-particle number density of systematically small particle size compared to most cirrus clouds. Another curious and relevant observation pertaining to these clouds is a lifetime that exceeds, sometimes on the order of days, the norm for thunderstorm blowoff or cyclone cirrus shields.

In this report we will present case studies illustrating the optical properties of these clouds, show the connection to the causal pyroconvection, volcanic injection, or dust storm, and follow the cloud lifecycle. Our analysis will combine satellite views from polar orbiting and geostationary nadir imagers, CloudSat space-based cloud radar, and CALIPSO lidar in space.