Abstract

An armada of infrared^1-2 and other remote sensing equipment have been developed and deployed to characterize clouds from the ground, from airplanes, and from satellites. Perhaps someday we will be able to trust this equipment to provide all the information we need about clouds - we no longer will have to fly through them and directly record particle statistics - but first, we must properly invert cloud radiative signatures to obtain particle information. To invert, obvious common thought has been that we must have proper numerical models for the single scattering properties of generally nonspherical cloud particles. We have been using a cloud box in the laboratory to measure radiative properties of spherical water³ and hexagonal ice particles⁴ in a controlled environment where clouds can be well characterized. Our first spectral extinction measurements (2-18 µm) were performed on ice clouds grown near water saturation at a variety of temperatures so that a wide range of particle morphologies from nearly equi-axed columns to sector plates were observed.⁴ Then we measured spectral extinction by water clouds so we could learn how to invert, using Lorenz-Mie theory, the extinction spectra to obtain particle size spectra, with the larger goal in mind of eventually retrieving ice particle size spectra using the discrete dipole approximation.³ We decided to first apply the inversion based on the Lorenz-Mie theory for ice spheres to the measured ice spectra (mostly so we could grin and giggle at the anticipated poor results), and found much to our surprise that the retrieved IR spectra matched the measurements even better than the water cloud results! Though the retrieved ice sphere size spectra are qualitatively in accord with the measured results, we have not yet found a proper principle to guide us in converting from the geometry of a hexagonal ice crystal to the geometry of a sphere. Ok, so the Mie model seems to work for mimicking the infrared spectral extinction by ice crystals, but are the inverted size spectra reasonable and useful? Come to the talk and find out.

© 1997 Optical Society of America