Since no direct size distribution (SD) measurements have been able to resolve this issue, we have addressed this issue from the angle of remote sensing, either from ground or from satellite. This avoids the issue of whether small ice crystals are artifacts of shattering at the probe inlet. From ratios of the absorption optical depth at a moderate and strongly absorbing wavelength, the amplitude of the small crystal mode of the SD can be retrieved using the modified anomalous diffraction approximation (MADA). MADA predicts the absorption/emission phenomena of an ice cloud. Such a retrieval is possible since MADA is formulated in terms of the SD parameters. Retrieval results are consistent with in situ size spectra based on the FSSP and 2DC probes, which have been parameterized as a function of temperature and ice water content for mid-latitude cirrus. Retrieval results are also consistent with our parameterization of tropical' cirrus (of convective origin), which predicts very high concentrations of small crystals, especially at colder temperatures.
This study also provides evidence that a fundamental difference exists in the ice particle size spectra (and radiative properties) of tropical versus mid-latitude cirrus. This has to do with the opposite temperature dependence of the small particle mode. In fact, the tropical anvil SD scheme predicts that at temperatures < -50 deg.C, the small crystals strongly dominate the cloud radiative properties. More case studies are needed to resolve these issues.