Remote sensing of small ice crystal concentrations in relation to FSSP measurements

Considerable controversy exists regarding the concentrations of small (D < 70 µm) ice crystals in cirrus clouds based on in situ measurements. Aircraft probes that measure ice crystals undetected by the 2DC probe (D < 70 µm), such as the FSSP, CPI, and SID (developed in the UK), have inlets that may shatter large ice crystals. Thus the measured high concentrations of small crystals by these probes may be partially due to this postulated shattering phenomena. The issue of small crystals is critical to resolving the radiative role that cirrus play in climate feedback processes and the earth's radiation balance. For example, including these small crystals measured by the FSSP typically reduces effective diameters based on 2DC spectra by 45%.

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.