The reliability of the PMS FSSP in the presence of small ice crystals

Abstract

The performance of measurement systems for small ice crystals is an important issue because of the need to characterize cirrus cloud microphysics for climate research. In a cloud formed of small quasi-spherical ice particles, as found in recently formed aircraft contrails, the PMS FSSP probe gives reliable measurements consistent with other probes. This is in contrast to the expectation, based on earlier work by others, that the FSSP measurements are unreliable when ice is present in the cloud. In natural cirrus on the other hand, where there are irregular large ice particles, the FSSP indeed gives the exaggerated response reported earlier. Here simple criteria by which the presence of spherically shaped small ice particles can be recognized are proposed. These criteria provide a useful indication of the occasions in ice clouds when FSSP measurements can be relied on.