THE RELIABILITY OF THE PMS FSSP IN THE PRESENCE OF SMALL TEE CRYSTALS

The performance of measurement systems for small ice crystals is an im portant issue because of the need to characterize cirrus cloud microph ysics for climate research. In a cloud formed of small quasi-spherical ice particles, as found in recently formed aircraft contrails, the PM S FSSP probe gives reliable measurements consistent with other probes. This is in contrast to the expectation, based on earlier work by othe rs, that the FSSP measurements are unreliable when ice is present in t he cloud. In natural cirrus on the other hand, where there are irregul ar large ice particles, the FSSP indeed gives the exaggerated response reported earlier. Here simple criteria by which the presence of spher ically shaped small ice particles can be recognized are proposed. Ther e criteria provide a useful indication of the occasions in ice clouds when FSSP measurements can be relied on.