RADAR AND RADIATION PROPERTIES OF ICE CLOUDS

The authors derive relations of the equivalent radar reflectivity Z(e) and extinction coefficient alpha of ice clouds and confirm the theory by in situ aircraft observations during the First International Satel lite Cloud Climatology Project Regional Experiment. Equivalent radar r eflectivity Z(e) is a function of ice water content W and a moment of the size distribution such as the median volume diameter D-0. Stratifi cation of the data by Do provides a set of W-Z(e) relations from which one may deduce the dependence of particle density on size. This relat ion is close to that of Brown and Francis and provides confidence in t he methodology of estimating particle size and mass. The authors find that there is no universal W-Z(e) relation, due both to large scatter and systematic shifts in particle size from day to day and cloud to cl oud. These variations manifest the normal changes in ice crystal growt h. The result is that, with the exception of temperatures less than -4 0 degrees C, temperature cannot be used to reliably parameterize the p article size as has been previously suggested. To do so is to risk lar ge possible systematic errors in retrievals. Even if one could measure monthly averages of ice water content, this is inadequate to estimate the monthly radiative effect because of the nonlinearity between the two. The authors show that a sizable fraction of radiatively significa nt clouds would be missed at a radar threshold of -30 dBZ, the value p roposed for a spaceborne cloud-profiling radar.