Radar and Radiation Properties of Ice Clouds

The authors derive relations of the equivalent radar reflectivity Ze and extinction coefficient α of ice clouds and confirm the theory by in situ aircraft observations during the First International Satellite Cloud Climatology Project Regional Experiment. Equivalent radar reflectivity Ze is a function of ice water content W and a moment of the size distribution such as the median volume diameter D0. Stratification of the data by D0 provides a set of W − Ze relations from which one may deduce the dependence of particle density on size. This relation is close to that of Brown and Francis and provides confidence in the methodology of estimating particle size and mass. The authors find that there is no universal W − Ze relation, due both to large scatter and systematic shifts in particle size from day to day and cloud to cloud. These variations manifest the normal changes in ice crystal growth. The result is that, with the exception of temperatures less than −40°C, temperature cannot be used to reliably parameterize the particle size as has been previously suggested. To do so is to risk large 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 significant clouds would be missed at a radar threshold of −30 dBZ, the value proposed for a spaceborne cloud-profiling radar.