Measuring crystal size in cirrus using 35-and 94-GHz radars

Results are presented from a case study in which coincident 35- and 94-GHz radars located at Chilbolton, England, were used to measure crystal size in cirrus clouds. In the presence of larger crystals the 94-GHz radar scatter s sufficiently beyond the Rayleigh regime that the difference in reflectivi ty factor measured by the two can be directly related to size. This enables more accurate estimation of ice water content than would be possible using a single radar. The small crystals at the top of the cloud scatter in the Rayleigh regime at both wavelengths, which provides a valuable method of ca libration, but also means that sizing is not possible in this part of the c loud. Ice water content and median volume diameter were derived and compare d with the analyses of the U.K. Meteorological Office Unified Model. The sm allest measurable median volume diameter in this case study was around 200 mu m, although it is believed that with both radars sensitive down to -35 d BZ, it should be possible to measure median diameters of 100 mu m. Scatteri ng calculations have been carried out to determine the sensitivity of these measurements to crystal density, crystal aspect ratio, and the shape of th e size distribution. Density is found to be the most significant source of uncertainty, possibly introducing errors of 20% into retrieved diameter.