The potential of spaceborne dual-wavelength radar to make global measurements of cirrus clouds

Spaceborne millimeter-wave radar has been identified as a possible instrume nt to make global measurements in ice clouds, which have an important but p oorly understood role in the earth's radiation budget. In this paper, the a uthors explore the potential of a dual-frequency spaceborne radar to estima te crystal size in cirrus clouds and, hence, determine ice water content an d the shortwave extinction coefficient more accurately than would be possib le using a single radar. Calculations show that gaseous attenuation is not a serious problem for a nadir-pointing radar measuring down to cirrus altit udes at frequencies between 35 and 215 GI Iz, provided the frequencies are chosen to lie in the window regions of the atmospheric absorption spectrum. This enables one to exploit the significant benefits of using frequencies too high to be operated from the ground. Radar reflectivity at 35, 79, 94, 140, and 215 GHz has been calculated from aircraft ice particle size spectr a obtained during the European Cloud Radiation Experiment (EUCREX) and the Central Equatorial Pacific Experiment (CEPEX), and it is shown that overall the most promising dual-wavelength combination for measuring crystal size and ice water content is 79 and 215 GHz. For a minimum radar sensitivity of -30 dBZ, this combination can measure ice water content and median volume diameter with errors of between 10% and 30% when the reflectivity is greate r than -15 dBZ (equivalent to an ice water content of around 0.015 g m(-3)) . If only a single wavelength radar were affordable, then, for estimating i ce water content, 215 GHz would be the preferred choice. Since the two rada rs would be likely to use the same antenna, the authors also consider the e ffect of cloud inhomogeneities to introduce a random error into the reflect ivity ratio because of the different beamwidths at each frequency. It is fo und, using data from the cloud radars at Chilbolton, England, that this is more than 0.2 dB for frequency pairings that include 35 GHz but for all oth er combinations is less than 0.1 dB, which is comparable to the other error s in the system and much smaller than the typical values being measured. No nspherical crystals are shown to have a significant effect on the size meas ured by a nadir-pointing dual-wavelength radar, but the authors present evi dence that this can be largely eliminated by viewing at 45 degrees from nad ir.