Due to their short wavelength, millimeter radars are capable of detecting very small droplets with diameters of tens of microns. In addition to their high sensitivity, millimeter radars can be configured to have excellent temporal and spatial resolution and can operate with antennas that have a very narrow beamwidth. These factors result in sampling volumes that are very small compared with those of longer wavelength radars. This reduced sampling volume decreases the effects of the Doppler spectrum broadening due to turbulence. The capability of mm-wave radars for cloud detection and their portability make them a good tool for studying cloud microphysics and dynamics of boundary layer clouds and cirrus. Because of the deep Mie backscattering oscillations occurring in the raindrop particle size range, the 94-GHz radars are also an attractive choice for vertical air motion and drop size distribution measurements in rain, particularly when used in conjunction with centimeter wavelength Doppler radars.
This paper will provide an overview of recent activities involving the development of cloud radars and associated retrieval techniques and a review of results from process studies and long-term monitoring projects where mm-wavelength radar observations play a critical role in improving our understanding of cloud and precipitation processes.