Meteorological research applications of MM-wave radar

Abstract

MM-wave radar has now been developed well beyond that of simply providing qualitative information about the presence or location of clouds. Uncertainty about cloud properties leading to gross errors in climate model results has provided the impetus to develop mm-wave radars into reliable, quantitative tools for studying clouds. Besides depicting the small-scale (a few tens of meters) features of tenuous cirrus and low level stratus clouds, the 3 mm and 8 mm wavelength radars described here can examine the physical structure, dynamics and small-scale turbulence of clouds when used alone. Polarization capability of these radars is now generating new information about the deformity of cloud particles needed for calculations of radiation budgets of clouds. When used with other sensors such as lidar or radiometers, additional cloud microphysical information can be retrieved. We discuss here two different ways to calculate ice mass content profiles from radar/lidar data and from radar/IR radiometer data. Mm-wave radar is most suited for these calculations because of complications introduced by 1) Bragg (refractivity) scatter, 2) the lower resolution, and 3) ground clutter effects at longer wavelengths. Combining radar and microwave radiometer data is shown to provide liquid water profiles in warm marine stratus clouds. The small size and weight of mm-wave radars make them particularly suitable for use on aircraft and satellite platforms and we show recent results from an airborne system to make that point. The technology has now advanced to the point where unattended, vertically-pointing, Doppler mm-wave radars will soon be commonly used in research applications.