Detection of Weakly Precipitating Winter Clouds by a NOAA 404-MHz Wind Profiler

Abstract

Recent studies have demonstrated that the 404-MHz wind profilers of the National Oceanic and Atmospheric Administration WPDN (Wind Profiler Demonstration Network) can detect precipitation under most circumstances. Their ability to detect nonprecipitating and weakly precipitating clouds, however, has remained the subject of debate. To address this question, a 35-GHz Ka-band cloud-sensing radar was operated side by side, with a WPDN profiler in Colorado during the winter of 1993. The short wavelength (0.87 cm), finescale resolution, and excellent sensitivity of the Ka-band system to small hydrometeors make it very well suited for detailed measurements of clouds and weak precipitation. Comparisons of data from the two instruments show that in addition to detecting precipitation, the profiler did indeed detect nonprecipitating ice clouds under some circumstances that can be approximately delineated by profiler reflectivity and vertical velocity thresholds. These thresholds are a function of height for the case examined. A weak cloud of given intensity is easier for the profiler to detect if it is located high in the troposphere rather than close to the ground, because it contrasts more strongly against the background of clear-air reflectivity, which generally decreases sharply with height The apparent mechanism of cloud detection by the profiler is Rayleigh backscattering from ice crystals that are larger than typical cloud droplets but have minimal fall speeds.