Quantitative precipitation estimation and error analysis with a UHF wind profiling radar and a two-dimensional video disdrometer

During periods of precipitation, vertically pointed wind profiling radars can be used to directly measure the drop size distribution (DSD) through the Doppler velocity spectrum. This is achieved by directly mapping the Doppler spectrum from velocity space into diameter space. In the absence of vertical ambient air motion and if Rayleigh scatter is the dominant contribution to the radar signal, the velocities detected by the radar are primarily due to falling hydrometeors. Under these conditions, the DSD is retrieved from the Doppler spectrum by applying an appropriate expression that relates drop diameter to terminal fall speed. Precipitation parameters such as rainfall rate, radar reflectivity factor, liquid water content, mass-weighted mean drop diameter, and median volume drop diameter can be calculated from the retrieved DSD. Unlike in-situ instruments located at the surface, measurements from the profiler can be used to investigate the evolution of these parameters with height.

Several factors complicate the DSD retrieval process. First, the retrieval method relies on the assumption that there is no significant vertical ambient air motion. The presence of undetected updrafts and downdrafts will bias the retrieved number concentration. Second, there are many choices for fall speed relationships. Since the retrieval method assumes a single fall speed relationship, different expressions will result in different retrievals. Third, an inherent artifact of the DSD retrieval process prohibits accurate retrievals of the number concentration of very small drops, but the smallest diameter at which number concentration information should be included in DSD calculations is somewhat subjective. Fourth, the presence of ground clutter at the lower sampling heights introduces further complications to retrieving information about the smallest drops. Each of these considerations introduces errors into DSD retrievals that propagate through the precipitation estimations. These factors are examined and error analysis is presented.

The present study focuses on precipitation systems passing over central Oklahoma. The principal measurements are made using a 915-MHz boundary layer radar (BLR) and a two-dimensional video disdrometer (2DVD) located near the BLR. Emphasis is placed on non-convective systems due to the assumption of a quiescent environment. Time-height development of several parameters associated with the DSD are presented. In particular, transitions across the melting layer are examined. Rainfall parameters including rainfall rate, radar reflectivity factor, mass-weighted mean diameter, and median volume diameter are compared between the lowest sampled height from the BLR and the 2DVD. This study is motivated by ongoing comparisons with the NOAA National Weather Severe Storms Laboratory polarimetric S-band weather radar KOUN.