A comparative study of drop size distribution retrieval using two video disdrometers and a UHF wind profiling radar

Knowledge of the drop size distribution (DSD) is critically important for the accurate retrieval of all rainfall parameters from weather radar data. Vertically pointing wind profiling radars can be used to directly measure the DSD by means of the Doppler velocity spectrum measured dudring periods of precipitation. In order to extract the DSD from Doppler velocity spectra, it is necessary to know how much of the observed motion is due to the hydrometeors and how much is due to the ambient air. The Bragg (from variations in the refractive index) and the Rayleigh (from hydrometeors) contributions to the spectrum can be separately resolved under the appropriate conditions. After correcting for air motion described by the Bragg scattering component, velocity spectra from the profiler can be used to generate height profiles of the DSD.

This study was conducted in Central Oklahoma at the National Severe Storms Laboratory (NSSL) site in Norman. Two separate two-dimensional video disdrometers (2DVD) of similar design (manufactured by Joanneum Research in Austria) and a UHF Doppler profiler (manufactured by Radian Inc., currently Vaisala Inc.) were located at NSSL during late April and early May of 2005 for calibration purposes. One disdrometer is owned and operated by NSSL and the other is maintained by the National Center for Atmospheric Research (NCAR). On 21 April 2005, thunderstorms initiated by a dryline in western Texas moved into Oklahoma overnight, and a stratiform region associated with the decaying storms passed over the NSSL site by morning.

Both disdrometers and the collocated vertically pointed profiler operating at 915 MHz were collecting data during the precipitation event. Observations from the two disdrometers and the profiler were used to extract the DSDs, which were then compared to examine agreement between the disdrometers and with the profiler.

This is a preliminary study in preparation for future work involving the polarimetric capabilities of the NSSL KOUN radar. Rainfall measurements from the KOUN radar will be compared to observations from a UHF wind profiler and 2DVD located 30 km from the weather radar site.