Application of Dual-Frequency Millimeter-Wave Doppler Spectra for the Retrieval of Drop Size Distributions and Vertical Air Motion in Rain

Abstract

Millimeter-wave Doppler spectra obtained from the dual-frequency Cloud Profiling Radar System (CPRS) are used to retrieve both the drop size distribution and the vertical air motion in rain. CPRS obtains collocated spectra at W and Ka bands through a single 1-m-diameter lens antenna. The vertical air motion is determined primarily from the 95-GHz Mie scattering from rain, whereas turbulence effects are minimized by correlating the drop size distributions measured at both the 95- and 33-GHz frequencies. The authors describe an iterative procedure that estimates the drop sizes and vertical motions with range and horizontal resolution of 60 m and temporal resolution of 2 s. Model drop size distributions are used to initiate the procedure, but the retrieved distributions and vertical air motions are seen to be independent of the particular model used. Data were gathered to test the procedure during the Ground-Based Remote Sensing Intensive Observation Period (GBRS IOP) sponsored by the Department of Energy Atmospheric Radiation Measurement (DOE ARM) program. The measurements represent the first simultaneous Doppler spectra of rain at these frequencies. The experiment took place in April 1995 at the Cloud and Radiation Testbed (CART) site in Lamont, Oklahoma. Radiosonde and surface measurements of temperature and pressure were used in the retrieval algorithm. Rain events from stratiform and transition region (i.e., decaying from the convective region toward the stratiform region of a storm) clouds were observed and are analyzed in this paper. The rain rate for the stratiform rain case was relatively uniform with small amounts of vertical air motion. Variations of the vertical winds for the transition region case, however, were larger and more frequent and were accompanied by short intense downbursts. The algorithm?s results are best for rain rates higher than 1 mm h?1.