Application of dual-frequency millimeter-wave Doppler spectra for the retrieval of drop size distributions and vertical air motion in rain

Millimeter-wave Doppler spectra obtained from the dual-frequency Cloud Prof iling Radar System (CPRS) are used to retrieve both the drop size distribut ion and the vertical air motion in rain. CPRS obtains collocated spectra at W and Ka bands through a single l-m-diameter lens antenna. The vertical ai r motion is determined primarily from the 95-GHz Mie scattering from rain, whereas turbulence effects are minimized by correlating the drop size distr ibutions measured at both the 95- and 33-GHz frequencies. The authors descr ibe an iterative procedure that estimates the drop sizes and vertical motio ns 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 indepen dent of the particular model used. Data were gathered to test the procedure during the Ground-Based Remote Sen sing Intensive Observation Period (GBRS IOP) sponsored by the Department of Energy Atmospheric Radiation Measurement (DOE ARM) program. The measuremen ts represent the first simultaneous Doppler spectra of rain at these freque ncies. The experiment rook place in April 1995 at the Cloud and Radiation T estbed (CART) site in Lament, Oklahoma. Radiosonde and surface measurements of temperature and pressure were used in the retrieval algorithm. Rain eve nts from stratiform and transition region (i.e., decaying from the convecti ve 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 o f the vertical winds for the transition region case, however, were larger a nd more frequent and were accompanied by short intense downbursts. The algo rithm's results are best for rain rates higher than I mm h(-1).