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Application of dual-frequency millimeter-wave Doppler spectra for the retrieval of drop-size distributions in precipitation

John Michael Firda, University of Massachusetts Amherst


This dissertation presents a procedure that uses dual-frequency millimeter-wave Doppler spectra to retrieve the drop-size distribution of rain. Using two frequencies rather than one makes it possible to extract information about the drop-size distribution with less dependence on models. The two data sets can be compared to each other in addition to existing models. Vertical air motion is determined by Mie scattering effects, and turbulence is removed by correlating the two retrieved distributions. An iterative procedure uses retrieved drop-size distributions to estimate vertical air motion, reducing the dependence on a model drop-size distribution. Drop-size distributions and vertical air motion are retrieved at high spatial and temporal resolutions. Data was gathered to test the algorithm with the University of Massachusetts' Cloud Profiling Radar System (CPRS) during April 1995 at 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 at these frequencies. The experiment took place in April of 1995 at the ARM Cloud and radiation Testbed (CART) site in Lamont, OK. The radar obtains colocated spectra at 33.12 and 94.92 GHz by using a single one-meter-diameter lens antenna. Radio-sonde and surface measurements of temperature and pressure, provided by the ARM-site, were used in the retrieval algorithm. Rain from stratiform and convective clouds was analyzed. The stratiform rain case had a relatively uniform rain-rate with small variations in mean velocity, implying that the vertical winds did not vary significantly over time. The convective case showed frequent variations in mean velocity with greater intensity than the stratiform case, as well as short cells of increased precipitation. The algorithm's results were reasonably good for rain-rates higher than 1 $mmhr\sp{-1}$, and vertical air motion estimates were found to be relatively insensitive to the initial drop-size distribution model used after iterating the algorithm with retrieved drop-size distributions.

Subject Area

Electrical engineering

Recommended Citation

Firda, John Michael, "Application of dual-frequency millimeter-wave Doppler spectra for the retrieval of drop-size distributions in precipitation" (1997). Doctoral Dissertations Available from Proquest. AAI9809331.