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Graduate Exam Abstract


Leyda Leon

Ph.D. Preliminary

April 10, 2009, 10:00-12:00

Shepardson Room 102

ESTIMATION AND CORRECTION OF WET ICE ATTENUATION FOR X-BAND RADAR


Abstract: In the past, single polarized X-band radars were primarily used (along with S-band radars) for hail detection, first by the Russians and then later for the National Hail Research Experiment. But they were not used alone because of the large attenuation at high frequencies until dual-polarized radars were developed. This fact has brought attention to correct for rain attenuation in order to exploit the advantages of dual-polarized data. Past developed methods to correct for the signal attenuation make use of the close relation between the differential propagation phase ‘phidp’ and path attenuation (or PIA). Their use is known to be successful in rain events, but in the presence of wet ice, these methods are no longer useful because the differential propagation phase is not affected by the isotropic wet ice. This factor was the basis to develop a different technique for estimating the attenuation due to rain and wet ice separately and correct for the wet ice induced attenuation. Here we evaluate a preliminary method that uses the Surface Reference Technique (SRT) alpha-adjustment method to correct for such attenuation. This method was first developed for the TRMM precipitation radar. We assume that S-band data is un-attenuated and is used as a reference. The difference in reflectivities at the end of the beam is attributed to the total attenuation (sum of rain and any wet ice) along the beam. The attenuation due the rain component, if any, is corrected for using the differential propagation phase. Then the alpha value in the A-Z relationship (with fixed beta) is adjusted such that the reflectivities at S-band and the rain-corrected reflectivity at X-band at the end of the beam are forced to match. This adjusted alpha is used to apportion the reflectivity backwards, which assumes the alpha parameter is constant along the beam. Using the adjustment value, specific attenuation due to wet ice is estimated separately from that of specific attenuation due to rain. This preliminary method has been applied to different datasets. It was evaluated in both simulated and measured radar data. Using the CSU-RAMS model a supercell was simulated. A radar emulator was used to simulate radar measurements from this supercell at both X-band and S-band. Results showed good agreement of both corrected reflectivity profiles and wet ice specific attenuation estimation. A dataset from IHOP that had mixed phase was analyzed too. It showed good agreement also, when comparing profiles; moreover wet ice attenuation contours showed agreement with high values of reflectivity as expected in wet ice situations. CASA data was analyzed along with both NEXRAD KTLX and KOUN data. For the light rain case (CASA/KTLX), the dual wavelength ratio at the end of the beam was low as expected for Rayleigh scattering. When corrected for wet ice, the specific attenuation showed agreement with high values in reflectivity at both bands. Finally, this method was applied to CP2 radar data. In the CP2 data analysis, Mie hail signals were noted and were eliminated for the purpose of this research.

Adviser: Dr. V.N. Bringi
Co-Adviser: n/a
Non-ECE Member: Dr. Steve Rutledge, Atmospheric Science Dept.
Member 3: Dr. V Chandrasekar
Addional Members: n/a

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