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Title:Field Studies of Raindrop Oscillations With Applications to Radar Meteorology
Author(s):Tokay, Ali
Doctoral Committee Chair(s):Beard, Kenneth V.
Department / Program:Atmospheric Sciences
Discipline:Atmospheric Sciences
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Physics, Atmospheric Science
Abstract:The characteristics and causes of raindrop oscillations were studied from two field experiments in Hawaiian and Illinois showers. The former experiment (HaRP) focused on the onset of raindrop oscillations as well as oscillations frequencies of small raindrops, whereas the latter experiment (ISWS) had a broader purpose to investigate the oscillation characteristics of moderate-to-large size raindrops including amplitudes as well as frequencies, and oscillation causes.
Raindrop oscillations were found to start at a diameter of d = 1.0 to 1.1 mm in both HaRP and ISWS experiments, consistent with 1991 laboratory observations of Beard, Kubesh and Ochs and with the expected onset of eddy shedding. Most of the oscillation frequencies ($>$95%) were near the fundamental harmonic (f$\sb2)$ with only a few observed near the first harmonic (f$\sb3)$ in both experiments. The oscillation frequencies around the fundamental harmonic showed a size dependency that crossed the theoretical value for f$\sb2$ at about d = 1.5 to 1.6 mm. The oscillation amplitude measurements determined from the vertical extent of the dash streaks showed good agreement with the 1988 data of Sterlyadkin, whereas the oscillation amplitudes estimated from the distortion indicated somewhat larger values for d $>$ 2mm. Eddy shedding was found to be the most likely cause of small raindrop oscillations because of the onset oscillations near 1.0 mm and weak forcing by drop collisions, turbulence, and wind shear.
The influence of raindrop oscillations on dual-polarization radar parameters was investigated for the observed and theoretical size distributions using axis ratios based on four different sets of calculations and measurements. Differential reflectivity (Z$\sb{\rm DR})$ decreased by as much as 0.74 dB from its value for non-oscillating raindrops. Similar change caused by raindrop oscillations were also found for the other dual-polarization parameters, such as the circular and linear depolarization ratios (CDR, LDR), differential phase shift ($\phi\sb{\rm DP}),$ and differential specific attenuation $(\delta$A).
Issue Date:1993
Description:181 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1993.
Other Identifier(s):(UMI)AAI9329183
Date Available in IDEALS:2014-12-17
Date Deposited:1993

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