Item – Theses Canada

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OCLC number
46507621
Author
Nissen, Robert P.(Robert Peter),1961-
Title
Effects of air pressure on raindrop size distributions : modelling and field data verification.
Degree
Ph. D. -- University of Toronto, 1996
Publisher
Ottawa : National Library of Canada = Bibliothèque nationale du Canada, 1996.
Description
2 microfiches.
Notes
Includes bibliographical references.
Abstract
A parameterization is developed based on previous laboratory drop collision experiments performed at 50 kPa by Fung (1984). It is then applied to a numerical model to follow the evolution of the drop size spectrum. The transitional and temporal equilibrium behaviors are examined with respect to drop coalescence and breakup processes and compared with the 100 kPa trends. Modelling shows that a 50 kPa equilibrium distribution is established with peaks at 0.3, 0.8, and 2.0 mm. This compares to peaks at diameters of 0.3, 1.0, and 1.7 mM at a pressure of 100 kPa. Generally fewer of the larger drops and more of the smaller drops are found at 50 kPa than at the 100 kPa equilibrium. The faster terminal velocity at 50 kPa is the most important factor accounting for differences in the behavior of drop size distribution between the two pressure levels. Comparisons are then made with actual raindrop spectral signatures measured by a Doppler radar. Further validation is performed with rain spectrometers at the ground (disdrometer) and aloft by aircraft (2-DP laser probe). In cold rain of light to moderate rainrate (1.5-3.0 mm h$\sp{-1}$) the raindrop size distribution is virtually constant below the melting layer, indicating that the spectra at the ground reflect the melted ice particles just above the 0$\sp\circ$C level. Drop diameters corresponding to Doppler mean terminal velocities undergo no change with height. Fall speed profiles determined from the radar data analyses are consistent with values derived from data obtained by the disdrometer and 2-DP probe instruments, which are based on different measurement principles. Air pressure affects the evolution of the raindrop spectrum in a minor fashion only. However, constraints imposed by realistic rainrates dictate that the observed fall speed profiles do not imply attainment of raindrop size spectra equilibria. The evolution times are too short.
ISBN
0612118193
9780612118195