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Type: Thesis
Title: Raindrop size distribution retrievals in the tropics and mid latitudes.
Author: Dolman, Bronwyn Kaye
Issue Date: 2010
School/Discipline: School of Chemistry and Physics
Abstract: Weather radar capabilities have improved dramatically over the last 50 years. Following World War II, surplus military radars were turned to the study of weather. Since then, they have evolved to the modern standard of rainfall estimations available to the general public in real time. Forecasters rely on weather radars not only for routine forecasting, but also for tracking rapidly evolving, potentially hazardous, severe weather events. These storms have the potential to cause flash floods and hence loss of crops, livestock, and human life. Most weather radars estimate rainfall by converting the measured reflectivity to a rainrate via an empirical relationship (Z-R relationship). There are limitations in the accuracy of the rainfall estimates derived from these scanning radars. Variations in the raindrop size distribution (DSD), that is, the spread of sizes of raindrops falling at a given location, affect the measured reflectivity, and thus the rain rate estimate. The DSD can vary both temporally and spatially, and also with latitude. Investigation of the DSD and its evolution can be used to investigate the effectiveness of Z-R relationships in varied meteorological conditions and locations. Well-established techniques exist for retrieving the DSD using vertically pointing VHF Doppler radars. These radars can simultaneously detect a clear-air echo due to fluctuations in temperature and humidity, and a precipitation echo. Mean vertical air motion and spectral width are estimated from the clear air spectrum, and used to correct the precipitation spectrum through a deconvolution procedure. The corrected precipitation spectrum is then converted to a size spectrum, and the DSD calculated. The DSD and associated integral parameters such as rainrate and liquid water content can then be used to infer the microphysical processes dominating the cloud and precipitation structure. This knowledge can then be used to investigate various Z-R relationships. This thesis presents DSD retrievals from VHF profilers located in Adelaide and Darwin. Each profiler is installed within the footprint of a scanning weather radar, allowing direct comparison of the same air space. These radars provide a unique opportunity to study the evolution of the DSD with the profiler, and use this to investigate variations in time and height of the Z-R relationship. The locations of the radars also permits investigation of the differing nature of DSD evolution in the tropics compared to the mid-latitudes. The TWP-ICE field campaign was conducted in Darwin and surrounding areas in January and February 2006. The campaign involved many instruments, both insitu and remote sensing, including a fleet of aircraft and ship. The University of Adelaide Atmospheric Physics Group installed a VHF wind profiler operating at 54.1 MHz near Darwin airport for the experiment. This radar sampled the same air space as a C-band polarimetric scanning radar (CPol), which performed horizontal scans at increasing elevations, along with vertical scans over the profiler site every 10 minutes. Results from 8 events, differing in age, type, dominant microphysical process and seasonal regime are presented in this thesis. A VHF profiler permanently located near Adelaide airport provides an observational capability similar to Darwin, but in the mid-latitudes where the processes dictating rainfall are vastly different. This radar also operates at 54.1 MHz, and is installed within the footprint of an operational weather watch radar. Pseudo vertical scans can be constructed from the successive horizontal scans allowing direct comparisons. This profiler is the first generation of the profiler in Darwin. It is not as powerful and cannot detect low intensity rainfall. Due to this and the drought South Australia experienced between 2006 and 2008, data from Adelaide are limited. Two events are presented. Seasonal trends in the tropics, that is break conditions as opposed to the monsoon, are compared and contrasted. These trends are then compared to the limited Adelaide data. By analysing the evolution in both time and height of the DSD, and the dependence on season and latitudinal location, this thesis leads to a better understanding of the microphysical processes dictating rainfall in the tropics and mid-latitudes.
Advisor: Vincent, Robert Alan
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, School of Chemistry and Physics, 2010
Keywords: rainfall; drop size distribution; wind profiler; tropics; TWP-ICE; mid latitudes; weather radar
Provenance: Copyright material removed from digital thesis. See print copy in University of Adelaide Library for full text.
Appears in Collections:Research Theses

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