Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Physics

Supervisor

Dr. Wayne Hocking

Abstract

Applications and results of a variety of studies carried out with the Costa Rican VHF profiler radar are included. This radar is unique because of the wide frequency bandwidth available (5 MHz). During the developing stages of the wind profiler radar new techniques of processing and radar engineering were used.

A thorough analysis of a data-dependent method (Capon's) commonly used in radar was carried out. This method is analyzed based on a reference established by Fourier theory. Disadvantages of using data-dependent methods with small filters were found. A small filter size will cause Capon's method to underestimate the number of spectral peaks when compared to using a larger filter size. The understimation will occur even when the filter size is larger than the number of peaks. When used to estimate a known Gaussian spectrum, Capon's method understimated the spectral width independently of the filter size.

The backscattered signal measured in radars is the convolution of the transmitted signal with the atmospheric profile of scatterers. The convolution integral model was used to calculate radar backscatter. The implementations were created to simulate radar backscatter using the transmitted signal and the electric permittivity profile of the atmosphere. Tests with realistic physical conditions were carried out to validate the model and verify the implementation. Appropriate range location and Doppler velocites were obtained from simple simulations in one, two and three dimensions. The convolution engine was later used along with a mathematical representation of atmospheric scatterers to study the simulated radar echoes. Correct radial velocites were obtained along with realistic radar effect like the beam broadening effect.

A Large eddy simulation (LES) model was used to simulate a full-physics atmosphere. The LES code follows the fluid dynamics equations. The simulated LES atmosphere was used to calculate the electric permittivity from atmospheric variables. A radar simulation involving the convolution of this electric permittivity profile and the radar pulse allowed us to simulate a radar inside the simulated atmosphere. The initial conditions of the simulation created a clear planetary boundary layer as well as a region of shear instabilities in the upper heights. Both regions generated turbulence during the simulation and allowed the radar simulation to measure it satisfactorily. Anisotropy was observed in the results when comparing vertical beam data to tilted beam data as usually observed in real measurements.

A long term experiment was carried out in Costa Rica to gather information about the tropical atmosphere. This was the first time this type of experiment was carried out in Costa Rica. The information provided a clear perspective of the phenomena found in the lower troposphere. Among others, the planetary boundary layer (PBL), thin layers, isolated patches of turbulence, oscillations and convective events were detected. The presence of layers over Costa Rica is well defined; during the dry season months (winter-spring time of the northern hemisphere) at least one layer can be observed up to 30% of the time, while it decreases below 10% of the time during the rainy season (summer-fall time of the northern hemisphere). The PBL shows great variability depending on the general conditions of the atmosphere; the average PBL top was located near 2 km during the dry season months and increased to almost 4 km during the rainy season. More examples are provided.


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