Tesis
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Browsing Tesis by Subject "Atmospheric temperature"
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Item Open Access A study of radar aspect sensitivity in the lower atmosphere(Cornell University, 2001) Chen, CharlesThe goal of this thesis is related to atmospheric temperature measurements using in situ techniques in tandem with a direct numerical simulation to better understand the zenith angle dependence of VHF (30-300 MHz) radar backscatter from the atmosphere. We begin our study with a high-resolution balloon-borne in situ temperature measurement made over Wichita, KS, in 1995. Very steep vertical temperature gradients were found at the edges of vertical potential steps, regions of near zero vertical potential temperature gradient. We use wavelet analysis to isolate the organized components of the signal and, after subtraction from the original signal, the residual signal is found to have the characteristics of isotropic turbulence. This confirms our hypothesis that the measured temperature profile is a superposition of coherent structures and a background isotropic turbulence. From a radar perspective, we show that this wavelet analysis allows us to predict the radar backscatter as a function of zenith angle from a high- resolution one-dimensional temperature measurement. Unfortunately, radar measurements were not available at this point. We next explore the cause of aspect sensitivity directly via a multi-instrument investigation of the lower atmosphere over the Jicamarca Radio Observatory (JRO) near Lima, Peru. The joint analysis of radar backscatter and in situ measurements of the temperature structure shows that a combination of Fresnel scattering and turbulence is the most likely explanation for aspect sensitive echoes. Furthermore, the strong backscatter seems to originate from vertical potential temperature steps; such as those observed over Wichita, KS. Finally, we show that the measured potential temperature steps and the structures seen in a direct numerical simulation (DNS) of a Kelvin-Helmholtz instability (KHI) are remarkably similar. Not only do we find good agreement between the observation and the simulation; the similarity is also seen in the wavelet spectrum, which is the behavior of the wavelet coefficient as a function of scale size. We extend the results from experimental observations and numerical simulation by predicting the characteristic radar backscatter and show that it is consistent with observations.Item Open Access Photoelectron transport and energy balance in the low-latitude ionosphere(Cornell University, 2012) Varney, Roger Hale; Kelley, MichaelTheoretical studies of ionospheric structure and dynamics require knowledge of the underlying thermal structure of the ionosphere since it affects the chemical reaction rates, recombination rates, and pressure gradients. Measurements of ionospheric temperatures have been made for decades with a variety of ground- and space-based techniques. This thesis is motivated in particular by the recent improvements in the temperature measurements made by the Jicamarca Radio Observatory (JRO) using the incoherent scatter radar (ISR) technique. Modern ionospheric models all have widely different treatments of ionospheric energetics, and none can produce satisfactory quantitative agreement with the JRO measurements even in quiet conditions. This thesis explores the energy balance calculations in the widely used, open source SAMI2 model in detail, and shows that it is the oversimplification of the treatment of nonlocal heating by photoelectrons in particular which is pre- venting this model from predicting JRO measurements. This thesis presents an extended version of the SAMI2 model, called SAMI2-PE, which includes a newly developed photoelectron transport model. The model uses finite volume methods which guarantee conservation of particles and energy, incorporates the magnetic field geometry and magnetic mirroring effects, and can be extended to any spatial, energy, and pitch-angle resolution. The new model shows promising agreement with the JRO measurements.