Measurements and modeling of the Great Plains low-level jet.

Abstract

This dissertation describes a project to elucidate the turbulent and momentum structure of the Great Plains Low-Level Jet (LLJ), primarily by the use of Doppler radar. Simple theoretical and numerical models of the LLJ are developed which are extensions of the Blackadar inertial oscillation theory. The results of this research are generally consistent with this theory. Turbulence is central to this theory and the core of this research is measurements of turbulence and wind speeds in actual LLJs, in addition to simple models. The use of clear-air Doppler radar data appropriate for LLJ study, requires an understanding of the nature of the clear-air echo. It is important to avoid migrating birds as radar targets as they are a large potential source of velocity bias. For this reason, considerable attention is given to the source of clear-air echo, and a couple cases are analyzed with high-resolution radars. This work strongly implies that the clear-air echo for the cases considered was primarily insects. This contrasts with much recent work (reviewed here) supporting the theory that migrating birds are a major source of clear-air echo. Using radar data believed to be mostly free from migrating bird contamination, this work describes and develops data reduction and quality control techniques so that high-quality profiles and time-height cross-sections can be routinely obtained. These techniques include dealiasing, minimizing the impact of ground clutter bias, obtaining a measure of large scale turbulence from a VAD, and using spectral width information to extract both a measure of small-scale turbulence as well as wind shear.