Periodic modulation of fine-scale turbulence by gravity waves above the nocturnal boundary layer: Experimental validation using unique in situ measurements.

摘要

This dissertation uses the unique in situ data provided by the Cooperative Institute for Research in Environmental Sciences (CIRES) Tethered Lifting System (TLS) to investigate the effects of gravity waves on the variability of small-scale turbulence statistics ( C2T and epsilon). The goal of the study is to validate experimentally that patches of enhanced turbulence intensity can be connected to the minima of the wave-modulated gradient Richardson number.; The first part of the thesis introduces the TLS and presents details of the calibration algorithms used to convert and merge the high-pass filtered and low-pass filtered voltages of the hot-wire sensor (sensitive to changes of wind speed U) and of the cold wire sensor (sensitive to changes of temperature T) into high-frequency (200 Hz or 1 kHz) calibrated velocity and temperature measurements. The high-frequency calibrated temperature signal has an absolute accuracy better than 1 K and an accuracy of its fluctuations better than 2%. The velocity calibrations consist of fitting the merged hot-wire voltages to the velocities provided by a Pitot-tube sensor using a modified King's law. The calibrated velocity signal has an absolute accuracy better than 1 ms-1 while the accuracy of the slope of the calibration curve is better than 5%. Next estimates of the small-scale turbulence statistics of C2T (i.e., the temperature structure constant) and epsilon (i.e., the energy dissipation rate) are produced from spectra of the high-frequency temperature and velocity fluctuations which are fitted to theoretical curves that model both a noise floor and the typical -5/3 slope of the cascade of energy in the inertial range. With 1-second spectra, those small-scale turbulence estimates are typically better than 15%.; In the second part of the thesis, time series of constant-altitude measurements of temperature and velocity in the residual layer of the nocturnal boundary layer are shown to exhibit in-phase wave-like fluctuations (0.5 C and 0.4 ms-1 amplitudes) with troughs that are synchronized with patches of enhanced C2T and epsilon. Because the TLS only flew one turbulence package at that time, the wave-modulated gradient Richardson number Ri could not be determined experimentally and therefore it is computed numerically using the matrix method to calculate the modal solutions of these ducted gravity waves. Results show that the computed wave-modulated Richardson number experiences purely sinusoidal wave-like fluctuations between ∼16 and ∼40 that are in phase with the temperature and velocity waves. Therefore, the minima of Ri coincide with the location of the patches of enhanced turbulence intensity, in agreement with the statements made by previous theoretical studies, namely, that a wave-modulated Richardson number will induce periodic changes of turbulence intensity even though Ri never goes below it critical value of 1/4.