Graduate Thesis Or Dissertation
 

Radar and Optical Remote Sensing of Submesoscale Frontal Features

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/rn3018227

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  • The coastal region is home to many types of surface fronts that exist on a submesoscale (1-10 $km$). While in situ observations capture point or transect observations of frontal behavior subsurface, their complex spatial patterns can be well-captured using remote sensing techniques, which provide synoptic views of the ocean surface with relatively low cost and ease of deployment. This work outlines advances in our understanding of two types of frontal features with surface expressions (nonlinear internal waves, and estuarine density fronts that develop horizontal shear instabilities) using X-band marine radar and optical remote sensing techniques. The dissertation is presented in a manuscript format, containing three first-author manuscripts and an appendix which is an additionally contributed to manuscript. The first and last chapters are an Introduction and Conclusion to the body of work. In Chapter 2, a novel data set of X-band radar and synchronous mooring observations is presented of packets of nonlinear internal waves as they propagate and shoal from shallow waters ($<$50 $m$) to shore. This work is novel in its ability to robustly characterize internal wave packet dynamics using remote sensing tools. The manuscript describes the field experiment in South-central California, presents an image processing technique for extracting cross-shore speed and angle, and details findings from combining radar with the subsurface view of the internal waves via deployed moorings. We compare radar-estimated to mooring-estimated internal wave speeds and confirm a cross-shore profile that deviates from linear theory. We additionally use the synchronous in situ data to perform a close analysis on three nearly-consecutive internal tides. We reveal inter-packet speed variability (internal wave dispersion) and an instance of internal wave polarity reversal observed in the radar and moorings. In Chapter 3, aerial image sequences from a small Unpiloted Aircraft System (sUAS) are presented of the Connecticut River ebb plume front, which propagates into a strong tidal cross-flow. Image analysis reveals temporal and spatial scales of the instabilities, which have roughly 20 $m$ wavelengths, 2 $m$ amplitudes, and propagate near 0.4 $m/s$. Additionally, the surface currents and horizontal shear across the plume front are quantified using imagery. In situ measurements from cross-front ship transects are also used to examine properties of the frontal shear zone. A non-hydrostatic numerical simulation of an ebb plume in an idealized tidal cross-flow was performed using comparable flow parameters and confirms the presence of shear instabilities, while also providing simulations of an idealized case with no tidal cross flow. We show that the results are consistent with previous theoretical work: shear instabilities are likely to emerge given the observed field and modeled conditions, and unlikely to emerge without a tidal cross-flow. Chapter 4 additionally details the presence of horizontal shear instabilities in remote sensing observations of estuarine fronts. In this work, remote sensing observations are presented of two different estuarine flood intrusion fronts exhibiting along-front propagating horizontal shear instabilities. At Mobile Bay, AL, an X-Band radar reveals horizontal shear instabilities of O(100 $m$) wavelengths during maximum flood currents along a tidally-consistent flood front. James River, VA, reveals horizontal shear instabilities along a flood intrusion front with relatively similar shape and behavior, however with wavelengths O(10 $m$). At both sites, the shear is characterized. A relationship between observed instability wavelength and the width of the frontal shear layer is found to be consistent with instability theory. Additionally, high resolution imagery reveals intriguing evidence of instability merging at the flood intrusion tip, as well as the existence of secondary instabilities along the primary instability braid. The appendix presents a look at a novel gravity current front on the inner shelf, which is birthed from an internal bore, and exhibits along-front propagating instabilities. This work combines concepts of frontal speed estimation detailed in Chapter 2. Additionally, we place the observed shear instabilities in the context of the gravity current flow regime in a similar manner as Chapters 3 and 4. This work is included as a relevant, secondary contribution to the aforementioned body of work.
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