Thwaites
Fredrik T.
Thwaites
Fredrik T.
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ArticleEkman veering, internal waves, and turbulence observed under Arctic sea ice(American Meteorological Society, 2014-05) Cole, Sylvia T. ; Timmermans, Mary-Louise ; Toole, John M. ; Krishfield, Richard A. ; Thwaites, Fredrik T.The ice–ocean system is investigated on inertial to monthly time scales using winter 2009–10 observations from the first ice-tethered profiler (ITP) equipped with a velocity sensor (ITP-V). Fluctuations in surface winds, ice velocity, and ocean velocity at 7-m depth were correlated. Observed ocean velocity was primarily directed to the right of the ice velocity and spiraled clockwise while decaying with depth through the mixed layer. Inertial and tidal motions of the ice and in the underlying ocean were observed throughout the record. Just below the ice–ocean interface, direct estimates of the turbulent vertical heat, salt, and momentum fluxes and the turbulent dissipation rate were obtained. Periods of elevated internal wave activity were associated with changes to the turbulent heat and salt fluxes as well as stratification primarily within the mixed layer. Turbulent heat and salt fluxes were correlated particularly when the mixed layer was closest to the freezing temperature. Momentum flux is adequately related to velocity shear using a constant ice–ocean drag coefficient, mixing length based on the planetary and geometric scales, or Rossby similarity theory. Ekman viscosity described velocity shear over the mixed layer. The ice–ocean drag coefficient was elevated for certain directions of the ice–ocean shear, implying an ice topography that was characterized by linear ridges. Mixing length was best estimated using the wavenumber of the beginning of the inertial subrange or a variable drag coefficient. Analyses of this and future ITP-V datasets will advance understanding of ice–ocean interactions and their parameterizations in numerical models.
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ThesisDevelopment of an acoustic vorticity meter to measure shear in ocean-boundary layers(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1995-09) Thwaites, Fredrik T.This thesis describes the analysis and development of an acoustic vorticity meter to measure shear in ocean-boundary layers over smaller measurement volumes than previously possible. A nonintrusive measurement of vorticity would filter out irrotational motion such as surface waves and currents that can swamp small scale measurements of shear. The thesis describes the desired geophysical measurements and translates this oceanographic context into design goals. The instrument was designed, built, tested, and deployed. It measures three-axis vorticity at 0.83 and 2.45 meters below the ocean surface with measurement volumes of 0.45 meters on a side. The instrument forms a buoy that is inertially instrumented to calculate and remove buoy motion from the measurements. The instrument uses a complementary filter algorithm to estimate attitude and motion from low-power, inexpensive, strapdown rate gyros, accelerometers, and fluxgate magnetometers. The instrument performance has been measured to have a vorticity bias of not more than 1 x 10-2 per second in a mean flow of 0.7 meters per second, a bias of not more than 1 x 10-2 per second in the down-wave and vertical directions in typical ocean waves, and a 30 decibel spectral rejection of surface wave velocity. Two instrument deployments are described to show the potential of the system. The instrument has measured shear in the upper-ocean-boundary layer, and these measurements are compared to concurrently measured wind stress and stratification. The instrument was also deployed, tethered in the thermocline, in an area of high internal wave activity. Richardson-number time series were measured and compared favorably to concurrently measured Richardson numbers made over a larger spatial scale.