Miller
Una Kim
Miller
Una Kim
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ArticleMoored turbulence measurements using pulse-coherent doppler sonar(American Meteorological Society, 2021-09-01) Zippel, Seth F. ; Farrar, J. Thomas ; Zappa, Christopher J. ; Miller, Una ; St. Laurent, Louis C. ; Ijichi, Takashi ; Weller, Robert A. ; McRaven, Leah T. ; Nylund, Sven ; Le Bel, DeborahUpper-ocean turbulence is central to the exchanges of heat, momentum, and gases across the air–sea interface and therefore plays a large role in weather and climate. Current understanding of upper-ocean mixing is lacking, often leading models to misrepresent mixed layer depths and sea surface temperature. In part, progress has been limited by the difficulty of measuring turbulence from fixed moorings that can simultaneously measure surface fluxes and upper-ocean stratification over long time periods. Here we introduce a direct wavenumber method for measuring turbulent kinetic energy (TKE) dissipation rates ϵ from long-enduring moorings using pulse-coherent ADCPs. We discuss optimal programming of the ADCPs, a robust mechanical design for use on a mooring to maximize data return, and data processing techniques including phase-ambiguity unwrapping, spectral analysis, and a correction for instrument response. The method was used in the Salinity Processes Upper-Ocean Regional Study (SPURS) to collect two year-long datasets. We find that the mooring-derived TKE dissipation rates compare favorably to estimates made nearby from a microstructure shear probe mounted to a glider during its two separate 2-week missions for O(10−8) ≤ ϵ ≤ O(10−5) m2 s−3. Periods of disagreement between turbulence estimates from the two platforms coincide with differences in vertical temperature profiles, which may indicate that barrier layers can substantially modulate upper-ocean turbulence over horizontal scales of 1–10 km. We also find that dissipation estimates from two different moorings at 12.5 and at 7 m are in agreement with the surface buoyancy flux during periods of strong nighttime convection, consistent with classic boundary layer theory.
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ArticleScaling of moored surface ocean turbulence measurements in the Southeast Pacific Ocean(American Geophysical Union, 2022-12-17) Miller, Una Kim ; Zappa, Christopher J. ; Zippel, Seth F. ; Farrar, J. Thomas ; Weller, Robert A.Estimates of turbulence kinetic energy (TKE) dissipation rate (ε) are key in understanding how heat, gas, and other climate‐relevant properties are transferred across the air‐sea interface and mixed within the ocean. A relatively new method involving moored pulse‐coherent acoustic Doppler current profilers (ADCPs) allows for estimates of ε with concurrent surface flux and wave measurements across an extensive length of time and range of conditions. Here, we present 9 months of moored estimates of ε at a fixed depth of 8.4 m at the Stratus mooring site (20°S, 85°W). We find that turbulence regimes are quantified similarly using the Obukhov length scale (LM) $({L}_{M})$ and the newer Langmuir stability length scale (LL) $({L}_{L})$, suggesting that ocean‐side friction velocity u∗ $\left({u}_{\ast }\right)$ implicitly captures the influence of Langmuir turbulence at this site. This is illustrated by a strong correlation between surface Stokes drift us $\left({u}_{s}\right)$ and u∗ ${u}_{\ast }$ that is likely facilitated by the steady Southeast trade winds regime. In certain regimes, u∗3κz $\frac{{u}_{\ast }^{3}}{\kappa z}$, where κ $\kappa $ is the von Kármán constant and z $z$ is instrument depth, and surface buoyancy flux capture our estimates of ε $\varepsilon $ well, collapsing data points near unity. We find that a newer Langmuir turbulence scaling, based on us ${u}_{s}$ and u∗ ${u}_{\ast }$, scales ε well at times but is overall less consistent than u∗3κz $\frac{{u}_{\ast }^{3}}{\kappa z}$. Monin‐Obukhov similarity theory (MOST) relationships from prior studies in a variety of aquatic and atmospheric settings largely agree with our data in conditions where convection and wind‐driven current shear are both significant sources of TKE, but diverge in other regimes.