Zippel Seth F.

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Zippel

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Seth F.

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0000-0002-6287-9896

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Wave breaking turbulence at the offshore front of the Columbia River Plume

(John Wiley & Sons, 2014-12-19) Thomson, James M. ; Horner-Devine, Alex R. ; Zippel, Seth F. ; Rusch, Curtis ; Geyer, W. Rockwell

Observations at the Columbia River plume show that wave breaking is an important source of turbulence at the offshore front, which may contribute to plume mixing. The lateral gradient of current associated with the plume front is sufficient to block (and break) shorter waves. The intense whitecapping that then occurs at the front is a significant source of turbulence, which diffuses downward from the surface according to a scaling determined by the wave height and the gradient of wave energy flux. This process is distinct from the shear-driven mixing that occurs at the interface of river water and ocean water. Observations with and without short waves are examined, especially in two cases in which the background conditions (i.e., tidal flows and river discharge) are otherwise identical.
Moored 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, Deborah

Upper-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.
Turbulence from breaking surface waves at a river mouth

(American Meteorological Society, 2018-02-22) Zippel, Seth F. ; Thomson, James M. ; Farquharson, Gordon

Observations of surface waves, currents, and turbulence at the Columbia River mouth are used to investigate the source and vertical structure of turbulence in the surface boundary layer. Turbulent velocity data collected on board freely drifting Surface Wave Instrument Float with Tracking (SWIFT) buoys are corrected for platform motions to estimate turbulent kinetic energy (TKE) and TKE dissipation rates. Both of these quantities are correlated with wave steepness, which has previously been shown to determine wave breaking within the same dataset. Estimates of the turbulent length scale increase linearly with distance from the free surface, and roughness lengths estimated from velocity statistics scale with significant wave height. The vertical decay of turbulence is consistent with a balance between vertical diffusion and dissipation. Below a critical depth, a power-law scaling commonly applied in the literature works well to fit the data. Above this depth, an exponential scaling fits the data well. These results, which are in a surface-following reference frame, are reconciled with results from the literature in a fixed reference frame. A mapping between free-surface and mean-surface reference coordinates suggests 30% of the TKE is dissipated above the mean sea surface.
Parsing the kinetic energy budget of the ocean surface mixed layer

(American Geophysical Union, 2022-01-10) Zippel, Seth F. ; Farrar, J. Thomas ; Zappa, Christopher J. ; Plueddemann, Albert J.

The total rate of work done on the ocean by the wind is of considerable interest for understanding global energy balances, as the energy from the wind drives ocean currents, grows surface waves, and forces vertical mixing. A large but unknown fraction of this atmospheric energy is dissipated by turbulence in the upper ocean. The focus of this work is twofold. First, we describe a framework for evaluating the vertically integrated turbulent kinetic energy (TKE) equation using measurable quantities from a surface mooring, showing the connection to the atmospheric, mean oceanic, and wave energy. Second, we use this framework to evaluate turbulent energetics in the mixed layer using 10 months of mooring data. This evaluation is made possible by recent advances in estimating TKE dissipation rates from long-enduring moorings. We find that surface fluxes are balanced by TKE dissipation rates in the mixed layer to within a factor of two.
Scaling 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.
Observations of turbulence and the geometry and circulation of windrows in a small bay in the St. Lawrence Estuary

( 2019-11-07) Zippel, Seth F. ; Maksym, Ted ; Scully, Malcolm E. ; Sutherland, Peter ; Dumont, Dany

Measurements of ocean turbulence, waves, and the geometry and circulation of windrows were made over 5 days in early March in a small bay in the St. Lawrence Estuary. Measurements were made from a small zodiac and from a SWIFT drifter. Two acoustic doppler velocity profilers (ADCPs) were used from the zodiac to measure water velocity and turbulent kinetic energy (TKE) dissipation rates near the surface. The acoustic backscatter from the ADCPs was used in conjunction with a GPS to map the location and spacing of wind aligned rows of bubbles. The SWIFT drifter provided measurements of waves, wind stress, and secondary measurements of TKE dissipation rates. Imagery of the surface was taken with a GoPro camera mounted on the zodiac, and with a DJI MavicPro quadcopter.
Data to accompany “Direct observation of wave-coherent pressure work in the atmospheric boundary layer”

(Woods Hole Oceanographic Institution, 2022-12-29) Zippel, Seth F. ; Edson, James B. ; Scully, Malcolm E. ; Keefe, Oaklin R.

As described in the methods section of “Direct Observation of Wave-coherent Pressure Work in the Atmospheric Boundary Layer”: Measurements were made from an open-lattice steel tower deployed in roughly 13 m water depth in Buzzards Bay, MA. Buzzards Bay is a 48 km by 12 km basin open on the SW side to Rhode Island Sound. The average depth is 11 m, with a tide range of 1 to 1.5 m, depending on the neap/spring cycles. Winds in Buzzards Bay are frequently aligned on the long-axis (from the NE or SW), and are commonly strong, particularly in the fall and winter. The tower was deployed near the center of the bay at 41.577638 N, 70.745555 W for a spring deployment lasting from April 12, 2022 to June 13th, 2022. Atmospheric measurements included three primary instrument booms that housed paired sonic anemometers (RM Young 81000RE) and high-resolution pressure sensors (Paros Scientific). The pressure sensor intakes were terminated with static pressure heads, which reduce the dynamic pressure contribution to the measured (static) pressure. The tower booms were aligned at 280 degrees such that the NE and SW winds would be unobstructed by the tower's main body. A fourth sonic anemometer (Gill R3) was extended above the tower such that it was open to all wind directions and clear of wake by the tower structure. A single point lidar (Riegl LD90-3i) was mounted to the highest boom, such that the lidar measured the water surface elevation underneath the anemometer and pressure sensors to within a few centimeters horizontally. All instruments were time synchronized with a custom "miniNode" flux logger, that aggregated the data streams from each instrument. Additional atmospheric and wave measurements on the tower included short-wave and long-wave radiometers (Kipp & Zonen), two RH/T sensors (Vaisala), and a standard lower-resolution barometer (Setra).
Buzzards Bay Air-Sea Interaction Tower (BB-ASIT) water column data from Fall 2022 deployment

(Woods Hole Oceanographic Institution, 2023-09-19) Scully, Malcolm E. ; Zippel, Seth F.

Data from an air-sea interaction tower are used to close the turbulent kinetic energy (TKE) budget in the wave-affected surface layer of the upper ocean. Under energetic wind forcing with active wave breaking, the dominant balance is between the dissipation rate of TKE and the downward convergence in vertical energy flux. The net energy flux is downward, primarily driven by pressure work, and the TKE transport is upward, opposite to the downgradient assumption in most turbulence closure models. The sign and the relative magnitude of these energy fluxes are hypothesized to be driven by a weak interaction between the vertical velocity of Langmuir circulation (LC) and the kinetic energy and pressure of wave groups that is the result of small scale wave-current interaction. Consistent with previous modeling studies, the data suggest that the horizontal current anomaly associated with LC refracts wave energy away from downwelling regions and into upwelling regions, resulting in negative covariance between the vertical velocity of LC and the pressure anomaly associated with the wave groups. The asymmetry between downward pressure work and upward TKE flux is explained by the Bernoulli response of the sea-surface, which results in groups of waves having a larger pressure anomaly than the corresponding kinetic energy anomaly, consistent with group-bound long wave theory.
Direct observation of wave-coherent pressure work in the atmospheric boundary layer

(American Meteorological Society, 2024-01-23) Zippel, Seth F. ; Edson, James B. ; Scully, Malcolm E. ; Keefe, Oaklin R.

Surface waves grow through a mechanism in which atmospheric pressure is offset in phase from the wavy surface. A pattern of low atmospheric pressure over upward wave orbital motions (leeward side) and high pressure over downward wave orbital motions (windward side) travels with the water wave, leading to a pumping of kinetic energy from the atmospheric boundary layer into the waves. This pressure pattern persists above the air–water interface, modifying the turbulent kinetic energy in the atmospheric wave-affected boundary layer. Here, we present field measurements of wave-coherent atmospheric pressure and velocity to elucidate the transfer of energy from the atmospheric turbulence budget into waves through wave-coherent atmospheric pressure work. Measurements show that the phase between wave-coherent pressure and velocity is shifted slightly above 90° when wind speed exceeds the wave phase speed, allowing for a downward energy flux via pressure work. Although previous studies have reported wave-coherent pressure, to the authors’ knowledge, these are the first reported field measurements of wave-coherent pressure work. Measured pressure work cospectra are consistent with an existing model for atmospheric pressure work. The implications for these measurements and their importance to the turbulent kinetic energy budget are discussed.
Langmuir turbulence controls on observed diurnal warm layer depths

(American Geophysical Union, 2023-05-24) Wang, Xingchi ; Kukulka, Tobias ; Farrar, J. Thomas ; Plueddemann, Albert J. ; Zippel, Seth F.

The turbulent ocean surface boundary layer (OSBL) shoals during daytime solar surface heating, developing a diurnal warm layer (DWL). The DWL significantly influences OSBL dynamics by trapping momentum and heat in a shallow near‐surface layer. Therefore, DWL depth is critical for understanding OSBL transport and ocean‐atmosphere coupling. A great challenge for determining DWL depth is considering wave‐driven Langmuir turbulence (LT), which increases vertical transport. This study investigates observations with moderate wind speeds (4–7 m/s at 10 m height) and swell waves for which breaking wave effects are less pronounced. By employing turbulence‐resolving large eddy simulation experiments that cover observed wind, wave, and heating conditions based on the wave‐averaged Craik‐Lebovich equation, we develop a DWL depth scaling unifying previous approaches. This scaling closely agrees with observed DWL depths from a year‐long mooring deployment in the subtropical North Atlantic, demonstrating the critical role of LT in determining DWL depth and OSBL dynamics.
Vertical energy fluxes driven by the interaction between wave groups and Langmuir turbulence

(American Meteorological Society, 2024-06-26) Scully, Malcolm E. ; Zippel, Seth F.

Data from an air–sea interaction tower are used to close the turbulent kinetic energy (TKE) budget in the wave-affected surface layer of the upper ocean. Under energetic wind forcing with active wave breaking, the dominant balance is between the dissipation rate of TKE and the downward convergence in vertical energy flux. The downward energy flux is driven by pressure work, and the TKE transport is upward, opposite to the downgradient assumption in most turbulence closure models. The sign and the relative magnitude of these energy fluxes are hypothesized to be driven by an interaction between the vertical velocity of Langmuir circulation (LC) and the kinetic energy and pressure of wave groups, which is the result of small-scale wave–current interaction. Consistent with previous modeling studies, the data suggest that the horizontal velocity anomaly associated with LC refracts wave energy away from downwelling regions and into upwelling regions, resulting in negative covariance between the vertical velocity of LC and the pressure anomaly associated with the wave groups. The asymmetry between downward pressure work and upward TKE flux is explained by the Bernoulli response of the sea surface, which results in groups of waves having a larger pressure anomaly than the corresponding kinetic energy anomaly, consistent with group-bound long-wave theory.

Zippel Seth F.

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