Direct Turbulence Measurements Research Articles

Winter Vertical Diffusion Rates in the Arctic Ocean, Estimated From 7Be Measurements and Dissipation Rate Profiles

AbstractOcean turbulent mixing is a key process affecting the uptake and redistribution of heat, carbon, nutrients, oxygen and other dissolved gasses. Vertical turbulent diffusivity sets the rates of water mass transformations and ocean mixing, and is intrinsically an average quantity over process time scales. Estimates based on microstructure profiling, however, are typically obtained as averages over individual profiles. How representative such averaged diffusivities are, remains unexplored in the quiescent Arctic Ocean. Here, we compare upper ocean vertical diffusivities in winter, derived from the 7Be tracer‐based approach to those estimated from direct turbulence measurements during the year‐long Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC) expedition, 2019–2020. We found that diffusivity estimates from both methods agree within their respective measurement uncertainties. Diffusivity estimates obtained from dissipation rate profiles are sensitive to the averaging method applied, and the processing and analysis of similar data sets must take this sensitivity into account. Our findings indicate low characteristic diffusivities around 10−6 m2 s−1 and correspondingly low vertical heat fluxes.

Contamination of Finescale Strain Estimates of Turbulent Kinetic Energy Dissipation by Frontal Physics

Abstract Finescale strain parameterization (FSP) of turbulent kinetic energy dissipation rate has become a widely used method for observing ocean mixing, solving a coverage problem where direct turbulence measurements are absent but CTD profiles are available. This method can offer significant value, but there are limitations in its broad application to the global ocean. FSP often fails to produce reliable results in frontal zones where temperature–salinity (T/S) intrusive features contaminate the CTD strain spectrum, as well as where the aspect ratio of the internal wave spectrum is known to vary greatly with depth, as frequently occurs in the Southern Ocean. In this study we use direct turbulence measurements from Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) and glider microstructure measurements from Autonomous Sampling of Southern Ocean Mixing (AUSSOM) to show that FSP can have large biases (compared to direct turbulence measurement) below the mixed layer when physics associated with T/S fronts are meaningfully present. We propose that the FSP methodology be modified to 1) include a density ratio (Rρ)-based data exclusion rule to avoid contamination by double diffusive instabilities in frontal zones such as the Antarctic Circumpolar Current, the Gulf Stream, and the Kuroshio, and 2) conduct (or leverage available) microstructure measurements of the depth-varying shear-to-strain ratio Rω(z) prior to performing FSP in each dynamically unique region of the global ocean. Significance Statement Internal waves travel through the ocean and collide, turbulently mixing the interior ocean and homogenizing its waters. In the absence of actual turbulence measurements, oceanographers count the ripples associated with these internal waves and use them estimate the amount of turbulence that will transpire from their collisions. In this paper we show that the ripples in temperature and salinity that naturally occur at sharp fronts masquerade as internal waves and trick oceanographers into thinking there is up to 100 000 000 times more turbulence than there actually is in these frontal regions.

Turbulent Mixing in the Barrier Layer of the Equatorial Pacific Ocean

AbstractThe barrier layer (BL) is an expansive halocline layer commonly found in the western equatorial Pacific, long been thought to inhibit entrainment of colder thermocline water into the surface mixed layer (ML), consequently facilitating the development of El Niño. But here we find frequent turbulent mixing in the BL from both direct turbulence measurements and indirect mixing estimates within an 11‐year‐long Argo profile data set. The observed BL mixing is as strong as in the ML, yielding effective heat transfers across the isothermal layer (IL) base. The estimated BL mixing is ubiquitous, with occurrence ranging 20%–60% spatially and peaking at around 160°W, 0°N; it occurs more frequently in La Niña than El Niño years. The BL mixing is associated with thicker ML, BL, and IL, weaker BL stratification, and lower temperature and higher salinity in the IL. How the BL mixing may impact the El Niño development deserves further exploration.

Direct Measurements of Turbulence in the Upper Western Pacific North Equatorial Current over a 25-h Period.

Measurements of the turbulent kinetic energy dissipation rate (ε) were conducted by a free-fall microstructure profiler in the western Pacific North Equatorial Current (WPNEC) during a continuous period of 25 h, from the sea surface to about 160 m depth. In the mixed layer (ML), ε values were typically on the order of 10−8∼10−7 W kg−1, and an obvious diurnal cycle existed in the upper 40 m of the surface mixing layer. Below the ML, ε was reduced to 10−9∼10−8 W kg−1 with some patches of high ε reaching 10−7.5 W kg−1. The barrier layer was identified in the nighttime with a maximum thickness of 20 m, and it was eroded by the advection of freshwater within the lower part of the isothermal layers associated with an anticyclonic eddy in the afternoon. A simple scaling relevant to shear (S2) instability and stratification (N2) that can predict turbulent dissipation rates in the transition layer, between the well-mixed layer and the thermocline below, was obtained through the scaling . Besides turbulence, double-diffusive processes also contributed to the vertical mixing levels in the upper WPNEC.

Turbulent Scales Observed in a River Plume Entering a Fjord

AbstractObservations of turbulent mixing are quantified in the near‐field region (first several kilometers) of a controlled river flow entering Doubtful Sound, New Zealand. Measurement techniques include direct turbulence measurements using microstructure profilers in both Eulerian and quasi‐Lagrangian perspectives and turbulent overturn analysis. Over 500 profiles of vertical microstructure were collected within the inner fjord. With high flow speeds of the surface plume over 2 m/s and some of the strongest stratification (N2∼10−1 s−2) recorded in the coastal ocean, turbulent overturning (Thorpe, LT) scales are used to indirectly infer turbulence dissipation rates (ϵ) and compared with direct measurements in a quasi‐idealized laboratory setting. There exists an inverse relationship between directly measured estimates of ϵ and distance from the plume discharge point. Peak values of ϵ>10−2 W/kg are observed within 500 m from the discharge point, among the highest turbulence dissipation rates recorded in oceanic shear flows. Thorpe scales range from below 1 cm in the pycnocline to a few meters in the weakly stratified ambient water below the plume. Stratification within the surface layer constrains the vertical dimension, limiting LT. Therefore, LT is an unreliable estimator of L0, where LO is the Ozmidov scale and is used to infer ϵ. Care must thus be taken when applying this method of overturn analysis in an environment where stratification or physical boundaries limit the vertical extent of overturns.

Estimates of the turbulence intensity and power density of an asymmetrical tidal flow under variability of wind forcing

A high-frequency (1.2 MHz) four-beam Acoustic Doppler Current Profiler (ADCP) moored on the seabed has been used for direct measurements of turbulence in a shallow coastal zone of the eastern English Channel. From the measurements conducted, 5 tidal cycles covering calm and storm periods were selected. Impacts of the tidal cycle asymmetry and the variability of wind forcing on the turbulence intensity, Reynolds stresses, and the power density of the flow are assessed quantitatively. A comparison of the energy characteristics of the tidal flow during calm and storm periods revealed that the power density of the stream during the storm was about half of that during the calm period. Wave bias correction of Reynolds stresses allows estimating a contribution of small-scale turbulence to its total intensity.

High spatial variability of gas transfer velocity in streams revealed by turbulence measurements

ABSTRACTStreams are major sources of carbon dioxide (CO2) and methane (CH4) to the atmosphere, but current large-scale estimates are associated with high uncertainties because knowledge concerning the spatiotemporal control on stream emissions is limited. One of the largest uncertainties derives from the choice of gas transfer velocity (k600), which describes the physical efficiency of gas exchange across the water–atmosphere interface. This study therefore explored the variability in k600 and subsequent CO2 and CH4 emission rates within and across streams of different stream order (SO). We conducted, for the first time in streams, direct turbulence measurements using an acoustic Doppler velocimeter (ADV) to determine the spatial variability in k600 across a variety of scales with a consistent methodology. The results show high spatial variability in k600 and corresponding CO2 and CH4 emissions at small spatial scales, both within stream reaches and across SO, especially during high discharge. The k600 was positively related to current velocity and Reynolds number. By contrast, no clear relationship was found between k600 and specific stream characteristics such as width and depth, which are parameters often used in empirical models of k600. Improved understanding of the small-scale variability in the physical properties along streams, especially during high discharge, is therefore an important step to reduce the uncertainty in existing gas transfer models and emissions for stream systems. The ADV method was a useful tool for revealing spatial variability in this work, but it needs further development. We recommend that future studies conduct measurements over shorter time periods (e.g., 10–15 min instead of 40 min) and at more sites across the reach of interest, and thereby derive more reliable mean-reach k600 as well as more information about controls on the spatial variability in k600.

Observations of vertical turbulent nitrate flux during summer in the Great Australian Bight

To assist with developing an understanding of the nutrient supply dynamics in the Great Australian Bight (GAB) the vertical turbulent nitrate flux was calculated at stations along two cross-shelf transects using direct measurements of turbulence and nitrate concentrations. Coincident hydrographic profiling conducted at the onset of the summertime upwelling season revealed upwelling onto the shelf in the eastern GAB (eGAB) and downwelling at the shelf slope in the central GAB (cGAB). Under these conditions, examination of the strength of competing vertical temperature and salinity gradients using the Turner angle suggested a high potential for double diffusive convective mixing. However, corresponding estimates of the mixing efficiency derived from microstructure profiling indicated turbulence was the main mixing process driving vertical fluxes across the region. The average upward nitrate flux between the surface mixed layer depth(MLD) and the base of the euphotic layer (92 m depth) was O(10−6−10−5) mmol N m−2 s−1 in the cGAB, with the peak flux observed at the upper-slope station (~ 400 m total water depth). In the eGAB, the magnitude of flux estimates at the shelf-slope and offshore stations were similar to those measured in the cGAB. An enhanced nitrate flux O(10−4) mmol N m−2 s−1 was observed on the shelf in the eGAB (~ 100 m total water depth) as the result of enhanced turbulence and an increased vertical nitrate gradient. Application of Redfield stoichiometry suggests vertical turbulent nitrate fluxes play a significant role in supporting up to 50% of the primary production in the GAB. We hypothesize that mixing processes resulting from interactions between upwelled and downwelled water masses and new water masses formed on the shelf during summer play an important role in maintaining the bands of subsurface chlorophyll a maxima observed below the MLD at the nitricline during periods of strong stratification. The snapshot provided by this study, emphasizes the need to better understand the variability and influence of mixing processes on nutrient supply and productivity across the GAB marine ecosystem.

Upper layer diapycnal mixing and nutrient flux in the subtropical frontal region of the Indian sector of the Southern Ocean

Abstract Upper layer diapycnal mixing in the Subtropical Front (STF) was estimated using microstructure shear profiles collected from the Indian sector of the Southern Ocean (ISSO) during the austral summer of 2012. Observations were made in the northern and southern boundary of the highly mesoscale turbulent STF, which is characterized by the presence of the dynamic Agulhas Return Current. During the observational period, the STF was populated with alternating cyclonic and anticyclonic eddies. In this mesoscale turbulent region, the average eddy diffusivity at the base of the euphotic zone was 5.5 × 10−5 m2 s−1. The average diapycnal nitrogen flux at the base of the euphotic zone, calculated using direct turbulence measurements, and nitrate (NO3) and nitrite (NO2) concentrations, was 6.4 × 10−5 μ mol m−2 s−1. The satellite-derived primary production in the STF was ~1000 mg C m−2 day−1. The observed diapycnal nutrient flux could only sustain

First Evidence of Coherent Bands of Strong Turbulent Layers Associated with High-Wavenumber Internal-Wave Shear in the Upstream Kuroshio

The upstream Kuroshio flows through Okinawa Trough and the Tokara island chain, the region near the continental shelf of the East China Sea and shallow seamounts, where the Kuroshio can induce strong mixing over the shallow topography. Also, tidal currents over the rough topography may produce internal tides, and associated turbulence. The previous observations show energetic high vertical wavenumber near-inertial wave shear in the Kuroshio thermocline, which implies strong turbulent mixing. However, direct turbulence measurements in this region are very scarce. Using high lateral resolution (1–2 km) direct turbulence measurements, we show here, for the first time, that strong turbulent layers form spatially coherent banded structures with lateral scales of >O(10 km), associated with bands of near-inertial wave/diurnal internal tide shear of high vertical wavenumber in the upstream Kuroshio. The turbulent kinetic energy dissipation rates within these turbulent layers are >O(10−7 W kg−1), and estimated vertical eddy diffusivity shows >O(10−4 m2 s−1) on average. These results suggest that the high vertical wavenumber near-inertial waves propagating in the upstream Kuroshio could have large impacts on the watermass modifications, momentum mixing, nutrient supply, and associated biogeochemical responses in its downstream.

Direct Measurements of Turbulence and Flow Characteristics of a Vertical Axis Wind Turbine Using the Eddy Covariance Method

Turbulence intensity measurements of vertical axis wind turbine (VAWT) are essential to detemineitsefficiency and performance. In this study, the eddy covariance (EC) method was used to characterise the flow and directly measure turbulence of a VAWT in a controlled indoor laboratory using an ultrasonic anemometer (at 10 Hz) at specific grid positions of upwind and downwind (in the wake) of the VAWT. In constrast to numerical simulations, this method has the potential to accurately quantify turbulence of wind turbines and thus able to describe the flow patterns around a VAWTexperimentally. Results show that at the upwind position, some of the swept area of the VAWT obstructed the flow due to the counter current flow generated by the VAWT rotors’ rotation while causing some flows to be diverted and concentrated to the concurrent flow area of the VAWT. In the wake of the VAWT, flow velocities and turbulence decreased and dispersed while at the centre line, vertical turbulence decreased towards the bottom (ground) swept area.This downward transfer of turbulence caused generally negative vertical momentum transfer. However, a slight increase of turbulence was observed at the ground surface due to flow deflection.

Evaluation of low wind modeling approaches for two tall-stack databases

The performance of the AERMOD air dispersion model under low wind speed conditions, especially for applications with only one level of meteorological data and no direct turbulence measurements or vertical temperature gradient observations, is the focus of this study. The analysis documented in this paper addresses evaluations for low wind conditions involving tall stack releases for which multiple years of concurrent emissions, meteorological data, and monitoring data are available. AERMOD was tested on two field-study databases involving several SO2 monitors and hourly emissions data that had sub-hourly meteorological data (e.g., 10-min averages) available using several technical options: default mode, with various low wind speed beta options, and using the available sub-hourly meteorological data. These field study databases included (1) Mercer County, a North Dakota database featuring five SO2 monitors within 10 km of the Dakota Gasification Company’s plant and the Antelope Valley Station power plant in an area of both flat and elevated terrain, and (2) a flat-terrain setting database with four SO2 monitors within 6 km of the Gibson Generating Station in southwest Indiana. Both sites featured regionally representative 10-m meteorological databases, with no significant terrain obstacles between the meteorological site and the emission sources. The low wind beta options show improvement in model performance helping to reduce some of the overprediction biases currently present in AERMOD when run with regulatory default options. The overall findings with the low wind speed testing on these tall stack field-study databases indicate that AERMOD low wind speed options have a minor effect for flat terrain locations, but can have a significant effect for elevated terrain locations. The performance of AERMOD using low wind speed options leads to improved consistency of meteorological conditions associated with the highest observed and predicted concentration events. The available sub-hourly modeling results using the Sub-Hourly AERMOD Run Procedure (SHARP) are relatively unbiased and show that this alternative approach should be seriously considered to address situations dominated by low-wind meander conditions.Implications: AERMOD was evaluated with two tall stack databases (in North Dakota and Indiana) in areas of both flat and elevated terrain. AERMOD cases included the regulatory default mode, low wind speed beta options, and use of the Sub-Hourly AERMOD Run Procedure (SHARP). The low wind beta options show improvement in model performance (especially in higher terrain areas), helping to reduce some of the overprediction biases currently present in regulatory default AERMOD. The SHARP results are relatively unbiased and show that this approach should be seriously considered to address situations dominated by low-wind meander conditions.

TOPOGRAPHICALLY INDUCED TURBULENCE ON CONTINENTAL SHELF OFF JOGASHIMA, SAGAMI BAY, JAPAN

ABSTRACT Direct turbulence measurements using TurboMAP over a “small ridge” on the continental shelf off Jogashima, Sagami Bay, Japan revealed an active layer of topographically induced turbulence. The active layer ( = 10-7 - 10-6 W kg-1) located at mid-depth of 200 – 250 m, occurred on alternate side of “small ridge” depending on the direction of the tidal current. This active layer was accompanied by rebounded-isopycnal surfaces 26.0 - 26.1 σθ , in which all in qualitative manner showed an internal hydraulic flow control on the shelf. The turbulence friction velocity was two order of magnitude lower than observed- in this active layer explaining a less role of bottom friction than hydraulic flow to the high turbulence layer. Keywords: topographically induced turbulence, continental shelf, internal hydraulic flow

Marginal instability and deep cycle turbulence in the eastern equatorial Pacific Ocean

Deep cycle mixing in the cold tongue of the equatorial Pacific Ocean is associated with a mean flow regime in which the gradient Richardson number Ri (a ratio of stratification to shear that affects the evolution of turbulence) fluctuates about a critical value near . This is the state of marginal instability (MI), a stable equilibrium between forcing by the trade winds (which works to reduce Ri) and turbulence (which works to increase Ri). Besides providing insight into the physics of deep cycle turbulence, MI is easily recognized in moored records of currents and density, and may therefore provide a valuable indicator of turbulence in historical data where direct turbulence measurements were not made. In this initial study, the seasonal cycle of MI is described. MI is present for 9 months of the year but disappears in March, April, and May, consistent with the recently discovered springtime minimum of equatorial turbulence.

Observations of vertical turbulent nitrate flux across the Kuroshio

AbstractThe vertical turbulent nitrate flux was calculated across the Kuroshio based on direct turbulence measurements and nitrate concentrations. Profile data were analyzed relative to the main axis of the Kuroshio front, the location of maximum surface velocity. Large upward nitrate flux on the northern side of the front, O(10−6) mmol N m−2 s−1, was observed from 300 m up to the base of the euphotic layer. Enhanced turbulence and a large vertical nitrate gradient contributed to this flux. On this side, high concentrations of chlorophyll‐a were also observed near the surface, and the large nitrate flux is consistent with a previous estimate of the rate of new production. In contrast, south of the axis, the nitrate flux was O(10–8–10−7) mmol N m−2 s−1 because of the small vertical nitrate gradient, although vertical diffusivity at the euphotic layer base remained relatively high.

Variability of turbulent quantities in the tidal bottom boundary layer: Case study in the eastern English Channel

Tidal current structure and turbulent quantities within a tidal bottom boundary layer (BBL) have been examined using an upward-looking acoustic Doppler current profiler (ADCP). The instrument was deployed on the seafloor, off the north-eastern French coast in the eastern English Channel over 12 tidal cycles and covered the period of the transition from mean spring to neap tide. Forcing regimes varied from calm to moderate storm conditions during the deployment. For the study of turbulent quantities in the BBL, we have chosen a calm period, when an effect of surface waves on the velocity structure was negligible. Stresses were found to vary regularly with the predominantly semidiurnal tidal flow, with the along-shore stress being generally greater during the flood flow (∼3.0Pa) than during the ebb flow (∼−1.5Pa). The turbulent kinetic energy (TKE) production rate, P, TKE density, Q, and its dissipation rate, ε, followed a nearly regular cycle with close to quarter-diurnal period. Near the seabed, peak values of P, Q and ε were found to be 0.5Wm−3, 0.5m2s−2 and, 0.04Wm−3, respectively, during the flood while, during the ebb, these quantities reached lesser values: 0.1Wm−3, 0.1m2s−2 and 0.03Wm−3, respectively. Near the bottom, eddy viscosity, Az, peak ranged from about 0.1m2s−1 during the flood to 0.03m2s−1 during the ebb flow. Away from the bottom, Az increased to reach a maximum near the mid-depth. Time–depth variation of the P/ε ratio indicated that the turbulence in the BBL, most of the time, was at a non-equilibrium state (P/ε≠1). The largest deviation from the equilibrium occurred during the flood, when P/ε exceeded about one decade near the bottom. During the ebb, P/ε was close to the equilibrium state, slightly decreasing with height above the bottom. Results are found to be in a good agreement with those of the other researches working on direct measurements of turbulence in tidal flows.

Effect of variable winds on current structure and Reynolds stresses in a tidal flow: analysis of experimental data in the eastern English Channel

Abstract. Wind and wave effects on tidal current structure and turbulence throughout the water column are examined using an upward-looking acoustic Doppler current profiler (ADCP). The instrument has been deployed on the seafloor of 18-m mean depth, off the north-eastern French coast in the eastern English Channel, over 12 tidal cycles, and covered the period of the transition from mean spring to neap tide, and forcing regimes varied from calm to moderate storm conditions. During storms, we observed gusty winds with magnitudes reaching 15 m s−1 and wave heights reaching up to 1.3 m. Analysis of velocity spectra revealed a noticeable contribution of wind-induced waves to spectral structure of velocity fluctuations within the subsurface layer. Near the surface, stormy winds and waves produced a significant intensification of velocity fluctuations, particularly when the sustained wind blew against the ebb tide flow. As during wavy periods, the variance-derived Reynolds stress estimates might include a wave-induced contamination, we applied the Variance Fit method to obtain unbiased stresses and other turbulent quantities. Over calm periods, the turbulent quantities usually decreased with height above the seabed. The stresses were found to vary regularly with the predominantly semidiurnal tidal flow. The along-shore stress being generally greater during the flood flow (~2.7 Pa) than during the ebb flow (~−0.6 Pa). The turbulent kinetic energy production rate, P, and eddy viscosity, Az, followed a nearly regular cycle with close to a quarter-diurnal period. As for the stresses, near the seabed, we found the maximum values of estimated quantities of P and Az to be 0.1 Wm−3 and 0.5 m2 s−1, respectively, during the flood flow. Over the storm periods, we found the highest unbiased stress values (~−2.6 Pa) during ebb when tidal currents were opposite to the southwesterly winds while, during the flood, the surface stresses slightly exceeded those estimated for a calm period. A comparison of obtained results gives a good agreement with those of other researchers working on direct measurements of turbulence in tidal flows.

Characteristics of observed turbulent mixing across the Antarctic Slope Front at 140°E, East Antarctica

The strength of mixing due to turbulence in the Antarctic Slope Front (ASF) region was investigated using CTD (conductivity-temperature-depth profilers) observations and direct measurements of turbulence conducted off Adelie Land, East Antarctica along 140°E from the 12th–14th February, 2005. The strongest horizontal gradient of the ASF was located below 300 m depth near the 1000 m isobath. The turbulent measurements revealed that the energy dissipation rate frequently exceeded 10−8 Wkg−1 on the continental shelf and upper slope regions. Turbulent diffusivities near the shelf break were higher than 10−3 m2s−1. Near the ASF the average turbulent heat flux was 5.7 Wm−2 and 1.1 Wm−2 across the temperature minimum layer to 250 m and from 300 to 600 m, respectively. The distribution of the high dissipation rate was consistently explained by the characteristic curve of the M2 internal wave emanating from the shelf break and continental slope. The water mass observed in the ASF below 300 m in the continental slope comprised Modified Circumpolar Deep Water and low salinity Shelf Water originating from either the upper layer of the Adelie Depression or the Adelie Bank, and produced by boundary mixing near the shelf break.

Intense mixing of the Faroe Bank Channel overflow

The continuous, swift flow of cold water across the sill of the Faroe Bank Channel, the deepest passage from the Nordic Seas to the North Atlantic Ocean, forms a bottom‐attached dense plume (overflow). The amount and distribution of entrainment and mixing that the overflow encounters during its descent influence the ventilation of the deep North Atlantic, however, remain poorly known due to lack of direct measurements. Using the first direct turbulence measurements, we describe the dynamic properties and mixing of the overflow plume as it descends toward the Iceland Basin. The vigorously turbulent plume is associated with intense mixing and enhanced turbulent dissipation near the bottom and at the plume‐ambient interface, but with a quiescent core. Our measurements show a pronounced transverse circulation consistent with rotating plume dynamics, a strong lateral variability in entrainment velocity, and a vertical structure composed of order 100 m thick stratified interface and comparably thick well‐mixed bottom boundary layer with significant transport and entrainment.

Descent and mixing of the overflow plume from Storfjord in Svalbard: an idealized numerical model study

Abstract. Storfjorden in the Svalbard Archipelago is a sill-fjord that produces significant volumes of dense, brine-enriched shelf water through ice formation. The dense water produced in the fjord overflows the sill and can reach deep into the Fram Strait. For conditions corresponding to a moderate ice production year, the pathway of the overflow, its descent and evolving water mass properties due to mixing are investigated for the first time using a high resolution 3-D numerical model. An idealized modeling approach forced by a typical annual cycle of buoyancy forcing due to ice production is chosen in a terrain-following vertical co-ordinate. Comparison with observational data, including hydrography, fine resolution current measurements and direct turbulence measurements using a microstructure profiler, gives confidence on the model performance. The model eddy diffusivity profiles contrasted to those inferred from the turbulence measurements give confidence on the skill of the Mellor Yamada scheme in representing sub-grid scale mixing for the Storfjorden overflow, and probably for gravity current modeling, in general. The Storfjorden overflow is characterized by low Froude number dynamics except at the shelf break where the plume narrows, accelerates with speed reaching 0.6 m s−1, yielding local Froude number in excess of unity. The volume flux of the plume increases by five-fold from the sill to downstream of the shelf-break. Rotational hydraulic control is not applicable for transport estimates at the sill using upstream basin information. To the leading order, geostrophy establishes the lateral slope of the plume interface at the sill. This allows for a transport estimate that is consistent with the model results by evaluating a weir relation at the sill.