Free-falling Microstructure Profiler Research Articles

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.

A wavelet-entropy based segmentation of turbulence measurements from a moored shear probe near the wavy sea surface

In this study, we explore the applicability of a wavelet-entropy based segmentation technique in reduction of motion-induced contaminations in time-domain from subsurface turbulence measurements made by a moving shear probe. After the quality screening of data, the Shannon entropy procedure is combined with a time-dependent adaptive wavelet thresholding method to split each 60-s long shear segment into a number of motion-reduced subblocks. The wavelet-entropy strategy leads to preventing the false detection effect caused by applying either wavelet de-noising or Shannon entropy alone for conditions where the turbulence (strongly) overlap with scales induced by waves or platform motions. The longest stationary subblock, with a size greater than 16-s, is then used to extract the Turbulent Kinetic Energy (TKE) dissipation rate, varepsilon. Efficiency of the proposed method is verified by comparing with varepsilon measurements made by a nearby free-falling microstructure profiler. While the quality of observations is constrained by a number of factors such as sensors’ angle of attack, and the wave kinematical and dynamical effects, results demonstrate significant improvements, by approximately a factor of 5–10, compared with varepsilon measurements from each 60-s segment using the Goodman et al. [13] method. Furthermore, the magnitudes of the motion-corrected varepsilon using the proposed method is largely consistent with the scaling suggested by Terray et al. [30].

Turbulent Mixing induced by Cold Water Intrusion over the Southwestern East Sea

ABSTRACT Seo, S.; Park, Y.-G.; Jeon, C.; Min, H. S.; Kim, D. G., and Park, J.-H., 2018. Turbulent mixing induced by cold water intrusion over the southwestern East Sea. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 476–480. Coconut Creek (Florida), ISSN 0749-0208. Along the southeastern coast of Korea the warm and salty Tsushima Warm Currents flows northeastward. During summer a southwestward cold water intrusion from the East Sea occurs along the bottom below the warm current. Thus, a strong shear and consequently strong turbulent mixing are expected along the boundary between the warm water and cold water. To quantify the mixing hydrographic surveys using a free falling microstructure profiler (VMP-500) were conducted. Enhanced dissipation of energy was indeed observed along the interface located between 15°C to 20°C layers. Along the bottom there was anot...

Turbulence Estimation Using Fast-Response Thermistors Attached to a Free-Fall Vertical Microstructure Profiler

AbstractEstimation of turbulence intensity with a fast-response thermistor is examined by comparing the energy dissipation rate from a Fastip Probe, model 07 (FP07), thermistor with from a shear probe, both of which are attached to a free-fall microstructure profiler with the fall rate of 0.6–0.7 m s−1. Temperature gradient spectra corrected with previously introduced frequency response functions represented by a single-pole low-pass filter yields with a bias that strongly depends on turbulence intensity. Meanwhile, the correction with the form of a double-pole low-pass filter derives less bias than of single-pole low-pass filter. The rate is compatible with when the double-pole correction with the time constant of 3 × 10−3 s is applied, and 68% of data are within a factor of 2.8 of in the wide range of = 10−10–3 × 10−7 W kg−1. The rate is still compatible with even in the anisotropy range, where the buoyancy Reynolds number is 20–100. Turbulence estimation from the fast-response thermistor is thus confirmed to be valid in this range by applying the appropriate correction to temperature gradient spectra. Measurements with fast-response thermistors, which have not been common because of their poor frequency response, are less sensitive to the vibration of profilers than those with shear probes. Hence, measurements could be available when a fast-response thermistor is attached to a CTD frame or a float, which extends the possibility of obtaining much more turbulence data in deep and wide oceans.

Vibration Analysis of the Free-Falling Microstructure Profiler

Free-falling microstructure profiler (FFMP) is the most effective platform for measuring ocean microstructure turbulence. Vibration is the key factor of influencing the accuracy of the measurement of the shear sensor mounted on the leading end of the FFMP. In the present work, vibration behavior of an FFMP called FFMP1000 was studied through fluid–structure interaction (FSI) simulations and field trials. Vibration characteristics and mechanism of the FFMP1000 were also discussed. Results showed that motion of the FFMP was like a compound pendulum oscillation, and was caused by vortex shedding at the trailing end of the FFMP. Empirical formulas used to predict the oscillation of the FFMP were deduced based on the characteristics of motion behavior and confirmed through sea trials. The present achievement provides scientific guidance for designing optimal hydrodynamic hull shape of the FFMP. It is also useful to estimate the low end detection limit of the FFMP and to modify the turbulence kinetic energy dissipation rate during ocean observations.

Turbulence and mixing in a freshwater-influenced tidal bay: Observations and numerical modeling

In situ observations and numerical simulations of turbulence are essential to understanding vertical mixing processes and their dynamical controls on both physical and biogeochemical processes in coastal embayments. Using in situ data collected by bottom-mounted acoustic Doppler current profilers (ADCPs) and a free-falling microstructure profiler, as well as numerical simulations with a second-moment turbulence closure model, we studied turbulence and mixing in the Xiamen Bay, a freshwater-influenced tidal bay located at the west coast of the Taiwan Strait. Dynamically, the bay is driven predominantly by the M2 tide, and it is under a significant influence of the freshwater discharged from the Jiulong River. It is found that turbulence quantities such as the production and dissipation rates of the turbulent kinetic energy (TKE) were all subject to significant tidal variations, with a pronounced ebb-flood asymmetry. Turbulence was stronger during flood than ebb. During the flooding period, the whole water column was nearly well mixed with the depth-averaged TKE production rate and vertical eddy viscosity being up to 5×10−6 W kg−1 and 2×10−2 m2 s−1, respectively. In contrast, during the ebb strong turbulence was confined only to a 5–8 m thick bottom boundary layer, where turbulence intensity generally decreases with distance from the seafloor. Diagnosis of the potential energy anomaly showed that the ebb-flood asymmetry in turbulent dissipation and mixing was due mainly to tidal straining process as a result of the interaction between vertically shared tidal currents and horizontal density gradients. The role of vertical mixing in generating the asymmetry was secondary. A direct comparison of the modeled and observed turbulence quantities confirmed the applicability of the second-moment turbulence closure scheme in modeling turbulent processes in this weakly stratified tidally energetic environment, but also pointed out the necessity of further refinements of the model.

The dispersal of phytoplankton populations by enhanced turbulent mixing in a shallow coastal sea

A single tidal cycle survey in a Lagrangian reference frame was conducted in autumn 2010 to evaluate the impact of short-term, episodic and enhanced turbulent mixing on large chain-forming phytoplankton. Observations of turbulence using a free-falling microstructure profiler were undertaken, along with near-simultaneous profiles with an in-line digital holographic camera at station L4 (50° 15′ N 4° 13′ W, depth 50m) in the Western English Channel. Profiles from each instrument were collected hourly whilst following a drogued drifter. Results from an ADCP attached to the drifter showed pronounced vertical shear, indicating that the water column structure consisted of two layers, restricting interpretation of the Lagrangian experiment to the upper ~25m. Atmospheric conditions deteriorated during the mid-point of the survey, resulting in values of turbulent dissipation reaching a maximum of 10−4Wkg−1 toward the surface in the upper 10m. Chain-forming phytoplankton >200μm were counted using the data from the holographic camera for the two periods, before and after the enhanced mixing event. As mixing increased phytoplankton underwent chain breakage, were dispersed by advection through their removal from the upper to lower layer and subjected to aggregation with other suspended material. Depth averaged counts of phytoplankton were reduced from a maximum of around 2050L−1 before the increased turbulence, to 1070L−1 after, with each of these mechanisms contributing to this reduction. These results demonstrate the sensitivity of phytoplantkon populations to moderate increases in turbulent activity, yielding consequences for accurate forecasting of the role played by phytoplankton in climate studies and also for the ecosystem in general in their role as primary producers.

Assessing shelf mixing using CTD, ADCP, and free falling shear probe turbulence data

CTD, ADCP and free-falling microstructure profiler data from Thermaikos Gulf of Northern Aegean Sea in Greece are analyzed in order to (a) gain an insight in the vertical diffusion processes of the area and (b) compare the methods of vertical diffusivity assessment based on microstructure profiler shear data and on potential density overturns. The analysis reveals the presence of mixing events at the top mixed layer as well as above the upper thermocline boundary. It is consistent with a notable change in the susceptibility of the water column to turbulent events, within a month's period as well as convective instability possibly related to riverine discharge. The comparison of the profiles of vertical shear of horizontal velocity and of density presumes isotropy of the turbulent field.

Hypolimnetic turbulence generation associated with superposition of large-scale internal waves in a strongly stratified lake: Lake Biwa, Japan

A 48-h microstructure experiment captured the variation of turbulence in Lake Biwa, Japan, during a strong stratification period, 1 week after a typhoon event. A free-fall microstructure profiler (TurboMAP) and a fine-scale profiler (F-probe) were deployed. An array of five ADCPs positioned close to the experiment site provided current measurements associated with the internal-wave field for a period of one week. Strong winds related to the passage of a typhoon close to the lake generated two low-frequency internal waves: a Kelvin wave and an inertial wave. Both waves were in the first baroclinic mode. This study pictures the superposition of two large-scale internal waves and a stronger current resulting from the two waves being in phase. The synchronization of the waves provoked enhanced shear in the hypolimnion, more than 10 m above the bottom, as well as high dissipation (10−7 W kg−1) and vertical diffusivity reaching 10−4 m2 s−1. The enhanced shear event seems to be related to the current amplitude since it occurred when the current amplitude was increased by the diel wind. Assuming the same turbulence intensity for the enhanced hypolimnetic shear events, this superposition may affect nutrient transfer in the hypolimnion. Also, we witnessed sediment resuspension, consisting of high turbidity and high fluorescence intensity, likely due to a combination of bottom stress and enhanced turbulence.

Measurements of turbulence in the zone of strong bottom currents in the strait of Denmark

A need for experimental data about the entrain� ment of waters from the Irminger Sea into the jet of cooler and denser waters from the Arctic Basin partic� ipating in the formation of deep waters of the North Atlantic became pressing in relation to the research into climatically important meridional water circula� tion in the North Atlantic within the International EU frame project "Thermohaline Overflow—at Risk?" (THOR) (www.euthor.eu). Turbulence near the interface of the intrusion and surrounding waters plays an important role in the entrainment. However, to the present, there has been little success in direct instru� mental measurements of the microstructure at large depths. The solution of this problem required develop� ment of a freefalling microstructure profiler (MP) that makes possible operative measurements of turbu� lent pulsations of hydrophysical fields at depths up to 3500 m. Such a profiler was made at the Atlantic Branch of the Institute of Oceanology with the support of THOR partners from Hamburg University and the Finnish Meteorological Institute. In this paper we describe a new promising method of microstructure measurements realized during the construction of the profiler and discuss preliminary scientific results of profiler application in the cruise of R/V

Multilayer biological structure and mixing in the upper water column of Lake Biwa during summer 2008

We carried out a 24-h station experiment at Lake Biwa (Japan) to measure mixing events and concurrent biological signals using a free-fall microstructure profiler (TurboMAP-L), conventional hydrographic measurement device (F-probe), and the Tracker acoustic profiling system (TAPS). A clearly defined three-layer physical system was observed. Two layers were actively mixed: the surface-mixed layer and the subsurface-mixed layer. Both winds and night-time convection create the surface-mixed layer, and vertical shear due to a counterclockwise gyre maintains turbulence in the subsurface mixing layer. A strongly stratified layer between these two mixing layers is almost turbulence free, so no material flux is expected. A local oxygen maximum layer, a local oxygen minimum layer, and layers of increased chlorophyll and zooplankton abundance are all located in this strongly stratified layer. The data show the intricate influence of physical processes on the structure of biological systems and their combined influence on biogeochemical and trophic transfers in aquatic systems.

Correction to “Latitudinal dependence of diapycnal diffusivity in the thermocline observed using a microstructure profiler”

[1] In the paper “Latitudinal dependence of diapycnal diffusivity in the thermocline observed using a microstructure profiler” by T. Hibiya et al. (Geophysical Research Letters, 34, L24602, doi:10.1029/2007GL032323) there were textual errors inadvertently overlooked during the production proofing process. [2] The second sentence in Paragraph 5 should read as follows. In the present study, we deployed the velocity microstructure profiler TurboMAP-D to a depth below the main thermocline at 53 locations in the interior Pacific Ocean between 53°N and 10°S (Figure 1). [3] Paragraph 9 should read as follows. Figure 3 shows the average diapycnal diffusivities over the depth range of ∼1000 m to ∼1600 m inferred from microstructure observations (hereinafter referred to as KρMSP) and those inferred by incorporating XCP data into Gregg's fine-scale parameterization [Gregg, 1989] (hereinafter referred to as KρPRM). [4] The third sentence of paragraph 10 should read as follows. The latitudinal distribution of KρMSP thus obtained from microstructure observations is in qualitative agreement with predictions based on parameterized estimates KρPRM at mid- and high-latitudes (poleward of 20°N) [Nagasawa et al., 2002; Hibiya and Nagasawa, 2004; Hibiya et al., 2006] and previous microstructure observations at low latitudes (equatorward of 20°N) [Gregg et al., 2003], although fine-scale parameterizations overestimate diapycnal diffusivities by a factor of 2–3. [5] The first sentence of paragraph 12 should read as follows. By deploying the un-tethered free-fall microstructure profiler called “TurboMAP-D” at 53 locations in the interior Pacific Ocean between 53°N and 10°S, we have examined the efficacy of the latitudinal dependence of diapycnal diffusivity in the main thermocline predicted based on parameterized estimates at mid- and high-latitudes [Nagasawa et al., 2002; Hibiya and Nagasawa, 2004; Hibiya et al., 2006] and previous microstructure observations at low-latitudes [Gregg et al., 2003]. [6] The second sentence of paragraph 13 should read as follows. One of the strong candidates that might make up for the deficiency of diapycnal mixing in the main thermocline is thought to be intense diapycnal mixing above abyssal rough bottom topography [Polzin et al., 1997; Ledwell et al., 2000]. [7] The pagination for Furuichi et al. [2005] was incorrect. The reference should read as follows.Furuichi, N., T. Hibiya, and Y. Niwa (2005), Bispectral analysis of energy transfer within the two-dimensional oceanic internal wave field, J. Phys. Oceanogr., 35(11), 2104–2109. [8] Ledwell et al. [1993] reference should be replaced with the following reference.Ledwell, J. R., E. T. Montgomery, K. L. Polzin, L. C. St. Laurent, R. W. Schmitt, and J. M. Toole (2000), Evidence for enhanced mixing over rough topography in the abyssal ocean, Nature, 403, 179–182. [9] The journal in Munk [1966] reference was incorrect. The reference should read as follows.Munk, W. (1966), Abyssal recipes, Deep Sea Res., 13(4), 707–730.

Estimating micro-scale intermittency of fluorescence fields from conventional CTD measurements

A free-fall microstructure profiler (TurboMAP) reveals highly intermittent features in the fluorescence field. Conventional CTDs that mount a fluorometer are usually deployed with a tethered cable, and the data are averaged or smoothed over a given time or water depth. Because of the nature of the operation and the data processing involved, intermittent features of fluorescence fields are masked. We have developed a statistical technique to extrapolate the intermittency using a lognormal mixture probability density function (mixture pdf). The statistics obtained from the mixture pdf are consistent with microstructure data, thus the conventional CTD can provide statistics appropriate to a micro-scale fluorescence field.

A New Free-Fall Profiler for Measuring Biophysical Microstructure

This paper evaluates the performance of a newly developed free-falling microstructure profiler. The instrument is equipped with standard turbulence sensors for measuring turbulent velocity shear and temperature gradient, as well as bio-optical sensors for measuring in situ chlorophyll and turbidity variations. Simultaneous measurements with this profiler and an acoustic Doppler velocimeter were carried out in a flow tank, and data from both instruments agreed well. Turbulence spectra computed from both instruments agreed with the Kolmogorov inertial subrange hypothesis over approximately two decades in wavenumber space. Data from field tests conducted with the profiler showed that turbulence spectra measured in situ agreed with the empirical Nasmyth spectrum when corrections were made for the shear probe’s spatial averaging. Dissipation rates as low as 5 3 10210 Wk g 21 were resolved when certain precautions were taken to avoid spectral bias caused by instrument vibrations. By assuming a universal form of the turbulence spectrum, turbulent kinetic energy dissipation rates below 5 3 10210 Wk g 21 can be estimated. The optical sensors resolved centimeter-scale structures of in vivo fluorescence and backscatter in field measurements.

Observations of Stratified Turbulent Mixing in an Estuary: Neap-to-spring Variations During High River Flow

Observations of turbulent mixing, stratification and currents were made with a free-falling microstructure profiler and a 600-kHz ADCP in the Hudson River off Manhattan in May 1994. The ship was anchored in a relatively straight and uniform channel about 15m deep. Over 4 days of intense observations from 19 to 24 May 1994, the progression from neap tides to spring tides caused a severe reduction in stratification from an initial top-to-bottom salinity difference of 18 to 3–6. While low gradient Richardson numbers,Ri, occurred only in the weakly stratified bottom layer on flood during neap tides, lowRispanned the water column on the second half of ebb during spring tides. Turbulent dissipation rates varied roughly inversely toRi. During neap conditions, turbulent mixing was intense only in the bottom layer on flood, where eddy diffusivities,Kρ, reached more than 10−2m2s−1, while mixing was weak in the centre of the halocline near 5m. During spring tides, strong mixing occurred throughout the water column during the latter part of ebb with typicalKρ≈5×10−3m2s−1and typical vertical salt flux of 0·0015kg m−2s−1. If it existed in isolation, this flux would eliminate the haline stratification on a time scale of 4h. Ebb-time bands of lowRiand intense mixing showed increasing phase lag with increasing distance from the bottom. The spring-tide, ebb-time internal turbulent stress was relatively uniform with depth with a typical magnitude of 0·4Pa (4dyne cm−2). Turbulent overturning (Thorpe) scales ranged from below 1cm in the neap-tide halocline to a few metres in the weakly stratified bottom layer on flood. The strong spring-tide, ebb-time mixing had Thorpe scales of typically 0·5m throughout the water column. Ozmidov scales tracked Thorpe scales except in the weakly stratified layer above the bottom.