arXiv subjects

Hans van Haren

Publications and source records attributed to Hans van Haren.

At least 37 records · Page 2Linked to original sources

arXiv2023

Convection and intermittency noise in water temperature near a deep Mediterranean seafloor

Turbulent and internal wave motions are important for the exchange of momentum, heat and suspended matter in the deep-sea which is generally stably stratified in density. Turbulence-generation models involve shear of vertical current differences that deforms the stratified waters, and convection that is drive by (unstable) buoyancy. Shear-generation is found more general in the well-stratified ocean-interior, while convection is known to occur near the sea-surface, e.g. via nighttime cooling. Far below the surface, the Western-Mediterranean Sea is very weakly stratified and offers opportunity to observationally study deep-sea convection. An opportunistic small set of high-resolution temperature sensors demonstrates not only classic internal-wave-induced turbulence, but also convection attributed to geothermal heating and spectral properties that relate to various chaos-theory models such as 1/sigma pink noise (sigma denoting frequency), mainly found lying at (0.01 m above) the seafloor, and 1/sigma^2 Brownian noise, mainly found on a moored line at about 100 m above the seafloor. Near-inertial temperature variations are observed to occur down to the seafloor thereby disturbing the local convective turbulence regime to shear-dominated one temporarily. The integral turbulence time-scale is generally smaller (with dominant higher frequency motions) at the seafloor than about 100 m above it.

physics.flu-dyn↗

arXiv2022

Turbulence observations using moored temperature sensors in weakly stratified deep West-Mediterranean waters

In the stably stratified ocean, small-scale turbulence is important for vertical exchange and hence for the mixing of water masses and suspended matter. To observationally study turbulent motions and the buoyancy- and shear-generators behind them, a 100-m tall array of high-resolution temperature (T) sensors was moored at a 2480-m deep seafloor near the steep continental slope of the Western Mediterranean Sea. The area is dominated by boundary flow, (sub-)mesoscale eddies, internal waves, wintertime dense water formation, and very weak vertical density stratification. Various physical oceanographic processes are observed in detail through a seasonal cycle, including: Low shear-driven turbulence in weak stratification in late-summer and autumn, large buoyancy-driven turbulence by geothermal heating from below alternated with inertial internal wave motions in winter, and moderate buoyancy-like turbulence pushed with stratified waters from above in late-winter and spring.

physics.ao-ph↗

arXiv2022

Internal wave and turbulence observations with very high-resolution temperature sensors along the Cabauw mast

Knowledge about the characteristics of the atmospheric boundary layer is vital for the understanding of redistribution of air and suspended contents that are particularly driven by turbulent motions. Despite many modelling studies, detailed observations are still demanded of the development of turbulent exchange under stable and unstable conditions. In this paper, we present an attempt to observationally describe atmospheric internal waves and their associated turbulent eddies in detail, under varying stable conditions. Therefore, we mounted 198 high-resolution temperature T-sensors with 1-m spacing on a 200-m long cable. The instrumented cable was attached along the 213 m tall meteorological mast of Cabauw, the Netherlands, during late-summer 2017. The mast has standard meteorological equipment at extendable booms at 6 levels in height. A sonic anemometer is at 60 m above ground. The T-sensors have a time constant in air of tau = 3 s and an apparent drift about 0.1 degC/mo. Also due to radiation effects, short-term measurement instability is 0.05 degC/h during nighttime and 0.5 degC/h during daytime. These T-sensor characteristics hamper quantitative atmospheric turbulence research, due to a relatively narrow inertial subrange of maximum one order of magnitude. Nevertheless, height-time images from two contrasting nights show internal waves up to the buoyancy period of about 300 s, and shear and convective deformation of the stratification over the entire 197 m range of observations, supported by nocturnal marginally stable stratification. Moderate winds lead to 20-m tall convection across weaker stratification, weak winds to episodic <10-m tall shear instability across larger stratification.

physics.ao-ph↗

arXiv2022

Foucault-Wheatstone device in the deep-sea

A pressure sensor, located for 4 months in the middle of a 1275-m long taut deep-sea mooring in 2380 m water-depth above a seamount with sub-surface top-buoys and seafloor anchor-weight, demonstrates deterministic spectral peaks at sub-harmonics of the local near-inertial frequency. None of these frequencies can be associated with oceanographic motions. No corresponding peaks are found in spectra of other observables like water-flow (differences), temperature, and pressure in the top-buoys of the mooring. The mid-cable pressure sensor was mounted on a nearly 1-kN weighing non-swiveled frame. Its data seem to reflect a resonant mechanical oscillation of the tensioned mooring cable under repeated short-scale Strouhal vibrations induced by water-flow. Equivalent vertical motions of 0.5-m amplitudes are found dominant at frequency f*/4, with monotonically decreasing peaks at f*/2, 3f*/4 and f* = 1.083f, where f denotes the local inertial frequency of Earth rotation. The observations provide, physically, proof of the Earth rotation and, oceanographically, of the relative vortex-rotation dzeta = 0.083f induced by (sub-)mesoscale eddies associated with a nearby seamount, so that effective near-inertial frequency f* = f + dzeta. As instrumentation only shows mid-cable resonant signals it is not considered as a Foucault pendulum, but more likely an Earth-rotation equivalent of the Wheatstone device with the mooring cable acting as elastic springs. While the f*/2 motion reflects the Foucault principle of linear orbits fixed in an inertial frame of reference, the dominant sub-harmonic f*/4 suggests resemblance with the precession of a spherical oscillator at half the angular speed of rotation.

physics.ao-ph↗

arXiv2022

A simple model for an internal wave spectrum dominated by non-linear interactions

Ocean motions at frequencies of the internal wave band are generally associated with freely propagating waves that are supported by stable vertical stratification in density. Previous analyses of yearlong current observations from the Bay of Biscay showed that a finestructure of semidiurnal tidal and near-inertial higher harmonics fills the spectrum. Here, a simple model is presented of forced nondispersive motions with forward energy cascade. The model fits the spectral shape of higher harmonics well within statistical significance and shows that such interactions imply maximum wave steepness in a balance between forcing and turbulent mixing. The single fitting parameter is related to the barotropic tidal flow speed, which thereby sets a nonlinear limit to baroclinic current scales without generating non-linear higher harmonics directly.

physics.ao-ph↗

arXiv2022

A note on the use of equidistant contours for presenting scientific data

The passionate plea for the use of scientific colour maps misses some aspects in the visual presentation of scientific data. While a linear colour map based on scientific human colour perception is useful for the presentation of some images, like the three examples given of the topography of the earth, an apple and a passport photograph, scientific data are not presented. In this note it will be shown that there is more in scientific oceanographic data as they are presented in forms varying from historic equidistant contours, via a linear black-(grey)-white b&w map, a linear colour map and a nonlinear colour map. From an objective perspective, equidistant contouring is the best means for presenting scientific information in a relatively unbiased way. Nonlinear colour maps may add information to that by highlighting certain aspects also by varying the colour range if needed. Such information is not available from linear colour maps. Finally, the aesthetic aspect of visual data presentation is discussed.

cs.HC↗

arXiv2021

Half a cubic hectometer mooring array of 3000 temperature sensors in the deep sea

The redistribution of matter in the deep-sea depends on water-flow currents and turbulent exchange, for which breaking internal waves are an important source. As internal waves and turbulence are essentially three-dimensional 3D, their dynamical development should ideally be studied in a volume of seawater. However, this is seldom done in the ocean where 1D-observations along a single vertical line are already difficult. We present the design, construction and successful deployment of a half-cubic-hectometer (480,000 m3) 3D-T mooring array holding 2925 high-resolution temperature sensors to study weakly density-stratified waters of the 2500-m deep Western Mediterranean. The stand-alone array samples temperature at a rate of 0.5 Hz, with precision <0.5 mK, noise level <0.1 mK and expected endurance of 3 years. The independent sensors are synchronized inductively every 4 h to a single standard clock. The array consists of 45 vertical lines 125 m long, at 9.5 m horizontally from their nearest neighbor. Each line is held under tension of 1.3 kN by a buoyancy element that is released chemically one week after deployment. All fold-up lines are attached to a grid of cables that is tensioned in a 70 m diameter ring of steel tubes. The array is build-up in harbor-waters, with air filling the steel tubes for floatation. The flat-form array is towed to the mooring site under favorable sea-state conditions. By opening valves in the steel tubes, the array is sunk and its free-fall is controlled by a custom-made drag-parachute reducing the average sinking speed to 1.3 m s-1 and providing smooth horizontal landing on the flat seafloor.

physics.ao-ph↗

arXiv2021

Convection under internal waves in a mountain lake

Turbulent mixing processes in deep alpine Lake Garda (I) have not extensively been observed. Knowledge about drivers of turbulent fluxes are important for insights in the transport of matter, nutrients and pollutants, in the lake and in natural water bodies in general. In this paper, the occurrence of internal wave induced turbulent convection is addressed as opposed to the more common shear-induced turbulence in a density stratified environment. Observations are analyzed from a dedicated yearlong mooring holding 100 high-resolution temperature sensors at 1.5 m intervals under a single current meter in the deeper half of the 344 m deep lake-center. Episodically, the weakly density stratified waters in the lower 50 m above the lake floor show spectral slope and coherence evidence of short-term (15 to 30 minutes) convective motions under internal waves that are supported by the stronger stratified waters above. The near-homogeneous conditions are not attributable to frictional Ekman dynamics, but to large-scale internal wave crests.

physics.ao-ph↗

arXiv2021

Further correcting pressure effects on SBE911 CTD-conductivity data from hadal depths

Hadal, more than 6000 m deep shipborne Sea-Bird Electronics SBE 911plus Conductivity Temperature Depth (CTD) data are obtained using two different systems in the vicinity of the Earths deepest point, in the Challenger Deep, Mariana Trench 14 years apart. Below 7000 m in very weakly density stratified waters, the salinity data from both sets show an artificial increase with depth of about 10-6 g kg-1 m-1 that is not covered by the SBE linear pressure correction to the conductivity data. With the aid of independent water sample data, salinity is corrected and an additional algorithm for the pressure correction on conductivity data is formulated. The corrected salinity data still weakly increase with depth, together with a decrease in temperature, which may point at an influx of dense modified Antarctic bottom water. The corrected density variations with depth are used in calculations of deep turbulence values.

physics.ao-ph↗

arXiv2020

Moored observations of turbulent mixing events in deep Lake Garda (I)

Deep water circulation and mixing processes in deep lakes are largely unknown, although they are responsible for the transport of matter, nutrients and pollutants. Such a lack of knowledge cannot be reliably provided by numerical hydrodynamic modelling studies because detailed observations are typically not available to validate them. To overcome some of these deficiencies, a dedicated yearlong mooring comprising 100 high-resolution temperature sensors and a single current meter were located in the deeper half of the 344 m deepest point of the subalpine Lake Garda (Italy). The observations show peaks and calms of turbulent exchange, besides ubiquitous internal wave activity. In late winter, northerly winds activate episodic deep convective overturning, the dense water being subsequently advected along the lake-floor. Besides deep convection, such winds also set-up seiches and inertial waves that are associated with about 100 times larger turbulence dissipation rates than that by semidiurnal internal wave breaking observed in summer. In the lower 60 m above the lake-floor however, the average turbulence dissipation rate is approximately constant in value year-around, being about 10 times larger than open-ocean values, except during deep convection episodes.

physics.ao-ph↗

arXiv2020

Challenger Deep internal wave turbulence events

Marine life has been detected in the ocean's trenches at great depths down to nearly 11 km. Such life is subject to particular environmental conditions of large static pressure exceeding 1000 atmosphere. While current flows are expected to be slow, waters cannot be stagnant with limited exchange of fresh nutrients needed to support life. For sufficient nutrient supply, the physics process of turbulent exchange is required. However, the environmental conditions hamper research in such waters. To study potential turbulent water motions, a string equipped with specially designed high-resolution temperature sensors was moored near the deepest point on Earth in the Challenger Deep, Mariana Trench for nearly three years. A preliminary analysis of a six-day period when the mooring was still demonstrates hundreds of meters slanted convection due to internal waves breaking from above. The associated turbulence dissipation rate with peak values hundred times above the background value is considered sufficient to maintain deep-trench life. Turbulence associates with one-ten thousandth of a degree temperature anomalies of about one hour duration.

physics.ao-ph↗

arXiv2020

Deep sea floor observations of typhoon driven enhanced ocean turbulence

The impact of large atmospheric disturbances on deep benthic communities is not well known quantitatively. Observations are scarce but may reveal specific processes leading to turbulent disturbances. Here, we present high-resolution deep-ocean observations to study potential turbulent mixing by large atmospheric disturbances. We deployed an array of 100-Hz sampling-rate geophysical broadband Ocean Bottom Seismometers (OBSs) on the seafloor. Within the footprint of this array we also deployed an oceanographic 1-Hz sampling-rate vertical temperature sensor string covering the water phase between 7 and 207 m above the seafloor at about 3000 m depth off eastern Taiwan between June 2017 and April 2018. All instruments registered Category 4 cyclone Typhoon Talims passage northeast of the array one day ahead of the cyclones closest approach when the cyclones eye was more than 1,000 km away. For 10 days, a group of near-inertial motions appeared most clearly in temperature. The registration reflects the importance of barotropic response to cyclones and the propagation of inertio-gravity waves in weak density stratification. In addition to internal tides, these waves drove turbulent overturns larger than 200 m that were concurrently registered by OBSs. The turbulent signals were neither due to seismic activity nor to ocean-surface wave action. Cyclones can generate not only microseisms and earth hums, as well as turbulence in the water column, producing additional ground motions. Quantified turbulence processes may help constrain models on sediment resuspension and its effect on deep-sea benthic life.

physics.ao-ph↗

arXiv2020

Slow persistent mixing in the abyss

Knowledge about deep-ocean turbulent mixing and flow circulation above abyssal hilly plains is important to quantify processes for the modelling of resuspension and dispersal of sediments in areas where turbulence sources are expected to be relatively weak. Turbulence may disperse sediments from artificial deep-sea mining activities over large distances. To quantify turbulent mixing above the deep-ocean floor around 4000 m depth, high-resolution moored temperature sensor observations have been obtained from the near-equatorial southeast Pacific (7°S, 88°W). Models demonstrate low activity of equatorial flow dynamics, internal tides and surface near-inertial motions in the area. The present observations demonstrate a Conservative Temperature difference of about 0.012°C between 7 and 406 meter above the bottom (hereafter, mab, for short), which is a quarter of the adiabatic lapse rate. The very weakly stratified waters with buoyancy periods between about six hours and one day allow for slowly varying mixing. The calculated turbulence dissipation rate values are half to one order of magnitude larger than those from open-ocean turbulent exchange well away from bottom topography and surface boundaries. In the deep, turbulent overturns extend up to 100 m tall, in the ocean-interior, and also reach the lowest sensor. The overturns are governed by internal-wave-shear and -convection. The turbulence inertial subrange is observed to extend into the internal wave frequency band. The associated mixing is not related to bottom friction processes but to internal wave breaking and near-inertial shear. The mixing facilitates long (hours to day) and high (exceeding 100 mab) dispersal of suspended sediments.

physics.ao-ph↗

arXiv2020

Deep caller for ocean acoustic releases

We relate about the custom-made modification of a Benthos deep-ocean acoustic release into a deep caller, a hydrophone for calling ocean acoustic releases that cannot be reached from a standard deck-unit. The self-contained deep caller can be lowered down to 12 km on any non-conducting winch cable. It may prove useful to retrieve sub-surface instrumentation like a seafloor-lander hidden behind large rocks or in a narrow canyon, or moorings in very deep topographic depressions. We used it to retrieve a 7 km long mooring from 10,910 m depth in the Challenger Deep, Mariana Trench.

physics.ao-ph↗

arXiv2019

Pacific shallow lagoon high-resolution temperature observations

The daily cycle of heating and cooling of the near-surface ocean may be quite different in a shallow lagoon with a few meters deep seafloor that can be heated directly by the sun. If important, the solar radiation will affect the local benthic communities. To study the physical processes associated with the daily cycle of south-Pacific lagoon Bora Bora, a vertical string of five high-resolution temperature sensors was moored at a 2-m deep site for 3 weeks. Besides the standard ocean warming (approximately during daytime) and cooling (approximately nighttime), the sensors show relatively highest temperature near the lagoon-floor during the warming phase and a weakly stable stratification towards the end of the cooling phase. During the warming phase, highly variable stratification is observed extending into the water column under calm weather and turbid waters, otherwise not. Under trade wind and clear waters, the lowest sensor(s) show(s) consistently higher temperature variability than sensors higher-up with spectral slopes indicative of shear- and/or convective turbulence. During the cooling phase, the lower sensor shows consistently very low variance (non-turbulent), while other sensors show a spectral slope around the buoyancy frequency evidencing weakly stratified waters supporting internal waves. These observations contrast with open-ocean near-surface observations of stable stratification during the warming phase and of turbulent free convection during the cooling phase. Thus, lagoons seem to more resemble the atmosphere than the ocean in daytime thermodynamics and possibly act as a natural solar pond with bottom conductive heating (when salinity compensates for unstable temperature variations).

physics.ao-ph↗

arXiv2019

Internal wave mixing in warming Lake Grevelingen

Seasonal hypoxia or even anoxia can occur in some local deep basins of coastal waters. Such low summertime oxygen contents especially affect benthic life. The seasonal coastal hypoxia is commonly related to biological increased respiration and to physical limited vertical turbulent exchange that is associated with increased vertical stable density stratification. However, the same stratification can support internal waves that may break and locally generate turbulence. Here, we investigate the physics of internal wave motions in saltwater Lake Grevelingen (SW-Netherlands) during warming in mid-spring. Grevelingen is refreshed by weak tidal motions through an open sluice in its dam to the North Sea. The outer North Sea has a surface tidal range of about 3 m, but the lake surface tidal range is negligible (<0.1 m). To quantify vertical turbulent exchange, high-resolution temperature sensors were moored in conjunction with a current meter in 36 m water depth for three days. The site is known for anoxic conditions near the bottom in summer. While a 3-day, 30-m mean eddy diffusivity of <[Kz]> = 4+/-2x10-5 m2 s-1 is found, the overall mean turbulence dissipation rate (turbulent flux) is <[epsilon]> = 1.1+/-0.6x10-7 m2 s-3. Turbulent mixing occurs episodically, via near-surface cooling during night and increased winds, via sparse shear-driven breaking of internal waves at the main pycnocline, and via sheared near-bottom currents. Shear-driven turbulence is not commonly found in fresh-water lakes. Just below the main pycnocline around mid-depth a layer of weak turbulence is observed, as in fresh-water lakes. The observed turbulent exchange is sufficient to warm the near-bottom waters over the course of summer, but insufficient to prevent the biology from over-consuming oxygen contents.

physics.ao-ph↗

arXiv2019

Deep-ocean inertial subrange small bandwidth coherence and Ozmidov-frequency separation

The inertial subrange of turbulence in a density stratified environment is the transition from internal waves to isotropic turbulence, but it is unclear how to interpret its extension to anisotropic stratified turbulence. Knowledge about stratified turbulence is relevant for the dispersal of suspended matter in geophysical flows, such as in most of the ocean. For studying internal-wave-induced ocean-turbulence moored high-resolution temperature (T-)sensors are used. Spectra from observations on episodic quasi-convective internal wave breaking above a steep slope of large seamount Josephine in the Northeast-Atlantic demonstrate an inertial subrange that can be separated in two parts: A large-scale part with relatively coherent portions adjacent to less coherent portions, and a small-scale part that is smoothly continuous (to within standard error). The separation is close to the Ozmidov frequency, and coincides with the transition from anisotropic/quasi-deterministic stratified turbulence to isotropic/stochastic inertial convective motions as inferred from a comparison of vertical and horizontal co-spectra. These observations contrast with T-sensor observations of shear-dominated internal wave breaking in an equally turbulent environment above the slope of a small Mid-Atlantic ridge-crest, which demonstrate a stochastic inertial subrange throughout.

physics.ao-ph↗

arXiv2019

Turbulent convection and high-frequency internal wave details in 1-m shallow waters

Vertically 0.042-m-spaced moored high-resolution temperature sensors are used for detailed internal wave-turbulence monitoring near Texel North Sea and Wadden Sea beaches on calm summer days. In the maximum 2 m deep waters irregular internal waves are observed supported by the density stratification during day-times warming in early summer, outside the breaking zone of <0.2 m surface wind waves. Internal-wave-induced convective overturning near the surface and shear-driven turbulence near the bottom are observed in addition to near-bottom convective overturning due to heating from below. Small turbulent overturns have durations of 5-20 s, close to the surface wave period and about one-third to one-tenth of the shortest internal wave period. The largest turbulence dissipation rates are estimated to be of the same order of magnitude as found above deep-ocean seamounts, while overturning scales are observed 100 times smaller. The turbulence inertial subrange is observed to link between the internal and surface wave spectral bands.

physics.ao-ph↗