Thermistor Strings Research Articles

A comparison between vertical motions measured by ADCP and inferred from temperature data

Abstract. Combined vertical current (w) and thermistor string data demonstrate that high-, near-buoyancy frequency internal "wave" trains along a pycnocline in a flat-bottom shelf sea consist for 2 periods of a dominant mode-1 non-linear part, while thereafter mainly of linear [mode-2, quadrupled frequency] waves, to first order. In a simple [linear] heat budget the use of unfiltered temperature gradient or its time mean changes results by only 10%. The observations also demonstrate that temperature is not always adequate to estimate vertical motions using the linear 1-D heat equation. In shallow seas, tidal-w estimated from temperature data can be an order of magnitude weaker than directly observed w, and thus do not represent free internal waves. In the ocean, not too far from the main internal wave topography source, tidal motions represent linear waves and are well described by temperature-inferred w. There however, temperature-inferred w and directly observed w differ strongly near the buoyancy frequency, at which w is dominated by non-linear waves, and near [sub]inertial frequencies, at which w is dominated by eddies and gyroscopic waves.

The NEREUS in‐lake wireless/acoustic chemical data network

We describe a system for the semi‐autonomous in situ measurement of lake chemical properties, with high temporal resolution and the potential to map chemical properties in 3 dimensions. The system comprises both fixed sensor locations on moored buoys and one (or potentially more) autonomous underwater vehicles (AUVs) serving as mobile underwater sensor platforms. Sensors include conventional water quality multiprobes and thermistor strings and the NEREUS underwater mass spectrometer for measurement of dissolved metabolic gases such as methane as well as permanent gases. Data are delivered to a shore station via a network that includes acoustic modems for communication with the AUV and a radio network operating in the license‐free ISM (industrial, scientific, and medical) band operating under the IEEE 802.11b protocol. The system is designed to use low‐cost commercial hardware and open‐source software wherever possible. We constructed a prototype system of 3 buoys, 1 AUV, and a shore station, and field tests have demonstrated its ability to measure and remotely display lake chemical data in real time.

Forced Resonant Oscillations as a Response to Periodic Winds in a Stratified Reservoir

The response of the Sau reservoir to a wind field characterized by having marked periodicities of 12 h and 24 h has been studied. Measurements of temperature, with a thermistor string, and currents, with an acoustic Doppler current meter, show that the reservoir also responds with the same water periodicities. During certain times of the stratified period, some of the natural oscillation modes of the reservoir are close to these forcing wind periods. In particular, in mid-July the vertical Mode V2 is close to 12 h and in mid- to end of September the vertical Mode V3 is close to 24 h. In these situations, these modes are selected out of the spectrum of possible internal waves and the reservoir behaves as a forced oscillator in resonance with the wind. The structure and the period of these vertical modes have been elucidated by using the 3D model ELCOM. Both modes are affected by the Earth's rotation at the widest part of the reservoir.

Reconstruction of snow/firn thermal diffusivities from observed temperature variation: application to iceberg C16, Ross Sea, Antarctica, 2004–07

Abstract.Two inverse methods are proposed as a means of estimating the thermal diffusivity of snow and firn from continuous measurements of their temperature. The first method is applicable to shallow depths where temperature experiences diurnal variations, and is based on the fact that phase and amplitude of these diurnal variations are functions of the thermal diffusivity. The second method is applicable to the deeper part of the firn layer, and is based on a simple least-squares estimation technique. The methods applied here differ from various methods used for borehole paleothermometry in that observations are continuous in time and performance constraints on model/data misfit can be applied over a finite temporal period. Both methods are tested on temperature records from thermistor strings operating in the upper 2.5 m of firn on iceberg C16 (Ross Sea, Antarctica) from 2004 to 2007. Results of the analysis show promise in identifying melting events and the movement and refreezing of meltwater within the snow/firn layer.

Echo intensity data as a directional antenna for observing processes above sloping ocean bottoms

Relative ‘echo intensity’ data (dI) from a bottom-mounted four-beam 300 kHz acoustic Doppler current profiler (ADCP) are used to infer propagation of vigorous processes above a continental slope. The 3- to 60-m horizontal beam spread and the 2-Hz sampling allow the distinction of different arrival times t i , i = 1,..., 4, at different distances in the acoustic beams from sharp changes in dI-content associated with frontal non-linear and turbulent bores or ‘waves’. The changes in dI are partially due to variations in amounts of resuspended material carried by the near-bottom turbulence and partially due to the fast variations in density stratification (‘stratified turbulence’), as inferred from 1-Hz sampled thermistor string data above the ADCP. Such bores are observed to pass the mooring up to 80 m above the bottom, having typical propagation speeds c = 0.15–0.5 m s−1, as determined from dI(t i ). Particle speeds in the immediate environment of a bore amount to |u|env=c ± 0.05 m s−1, the equality being a necessary condition for kinematic instability, whilst the maximum particle speeds amount |u|max = 1.2–2c. The dI-determined directions of up-, down- and alongslope processes are all to within ±10° of the ADCP’s beam-spread averaged current (particle velocity) data.

Using Smart Sensor Strings for Continuous Monitoring of Temperature Stratification in Large Water Bodies

A "smart" thermistor string for continuous long-term temperature profiling in large water bodies is described allowing highly matched yet low-cost spatial and temporal temperature measurements. The sensor uses the three-wire SDI-12 communications standard to enable a low-powered radio or data logger on supporting buoys to command measurements and retrieve high-resolution temperature data in digital form. Each "smart" temperature sensor integrates a thermistor element, measurement circuitry, power control, calibration coefficient storage, temperature computation, and data communications. Multiple addressable sensors at discrete vertical depths are deployed along a three-wire cable that provides power and allows data transfer at regular intervals. Circuit, manufacturing, and automated calibration techniques allow temperature measurements with a resolution of plusmn0.003degC, and with intersensor matching of plusmn0.006degC. The low cost of each sensor is achieved by using poor tolerance thermistor and circuit components in conjunction with a 15-bit charge-balance analog-to-digital converter. Sensor inaccuracies and temperature coefficients are corrected by a two-point calibration procedure made possible by a standard-curve generator within the sensor, based upon the method of finite differences. This two-point calibration process allows in-field sensor string calibration in stratified water bodies and provides a means to correct for long-term calibration drift without having to return the string to a laboratory

Exploration of Ellsworth Subglacial Lake: a concept paper on the development, organisation and execution of an experiment to explore, measure and sample the environment of a West Antarctic subglacial lake

Antarctic subglacial lakes have, over the past few years, been hypothesised to house unique forms of life and hold detailed sedimentary records of past climate change. Testing this hypothesis requires in situ examinations. The direct measurement of subglacial lakes has been considered ever since the largest and best-known lake, named Lake Vostok, was identified as having a deep water-column. The Subglacial Antarctic Lake Environments (SALE) programme, set up by the Scientific Committee on Antarctic Research (SCAR) to oversee subglacial lakes research, state that prior exploration of smaller lakes would be a “prudent way forward”. Over 145 subglacial lakes are known to exist in Antarctica, but one lake in West Antarctica, officially named Ellsworth Subglacial Lake (referred to hereafter as Lake Ellsworth), stands out as a candidate for early exploration. A consortium of over 20 scientists from seven countries and 14 institutions has been assembled to plan the exploration of Lake Ellsworth. An eight-year programme is envisaged: 3 years for a geophysical survey, 2 years for equipment development and testing, 1 year for field planning and operation, and 2 years for sample analysis and data interpretation. The science experiment is simple in concept but complex in execution. Lake Ellsworth will be accessed using hot water drilling. Once lake access is achieved, a probe will be lowered down the borehole and into the lake. The probe will contain a series of instruments to measure biological, chemical and physical characteristics of the lake water and sediments, and will utilise a tether to the ice surface through which power, communication and data will be transmitted. The probe will pass through the water column to the lake floor. The probe will then be pulled up and out of the lake, measuring its environment continually as this is done. Once at the ice surface, any water samples collected will be taken from the probe for laboratory analysis (to take place over subsequent years). The duration of the science mission, from deployment of the probe to its retrieval, is likely to take between 24 and 36 h. Measurements to be taken by the probe will provide data about the following: depth, pressure, conductivity and temperature; pH levels; biomolecules (using life marker chips); anions (using a chemical analyzer); visualisation of the environment (using cameras and light sources); dissolved gases (using chromatography); and morphology of the lake floor and sediment structures (using sonar). After the probe has been retrieved, a sediment corer may be dropped into the lake to recover material from the lake floor. Finally, if time permits, a thermistor string may be left in the lake water to take time-dependent measurements of the lake’s water column over subsequent years. Given that the comprehensive geophysical survey of the lake will take place in two seasons during 2007–2009, a two-year instrument and logistic development phase from 2008 (after the lake’s bathymetry has been assessed) makes it possible that the exploration of Lake Ellsworth could take place at the beginning of the next decade.

Propagation of impulsive broadband signals in a coastal ocean setting during the 1996 Shelfbreak Primer experiment

The 1996 Shelfbreak PRIMER experiment provided an opportunity to study the impulse response of a coastal ocean waveguide using broadband acoustic signals from explosive sources. Acoustic transmission paths in the vicinity of the shelfbreak included various orientations ranging from up‐slope to along‐slope, including interactions with a shelfbreak front. Acoustic receptions were acquired with a 16‐element vertical line array (VLA). In addition, thermistor string data acquired at the receiving array suggest that a group of high‐amplitude, internal waves traversed the test site during this portion of the experiment. Modal decomposition of broadband signal arrivals was performed to investigate variation of the impulse response during source deployments. Variations in the received spectra of individual normal modes were compared with numerical predictions for impulsive broadband signals propagating in range‐dependent environments constructed using oceanographic data available for the test site. The potential for mode coupling and three‐dimensional effects to shape the modal and spectral distribution of received energy is being investigated. Numerical and experimental results will be discussed. [Work supported by Office of Naval Research.]

Environmental effects on high-frequency propagation during the Makai experiment

The Makai experiment was conducted from 15 September to 2 October 2005 off Kauai Island to provide a comprehensive study of acoustic propagation in the 8–14-kHz band for diverse applications. Thermistor string measurements show how the interaction between the barotropic tides and the bathymetry generates a highly dynamic environment. Strong variability in the sound-speed profile has been observed, with diurnal and semidiurnal thermocline depth changes up to 50 m. Acoustic waves have been propagated at several ranges between two sources and receiver arrays, using fixed and drifted setup. The structure of the received impulse functions shows strong variability, although resolved arrivals are persistent.

A Look at the Surface-Based Temperature Inversion on the Antarctic Plateau

Abstract Data from radiosondes, towers, and a thermistor string are used to characterize the temperature inversion at two stations: the Amundsen-Scott Station at the South Pole, and the somewhat higher and colder Dome C Station at a lower latitude. Ten years of temperature data from a 22-m tower at the South Pole are analyzed. The data include 2- and 22-m temperatures for the entire period and 13-m temperatures for the last 2 yr. Statistics of the individual temperatures and the differences among the three levels are presented for summer (December and January) and winter (April–September). The relationships of temperature and inversion strength in the lowest 22 m with wind speed and downward longwave flux are examined for the winter months. Two preferred regimes, one warming and one cooling, are found in the temperature versus longwave flux data, but the physical causes of these regimes have not been determined. The minimum temperatures and the maximum inversions tend to occur not with calm winds, but with winds of 3–5 m s−1, likely due to the inversion wind. This inversion wind also explains why the near-surface winds at South Pole blow almost exclusively from the northeast quadrant. Temperature data from the surface to 2 m above the surface from South Pole in the winter of 2001 are presented, showing that the steepest temperature gradient in the entire atmosphere is in the lowest 20 cm. The median difference between the temperatures at 2 m and the surface is over 1.0 K in winter; under clear skies this difference increases to about 1.3 K. Monthly mean temperature profiles of the lowest 30 km of the atmosphere over South Pole are presented, based on 10 yr of radiosonde data. These profiles show large variations in lower-stratospheric temperatures, and in the strength and depth of the surface-based inversion. The near-destruction of a strong inversion at South Pole during 7 h on 8 September 1992 is examined using a thermal-conductivity model of the snowpack, driven by the measured downward longwave flux. The downward flux increased when a cloud moved over the station, and it seems that this increase in radiation alone can explain the magnitude and timing of the warming near the surface. Temperature data from the 2003/04 and 2004/05 summers at Dome C Station are presented to show the effects of the diurnal cycle of solar elevation over the Antarctic Plateau. These data include surface temperature and 2- and 30-m air temperatures, as well as radiosonde air temperatures. They show that strong inversions, averaging 10 K between the surface and 30 m, develop quickly at night when the sun is low in the sky, but are destroyed during the middle of the day. The diurnal temperature range at the surface was 13 K, but only 3 K at 30 m.

High sampling rate thermistor string observations at the slope of Great Meteor Seamount

Abstract. A high sampling rate (1 Hz) thermistor string has been built to accommodate the scientific need to accurately monitor high-frequency and vigorous internal wave and overturning processes in the ocean. The thermistors and their custom designed electronics can register temperature at an estimated precision of about 0.001° C with a response time faster than 0.25 s down to depths of 6000 m. With a quick in situ calibration using SBE 911 CTD an absolute accuracy of 0.005° C is obtained. The present string holds 128 sensors at 0.5 m intervals, which are all read-out within 0.5 s. When sampling at 1 Hz, the batteries and the memory capacity of the recorder allow for deployments of up to 2 weeks. In this paper, the instrument is described in some detail. Its performance is illustrated with examples from the first moored observations, which show Kelvin-Helmholtz overturning and very high-frequency (Doppler-shifted) internal waves besides occasionally large turbulent bores moving up the sloping side of Great Meteor Seamount, Canary Basin, North-Atlantic Ocean.

Cold intrusions in Lake Baikal: Direct observational evidence for deep-water renewal

We studied cold, deep-water intrusions in the South Basin of Lake Baikal on the basis of 2 yr of data (December 1995-November 1997) from near-bottom and near-surface thermistor strings, monthly conductivity-temperature-depth (CTD) profiles, and a near-bottom current meter, all collected near the South Basin maximum depth of 1,461 in. The data show intrusions into the greatest depths with temperatures of 0.08-0.20degreesC below ambient (similar to3.33 to similar to3.38degreesC at maximum depth). The intrusions were observed three times per year between January and June, when surface water is always cooler than deep water, with durations of a few (at least 1-3) days. The estimated water input ranged from 1 to 10 km(3) per event, and the annually accumulated volume was estimated to be 10-30 km(3), which is significantly less than the steady-state indirect estimates of 30-70 km(3) yr(-1) to the permanently stratified deep water (depth > 300 in). This indicates that not all of the cold intrusions reach the deepest area. Because the cooling of the near-bottom waters was not accompanied by a significant increase in ion or particle content and because deep sediment traps did not contain significant enrichments of minerogenic particles, we concluded that Selenga River inflow is not a possible source of the cold intrusions. Two CTD profiles in June 1996 and 1997 showed lower temperature throughout the deep water, as expected from thermobaric instabilities. The required downwelling is definitely not occurring in the pelagic interior but most probably by near-coast counterclockwise currents. The source of the regularly occurring deep intrusions is clearly cold surface water, but the actual mechanism is uncertain.

Internal- and surface-wave-induced fluctuations and frequency spreading in shallow water acoustic propagation over short ranges

During a 2001 ocean acoustics experiment approximately 40 miles west of San Diego, a moored, underwater source at 30-m depth transmitted a set of narrow-band tones in the 100–1000-Hz band to a 32-element, 992-m-long bottom hydrophone array deployed from R/P FLIP. Also deployed about FLIP were multiple, fixed vertical thermistor strings with ambient pressure sensors, and CTD casts. These data provide observations of the effects of a fluctuating ocean environment on acoustics in shallow water (180 m) over short ranges (2.5 km). A series of very long fast Fourier transforms of the received signals reveal time dependence in the frequency spreading about the transmitted frequency. Time series of spatial and temporal coherence estimates show temporal variations in the relatedness of the received signals. The environmental data are used along with physics-based models of ocean surface and internal waves to predict the signal fluctuation and frequency spreading characteristics. Results of comparison of acoustic observations with oceanographic-data/model-based predictions under various assumptions will be presented. [Work supported by ONR, Code 321(US).]

Classification of bioabsorption lines at low frequencies: The Van Holliday connection

We will describe preliminary results of the second Bioacoustic Absorption Spectroscopy experiment in the Santa Barbara Channel (SBC), BAS II, which was designed to measure the effects of bioabsorptivity on transmission loss (TL) at frequencies between 0.25 and 6 kHz between an ultrabroadband source and a 16-element vertical receiving array. A fisheries echo sounder and trawls provided bioacoustic parameters. Colocated cores and chirp sonar provided geoacoustic parameters (Turgut, unpublished). Temporal changes in temperature structure were measured with three orthogonal thermistor strings. Highest losses were observed at night when fish were dispersed, and at the average depth of the absorption layer, 13 m, in accord with echo-sounder data and simulations. Absorption lines were evident at the resonance frequencies of 15-cm sardines (1.1 kHz) and 11-cm anchovies (1.8 kHz), in accord with trawls, and 20-cm jack mackerel (0.7 kHz), a species known to inhabit the SBC. This classification is consistent with Holliday’s (1972) backscattering measurements of the resonance frequency of jack mackerel (samples collected by Purse Seiner corroborated his classification). Frequencies of absorption lines changed in accord with depth changes at twilight. [Work supported by the ONR and the Southwest Fisheries Science Center.]

Mixing of the Storfjorden overflow (Svalbard Archipelago) inferred from density overturns

Observations were made of the dense overflow from Storfjorden from a survey conducted at closely spaced stations in August 2002. The field data set consists of conventional conductivity‐temperature‐depth profiles and short‐term moored current meters and thermistor strings. Finestructure estimates were made by calculating Thorpe scales over identified overturns using 0.1‐dbar vertically averaged density profiles. Dissipation rate of turbulent kinetic energy per unit mass, ε, is estimated assuming proportionality between Thorpe and Ozmidov length scales. Vertical eddy diffusivity Kz is estimated using Osborn's model assuming a constant mixing efficiency. Survey‐averaged profiles suggest enhanced mixing near the bottom with values of Kz and ε, when averaged within the overflow, equal to 10 × 10−4 m2 s−1 and 3 × 10−8 W kg−1, respectively. Kz is found to decrease with increasing buoyancy frequency as N−1.2 (±0.3), albeit values of N covered only 0.5–8 cph (1 cph = 2π/3600 s−1). Values of heat flux obtained using Kz suggest that the plume gains a considerable amount of heat, 45 ± 25 W m−2, when averaged over the thickness of the plume, from overlying waters of Atlantic origin. This value is lower than but, considering the errors in estimates of Kz, comparable with 100 W m−2, the rate of change of heat in the overflow derived from sections across the sill and 80 km downstream.

Observations of a quasi shear‐free lacustrine convective boundary layer: Stratification and its implications on turbulence

Convection due to surface cooling has been investigated in the surface boundary layer of a small lake under near‐ideal conditions undisturbed by wind, large‐scale currents, and differential cooling. Successive temperature microstructure profiles revealed a stable stratification in the lower bulk of the convective boundary layer despite strong cooling over the entire layer. Lateral heat fluxes as well as heat exchange with the thermocline could be excluded as driving mechanism of the observed stratification. Acoustic Doppler current profiler (ADCP) measurements indicated an asymmetric pattern of vertical velocities: strong downward plumes were observed upon a background of slow upflow. We suggest that this asymmetry allows cold water from the super‐adiabatic surface layer to intrude at the base of the convective layer, thus causing the observed stratification. This in particular involves downward plumes that feature a relatively low entrainment rate in the center bulk of the convective layer. Corresponding large‐eddy simulation studies, as well as thermistor string and ADCP data, were found consistent with this scenario. The dissipation rate of turbulent kinetic energy (TKE) was estimated on the basis of the temperature microstructure profiles and from the ADCP data. Mean dissipation rates of only about 20% of the surface buoyancy flux were typically identified. This finding is attributed to respectively low buoyant production rates of TKE, attenuated by strong convective plumes penetrating through the stable stratification in the lower bulk of the convective layer into the thermocline.

Rapid fluctuations of the channel impulse response at midfrequencies in shallow water

A fixed source, fixed receiving array experiment was carried out to measure the stability of forward transmissions in shallow water (∼100 m deep) over a 6 km forward propagation path off San Diego, CA. The source was moored 6 m above the seafloor and the 12 aperture, 64-element vertical receiving array was deployed with the lowest element 4 m off the bottom. The source transmissions of interest here are the 2 kHz bandwidth, 1 s duration FM chirps which were transmitted continuously for 5 min at a time and have been matched filtered to yield the channel impulse response. In addition to CTDs taken in the region between the source and receiving array, a thermistor string at the receiving array site provided measurements of water column temperature fluctuations. The time-evolving structure of the channel impulse response clearly shows significant, environmentally induced fluctuations which also are evident in an arrival angle vs travel time spatial decomposition at the array. [Work supported by ONR.]

Internal wave band kinetic energy production: flat vs. sloping bottom

Velocity and temperature measurements obtained with acoustic Doppler current profilers and thermistor strings are used to evaluate the production of internal wave band kinetic energy mainly in the frequency band σ>15 cpd. Results from a flat 19 m deep, vigorous tidal environment in a shelf sea are compared with energy production in a bottom boundary layer above a continental slope. In the tidal environment, maximum production occurs in the near-bottom and near-surface layers. A distinct mid-depth maximum in KE production occurs during a period when wind speeds exceed 10 m s −1 and significant wave height ∼2 m. At the same time, no significant changes in the along-shore current speed take place but the cross-shore current, generated by strong stratification, is weakened. This suggests a direct energy input from the wind via surface waves into the water column turbulence. Maximum kinetic energy production in the frequency band σ>1.9 cpd, thus including the semidiurnal tide, occurs at mid-depth when strong stratification is present. The overall magnitude of internal wave band kinetic energy production agrees well with independent dissipation estimates obtained from microstructure profilers. Above the sloping bottom, KE production is somewhat larger than observed in the shallow tidal environment, despite rms currents being ∼50% smaller and wind effects being small. Above the sloping bottom KE shear production was comparable to buoyancy production. The latter was negligible at the shelf sea site.

Internal wave band eddy fluxes above a continental slope

Three weeks of velocity and temperature measurements from the bottom 45 m above the continental slope in the Bay of Biscay are used to evaluate the role of the internal wave band in boundary mixing near a sloping bottom. Utilizing acoustic Doppler current profilers and thermistor strings, internal wave band eddy fluxes of momentum and heat are estimated. The instrumentation is specifically designed to resolve internal wave band processes. Due to unresolved Doppler shifting, this wave band may include turbulence as well as internal waves. A very energetic and highly variable near-bottom environment is found. Periods of mixing and restratification alternate at the M 2 tidal frequency. Interpreting the observations in an Ekman sense, the three-week mean current is downwelling-favorable, which is consistent with existing boundary layer theories. However, a bi-directional flow associated with sloping boundary mixing is not found in the near-bottom layer, possibly due to observed strong stratification all the way to the bottom. We evaluated boundary layer dynamics and the effect of internal wave-band fluxes from two frequency ranges (σ ≥ 15 cpd and σ ≥ 1.9 cpd, including tides) on the three-week mean flow. The high-frequency range (σ ≥ 15 cpd) of the internal wave band supports significant momentum and buoyancy fluxes while the low-frequency range (σ ≥ 1.9 cpd) only supports significant momentum fluxes. Mean bottom-normal eddy diffusivities associated with anisotropic, nonlinear internal waves, are negative and O(-10 -2 m 2 s -1 ). Interpreting these negative eddy diffusivities as indication of a restratification process, high mixing efficiencies are expected throughout the mixing layer, which extends typically 20 m above the bottom. Mean eddy viscosities are positive in cross-slope direction and negative in alongslope direction, implying a strong anisotropy in the interaction between internal wave band eddies and the mean flow. Alongslope momentum is transferred from the internal tide to the mean flow. Buoyancy and pressure gradient forces, which we could not determine directly, may generate a buoyancy-driven secondary flow. The buoyancy equation is dominated by advection, possibly balanced by divergence of cross-slope and alongslope internal wave band fluxes.

Field observations of dilution on the Ipanema Beach outfall

Field observations of the Ipanema Beach, Rio de Janeiro, ocean sewage outfall are presented. Measurements of dilution and other wastefield characteristics were obtained by adding dye tracer to the effluent and measuring in-situ. Simultaneous measurements of oceanographic conditions were made by Acoustic Doppler Current Profilers, thermistor strings, and profiling instruments. Four experiments were performed, two during unstratified conditions when the plume was surfacing, and two during conditions of strong stratification when the plume was submerged. The minimum dilution varied from 30 to 130. The measurements reflect the worst case conditions as the campaigns were all made for weak currents.