Search Results
You are looking at 1 - 8 of 8 items for
- Author or Editor: Zachariah R. Hallock x
- Refine by Access: All Content x
Abstract
Changes in round-trip acoustc travel time (τ) measured between a bottom moored inverted echo sounder and the sea surface can be interpreted as changes in dynamic height (D) with suitable calibration information. The τ, D, and isotherm and isopycnal depths (Z) have been calculated using hydrographic (CTD) data from three regions: the Norwegian Current, the Sargasso Sea and the eastern Gulf Stream. Regressions of D and Z on τ were performed. The slope for the Norwegian Current is − 1.70 ± 0.01 [dyn cm (m s)−1], for the Sargasso Sea − 3.89 ± 0.16 [dyn cm (m s)−1] and for the Gulf Stream − 3.13 ± 0.07 [dyn cm (m s)−1]. The quasi-random scatter about regression curves is found to be primarily the result of variability in the seasonal thermocline, where present.
Abstract
Changes in round-trip acoustc travel time (τ) measured between a bottom moored inverted echo sounder and the sea surface can be interpreted as changes in dynamic height (D) with suitable calibration information. The τ, D, and isotherm and isopycnal depths (Z) have been calculated using hydrographic (CTD) data from three regions: the Norwegian Current, the Sargasso Sea and the eastern Gulf Stream. Regressions of D and Z on τ were performed. The slope for the Norwegian Current is − 1.70 ± 0.01 [dyn cm (m s)−1], for the Sargasso Sea − 3.89 ± 0.16 [dyn cm (m s)−1] and for the Gulf Stream − 3.13 ± 0.07 [dyn cm (m s)−1]. The quasi-random scatter about regression curves is found to be primarily the result of variability in the seasonal thermocline, where present.
Abstract
A hydrographic survey (CTD) was conducted in the vicinity of the Iceland–Faeroe Island oceanic front (IFF) north of the Faeroe Islands during October 1980. It consisted of CTD transects on three horizontal scales ranging from kilometers to hundreds of kilometers.
Intense interleaving of different water masses is found in the IFF in the presence of horizontal current shear. Significant alongfront variability on scales of about 50 km is present, consistent with earlier findings. Estimates of cross-front heat flux of 5.16 × 104 W m−2 and salt flux of 1.58 g m−2 s−1 are greater than those found for the Antarctic Polar Front but are of the same order as eddy heat flux across the IFF found by Willebrand and Meincke. Evidence suggests that intrusive interleaving in the IFF on 50 m vertical scales is driven by double-diffusive convection.
Abstract
A hydrographic survey (CTD) was conducted in the vicinity of the Iceland–Faeroe Island oceanic front (IFF) north of the Faeroe Islands during October 1980. It consisted of CTD transects on three horizontal scales ranging from kilometers to hundreds of kilometers.
Intense interleaving of different water masses is found in the IFF in the presence of horizontal current shear. Significant alongfront variability on scales of about 50 km is present, consistent with earlier findings. Estimates of cross-front heat flux of 5.16 × 104 W m−2 and salt flux of 1.58 g m−2 s−1 are greater than those found for the Antarctic Polar Front but are of the same order as eddy heat flux across the IFF found by Willebrand and Meincke. Evidence suggests that intrusive interleaving in the IFF on 50 m vertical scales is driven by double-diffusive convection.
Abstract
A theoretical model of expendable bathythermograph (XBT) fall rate is reviewed, and a new form of fall-rate equation is proposed to include new-surface transient effects. Comparisons are made of T-7 XBT and CTD (conductivity, temperature, and depth) depths of thermohaline features off Barbados. Fall-rate equation coefficients are derived and compared with the manufacturer-supplied coefficients. As other investigators have found, the Sippican equation consistently underestimates probe depth by as much as 35 m at 760 m. Analysis yields a new equation, Z=6.798t−0.002383t 2−4.01, for depths greater than about 10 m. Considerable probe-to-probe variability is noted and is found to be primarily the result of differences in the linear term or terminal velocity of the probes; variation in effective drag resulting from probe irregularities is the likely cause. Recommendations for additional work are made.
Abstract
A theoretical model of expendable bathythermograph (XBT) fall rate is reviewed, and a new form of fall-rate equation is proposed to include new-surface transient effects. Comparisons are made of T-7 XBT and CTD (conductivity, temperature, and depth) depths of thermohaline features off Barbados. Fall-rate equation coefficients are derived and compared with the manufacturer-supplied coefficients. As other investigators have found, the Sippican equation consistently underestimates probe depth by as much as 35 m at 760 m. Analysis yields a new equation, Z=6.798t−0.002383t 2−4.01, for depths greater than about 10 m. Considerable probe-to-probe variability is noted and is found to be primarily the result of differences in the linear term or terminal velocity of the probes; variation in effective drag resulting from probe irregularities is the likely cause. Recommendations for additional work are made.
Abstract
A new set of measurements made using an upward-looking broadband acoustic Doppler current profiler (ADCP) is analyzed for “surface velocity,” which previous investigations suggest is related to wind velocity. In the present case, use of a shallow instrument depth and 1-m-depth resolution are shown to yield both a surface return, identified by a maximum in backscatter intensity (similar to previous studies), and a subsequent return that corresponds to a maximum in speed. These returns have speeds (measured relative to the 2-m-depth current) that increase to a peak of 0.7–0.8 m s−1 at a wind speed of 4–5 m s−1 and decrease for higher wind speeds, a behavior similar to that found by Nakajima et al., who made ADCP measurements using the same incidence angle (20°) as in the present study. A new finding is that the return having maximum speed yields a direction that better approximates the wind direction (a mean difference of about 5°) over the range of wind speeds sampled (up to 17 m s−1). Suggestions are made for future investigations.
Abstract
A new set of measurements made using an upward-looking broadband acoustic Doppler current profiler (ADCP) is analyzed for “surface velocity,” which previous investigations suggest is related to wind velocity. In the present case, use of a shallow instrument depth and 1-m-depth resolution are shown to yield both a surface return, identified by a maximum in backscatter intensity (similar to previous studies), and a subsequent return that corresponds to a maximum in speed. These returns have speeds (measured relative to the 2-m-depth current) that increase to a peak of 0.7–0.8 m s−1 at a wind speed of 4–5 m s−1 and decrease for higher wind speeds, a behavior similar to that found by Nakajima et al., who made ADCP measurements using the same incidence angle (20°) as in the present study. A new finding is that the return having maximum speed yields a direction that better approximates the wind direction (a mean difference of about 5°) over the range of wind speeds sampled (up to 17 m s−1). Suggestions are made for future investigations.
Abstract
Internal-wave energetics derived from moored acoustic Doppler current profiler (ADCP) observations on the New Jersey shelf are described. Horizontal and vertical velocity components from three 40-day ADCP records acquired near the shelf edge south of the Hudson Canyon were bandpass filtered to isolate high-frequency internal waves. In this band (0.5–7.5 h−1) the vertical component of velocity is significant and is integrated to yield elevation anomaly. Using the ADCP data with buoyancy frequency profiles from nearby CTD station data, kinetic energy (KE), baroclinic potential energy (PE), and energy flux were calculated. Results show depth-averaged KE and PE are nearly equal and of order 10−3 J kg−1, and there are significant northwestward fluxes, generally parallel to the bathymetry gradient, during most of the record, with depth-integrated magnitudes sometimes exceeding 100 W m−1. Vertical energy fluxes are small relative to horizontal fluxes. Vertical profiles of time-averaged flux suggest a dominant first internal-wave mode. Depth-integrated, time-averaged fluxes range from 9 to 24 W m−1, with the highest values occurring 18 km southwest of the Hudson Canyon. Two-dimensional probability density functions are estimated for energy flux. Group velocity of a large-amplitude internal-wave packet is estimated at 0.36 m s−1.
Abstract
Internal-wave energetics derived from moored acoustic Doppler current profiler (ADCP) observations on the New Jersey shelf are described. Horizontal and vertical velocity components from three 40-day ADCP records acquired near the shelf edge south of the Hudson Canyon were bandpass filtered to isolate high-frequency internal waves. In this band (0.5–7.5 h−1) the vertical component of velocity is significant and is integrated to yield elevation anomaly. Using the ADCP data with buoyancy frequency profiles from nearby CTD station data, kinetic energy (KE), baroclinic potential energy (PE), and energy flux were calculated. Results show depth-averaged KE and PE are nearly equal and of order 10−3 J kg−1, and there are significant northwestward fluxes, generally parallel to the bathymetry gradient, during most of the record, with depth-integrated magnitudes sometimes exceeding 100 W m−1. Vertical energy fluxes are small relative to horizontal fluxes. Vertical profiles of time-averaged flux suggest a dominant first internal-wave mode. Depth-integrated, time-averaged fluxes range from 9 to 24 W m−1, with the highest values occurring 18 km southwest of the Hudson Canyon. Two-dimensional probability density functions are estimated for energy flux. Group velocity of a large-amplitude internal-wave packet is estimated at 0.36 m s−1.
Abstract
An experiment was performed to measure the near-surface current by aiming horizontally two of the beams from an acoustic Doppler current profiler (ADCP) deployed at 0.6-m depth from an anchored (but rolling) ship. The results compare favorably with independent current measurements made at 2-m depth but appear to resolve as well a vertical current shear associated with the shallow wind-drift layer. The approach, therefore, has potential for investigating the current profile in the upper meter or two of the water column.
Abstract
An experiment was performed to measure the near-surface current by aiming horizontally two of the beams from an acoustic Doppler current profiler (ADCP) deployed at 0.6-m depth from an anchored (but rolling) ship. The results compare favorably with independent current measurements made at 2-m depth but appear to resolve as well a vertical current shear associated with the shallow wind-drift layer. The approach, therefore, has potential for investigating the current profile in the upper meter or two of the water column.
Abstract
An anticyclonic mesoscale eddy, encountered during a hydrographic survey in the Sargasso Sea in September 1979, is described. Embedded in the eddy at a depth of 800 m was an isopycnal lens of anomalous water which extended 150 m vertically and ∼40 km horizontally. The source of the lens is not known but may be related to frontal processes in the area.
Abstract
An anticyclonic mesoscale eddy, encountered during a hydrographic survey in the Sargasso Sea in September 1979, is described. Embedded in the eddy at a depth of 800 m was an isopycnal lens of anomalous water which extended 150 m vertically and ∼40 km horizontally. The source of the lens is not known but may be related to frontal processes in the area.
Abstract
As part of the Naval Research Laboratory and Office of Naval Research sponsored Physics of Coastal Remote Sensing Research Program, an experiment was conducted in September–October 1996 off Virginia Beach. Ocean surface currents were measured using the high-frequency (25.4 MHz) mode of the Ocean Surface Current Radar at 20-min intervals at a horizontal resolution of 1 km over an approximate 30 km × 44 km domain. Comparisons to subsurface current measurements at 1–2 m beneath the surface from two broadband acoustic Doppler current profilers (ADCP) revealed good agreement to the surface currents. Regression analyses indicated biases of 4 and −3 cm s−1 for cross-shelf and along-shelf currents, respectively, where slopes were O(1) with correlation coefficients of 0.8.
Nine months of sea level heights from the NOAA National Ocean Survey Chesapeake Bay Bridge Tunnel tidal station revealed an energetic M 2 tidal component having an amplitude of 37.5 cm and a phase of 357°. The S 2 tidal constituent had an amplitude of 7 cm and a phase of 49°. By contrast, the diurnal band (K 1, O 1) tidal constituents were considerably weaker with amplitudes of 1–5 cm. From 19 days of HF-derived surface currents, the M 2 and S 2 tidal current amplitudes had a maximum of about 50 and 8 cm s−1 at the Chesapeake Bay mouth, respectively. Explained variances associated with these four tidal current constituents were a maximum of 60% at the mouth and decreased southward. Analyses at the ADCP moorings indicated that the semidiurnal tidal currents were predominantly barotropic with cross-shelf and along-shelf currents of 18 and 10 cm s−1. Energetic semidiurnal tidal currents were highly correlated over the HF-radar domain, and the phase angles indicated a consistent anticyclonic veering of the M 2 tidal current with along-shelf distance from the mouth. Normalized tidal current vorticities by the local Coriolis parameter (f), which represent a proxy for the Rossby number, were ±0.25f near the mouth and ±0.05f in the southern part of the domain for the M 2 constituent. Simulations from a linear, barotropic model were highly correlated with observed M 2 tidal currents at 85 points within the HF-radar domain, consistent with the premise of weakly nonlinear flows.
Abstract
As part of the Naval Research Laboratory and Office of Naval Research sponsored Physics of Coastal Remote Sensing Research Program, an experiment was conducted in September–October 1996 off Virginia Beach. Ocean surface currents were measured using the high-frequency (25.4 MHz) mode of the Ocean Surface Current Radar at 20-min intervals at a horizontal resolution of 1 km over an approximate 30 km × 44 km domain. Comparisons to subsurface current measurements at 1–2 m beneath the surface from two broadband acoustic Doppler current profilers (ADCP) revealed good agreement to the surface currents. Regression analyses indicated biases of 4 and −3 cm s−1 for cross-shelf and along-shelf currents, respectively, where slopes were O(1) with correlation coefficients of 0.8.
Nine months of sea level heights from the NOAA National Ocean Survey Chesapeake Bay Bridge Tunnel tidal station revealed an energetic M 2 tidal component having an amplitude of 37.5 cm and a phase of 357°. The S 2 tidal constituent had an amplitude of 7 cm and a phase of 49°. By contrast, the diurnal band (K 1, O 1) tidal constituents were considerably weaker with amplitudes of 1–5 cm. From 19 days of HF-derived surface currents, the M 2 and S 2 tidal current amplitudes had a maximum of about 50 and 8 cm s−1 at the Chesapeake Bay mouth, respectively. Explained variances associated with these four tidal current constituents were a maximum of 60% at the mouth and decreased southward. Analyses at the ADCP moorings indicated that the semidiurnal tidal currents were predominantly barotropic with cross-shelf and along-shelf currents of 18 and 10 cm s−1. Energetic semidiurnal tidal currents were highly correlated over the HF-radar domain, and the phase angles indicated a consistent anticyclonic veering of the M 2 tidal current with along-shelf distance from the mouth. Normalized tidal current vorticities by the local Coriolis parameter (f), which represent a proxy for the Rossby number, were ±0.25f near the mouth and ±0.05f in the southern part of the domain for the M 2 constituent. Simulations from a linear, barotropic model were highly correlated with observed M 2 tidal currents at 85 points within the HF-radar domain, consistent with the premise of weakly nonlinear flows.
