Search Results
You are looking at 1 - 10 of 13 items for
- Author or Editor: J. C. Larsen x
- Refine by Access: All Content x
Abstract
The tidal constituents K 1, O 1, P 1, Q 1, M 2, S 2, N 2, K 2 derived from sea level records at six sites spanning the Hawaiian Islands show a systematic variation in amplitude and phase from site to site that indicate that the tide is mainly diffracted about the Hawaiian Islands Ridge rather than diffracted about the individual islands. This variation enables us to construct cotidal charts.
We model the tide by a small set of plane waves for a uniform depth, horizontally unbounded ocean on a flat, uniformly rotating earth appropriate to a given latitude and model the ridge by an elliptically shaped cylindrical island. Proudman's (1914) approximate solution of this problem is used to determine the set of plane waves that least-squares fit the tidal observations. These plane waves enable us to estimate the tidal motion in the deep ocean beyond Hawaii. The rms (root-wean-square) fit of the model to the Hawaiian tidal constituents is <7% of the constituent amplitude. In fact, our tidal model, extrapolated out some 1200 km to Johnston Island, only differs from the tidal constituents observed there by <27% in amplitude and <18° in phase for the K 1, O 1, M 2, S 2, N 2 and K 2 constituent. We feel, therefore, that the cotidal charts derived here are reasonably valid for the ocean within a 1000 km radius of Honolulu.
Abstract
The tidal constituents K 1, O 1, P 1, Q 1, M 2, S 2, N 2, K 2 derived from sea level records at six sites spanning the Hawaiian Islands show a systematic variation in amplitude and phase from site to site that indicate that the tide is mainly diffracted about the Hawaiian Islands Ridge rather than diffracted about the individual islands. This variation enables us to construct cotidal charts.
We model the tide by a small set of plane waves for a uniform depth, horizontally unbounded ocean on a flat, uniformly rotating earth appropriate to a given latitude and model the ridge by an elliptically shaped cylindrical island. Proudman's (1914) approximate solution of this problem is used to determine the set of plane waves that least-squares fit the tidal observations. These plane waves enable us to estimate the tidal motion in the deep ocean beyond Hawaii. The rms (root-wean-square) fit of the model to the Hawaiian tidal constituents is <7% of the constituent amplitude. In fact, our tidal model, extrapolated out some 1200 km to Johnston Island, only differs from the tidal constituents observed there by <27% in amplitude and <18° in phase for the K 1, O 1, M 2, S 2, N 2 and K 2 constituent. We feel, therefore, that the cotidal charts derived here are reasonably valid for the ocean within a 1000 km radius of Honolulu.
Abstract
No Abstract Available.
Abstract
No Abstract Available.
Abstract
Motionally induced voltage differences offer one of the few observational methods sensitive to changes in large-scale ocean transports. They present a useful contrast to most oceanographic data by virtue of their natural spatial integration, temporal continuity, and potentially long duration. However, widespread oceanographic use of the voltages observable with seafloor cables has been impeded by uncertainties of interpretation. Interpretation in terms of volume transport fluctuations has proved successful in the Straits of Florida and for a short cable in the easternmost part of the Bering Strait. Still, a number of older experimental studies resulted in disappointment, the Bering Strait work has been little known, and the Florida success might be a special case. The question considered in this paper is: Does a linear relationship between net transport and voltage difference fluctuations hold for long, open-ocean cables? This question is addressed by using a numerical model based on two years of results from the WOCE Community Modeling Effort, which simulated the wind-driven and thermohaline circulation in the North Atlantic using mean monthly winds and realistic topography with a resolution sufficient to permit mesoscale eddies. The model includes the effects of spatial and temporal variations of seawater temperature and salinity, electric current loops, the effects associated with the meandering of ocean currents over realistic topography and sediment thickness, realistic earth conductivity, and the spatially varying geomagnetic field. The main result is that the relationship between voltage and net cross-cable transport fluctuations can be remarkably linear over long distances. In view of the difficulties of long-term, large-scale transport monitoring by other methods, the implication of this work is that well chosen and carefully interpreted voltage observations hold great promise. This should be explored through renewed modeling, observation, and interpretation efforts.
Abstract
Motionally induced voltage differences offer one of the few observational methods sensitive to changes in large-scale ocean transports. They present a useful contrast to most oceanographic data by virtue of their natural spatial integration, temporal continuity, and potentially long duration. However, widespread oceanographic use of the voltages observable with seafloor cables has been impeded by uncertainties of interpretation. Interpretation in terms of volume transport fluctuations has proved successful in the Straits of Florida and for a short cable in the easternmost part of the Bering Strait. Still, a number of older experimental studies resulted in disappointment, the Bering Strait work has been little known, and the Florida success might be a special case. The question considered in this paper is: Does a linear relationship between net transport and voltage difference fluctuations hold for long, open-ocean cables? This question is addressed by using a numerical model based on two years of results from the WOCE Community Modeling Effort, which simulated the wind-driven and thermohaline circulation in the North Atlantic using mean monthly winds and realistic topography with a resolution sufficient to permit mesoscale eddies. The model includes the effects of spatial and temporal variations of seawater temperature and salinity, electric current loops, the effects associated with the meandering of ocean currents over realistic topography and sediment thickness, realistic earth conductivity, and the spatially varying geomagnetic field. The main result is that the relationship between voltage and net cross-cable transport fluctuations can be remarkably linear over long distances. In view of the difficulties of long-term, large-scale transport monitoring by other methods, the implication of this work is that well chosen and carefully interpreted voltage observations hold great promise. This should be explored through renewed modeling, observation, and interpretation efforts.
Abstract
Hot wires respond to temperature as well as to velocity, whereas cold wires respond to velocity as well as to temperature. The static and dynamic response characteristics are summarized and it is shown that the frequency transfer functions for the four different responses in general are different. The influence of the transfer characteristics on measurements of turbulence statistics is discussed; it is shown that the nonideal response behavior influences, most strongly, statistics involving the correlation between velocity and temperature and here, most seriously, parameters involving small-scale turbulence.
Abstract
Hot wires respond to temperature as well as to velocity, whereas cold wires respond to velocity as well as to temperature. The static and dynamic response characteristics are summarized and it is shown that the frequency transfer functions for the four different responses in general are different. The influence of the transfer characteristics on measurements of turbulence statistics is discussed; it is shown that the nonideal response behavior influences, most strongly, statistics involving the correlation between velocity and temperature and here, most seriously, parameters involving small-scale turbulence.
Abstract
A high-resolution, barotropic model of the North Atlantic is used to study the variation of transport through the Straits of Florida on timescales from a few days to seasonal. The model is driven by wind and atmospheric pressure forcing derived from ECMWF twice daily analyses for the years 1985, 1986, 1987, and 1988. The model-computed transports are compared with the cable-derived estimates of daily mean transport. Atmospheric pressure forcing is found to have an insignificant effect on the model results and can be ignored. A visual comparison between the model-computed transport and the cable data shows many similarities. Coherence squared between the two time series has peaks between 0.4 and 0.5 and is significant at the 95% confidence level in the period range from 6 to 100 days, with a drop in coherence near 10 days. The model overestimates the autospectral energy in the period range of 4 to 20 days but underestimates the energy at longer periods. The authors find that remote forcing to the north of the straits does not significantly affect coherence squared and phase between the model-computed transport and the cable data but is necessary to explain the autospectral energy in the model-computed transports at periods greater than 10 days. The most significant failing of the model is its inability to capture 8–10 mo timescale events in the cable data. Interestingly, the World Ocean Circulation Experiment Community Modeling Effort, driven by synoptic wind forcing, does exhibit roughly 8-month timescale events, as seen in the cable data but missed by the barotropic model.
Abstract
A high-resolution, barotropic model of the North Atlantic is used to study the variation of transport through the Straits of Florida on timescales from a few days to seasonal. The model is driven by wind and atmospheric pressure forcing derived from ECMWF twice daily analyses for the years 1985, 1986, 1987, and 1988. The model-computed transports are compared with the cable-derived estimates of daily mean transport. Atmospheric pressure forcing is found to have an insignificant effect on the model results and can be ignored. A visual comparison between the model-computed transport and the cable data shows many similarities. Coherence squared between the two time series has peaks between 0.4 and 0.5 and is significant at the 95% confidence level in the period range from 6 to 100 days, with a drop in coherence near 10 days. The model overestimates the autospectral energy in the period range of 4 to 20 days but underestimates the energy at longer periods. The authors find that remote forcing to the north of the straits does not significantly affect coherence squared and phase between the model-computed transport and the cable data but is necessary to explain the autospectral energy in the model-computed transports at periods greater than 10 days. The most significant failing of the model is its inability to capture 8–10 mo timescale events in the cable data. Interestingly, the World Ocean Circulation Experiment Community Modeling Effort, driven by synoptic wind forcing, does exhibit roughly 8-month timescale events, as seen in the cable data but missed by the barotropic model.
Abstract
The integration of plug-in Fast Fourier Transform (FFT) boards in data acquisition computers allows a considerable development in the dynamic calibration of turbulence sensors. The spectral transfer function of a fast and sensitive turbulence sensor can be obtained in situ from a slow sensor having an absolute calibration, by computing in real time either the power spectra of the two signals or their complex cross-spectrum. The real-time spectral method allows calibration of sensors with relatively complex responses and, in most cases, nonlinear transfer functions. When used in conjunction with appropriate control and correction algorithms, this method can take care of numerous sources of error such as electronic noise, line pickup, and sensor malfunctions. This study shows that it can be extended to sensor arrays, including X-wire dual-component anemometers.
Abstract
The integration of plug-in Fast Fourier Transform (FFT) boards in data acquisition computers allows a considerable development in the dynamic calibration of turbulence sensors. The spectral transfer function of a fast and sensitive turbulence sensor can be obtained in situ from a slow sensor having an absolute calibration, by computing in real time either the power spectra of the two signals or their complex cross-spectrum. The real-time spectral method allows calibration of sensors with relatively complex responses and, in most cases, nonlinear transfer functions. When used in conjunction with appropriate control and correction algorithms, this method can take care of numerous sources of error such as electronic noise, line pickup, and sensor malfunctions. This study shows that it can be extended to sensor arrays, including X-wire dual-component anemometers.
Abstract
First results from the implementation of frequency domain interferometry (FDI) using an L-band frequency of 1290 MHz are presented. To our knowledge, FDI has not previously been applied to such high-frequency measurements. The experiment was conducted in September 1991 using the radar facility located in Søndre Strømfjord, Greenland. The Søndre Strømfjord radar is typically used for incoherent scatter measurements in the ionosphere, but these are some of the first lower-atmospheric results, namely, 8.6–13.4 km, since the new data-taking system was implemented. At the time of the experiment, the steerability of the 32-m dish antenna was hampered because of a faulty elevation-scanning bearing. Therefore, the measurements were taken from an approximately vertical direction for the duration of the experiment. The spectra and the correlation functions obtained from the FDI data are compared to previous results at other frequencies. The data show the Søndre Strømfjord radar is providing reliable wind measurements in the lower atmosphere and that FDI can be implemented at L band.
Abstract
First results from the implementation of frequency domain interferometry (FDI) using an L-band frequency of 1290 MHz are presented. To our knowledge, FDI has not previously been applied to such high-frequency measurements. The experiment was conducted in September 1991 using the radar facility located in Søndre Strømfjord, Greenland. The Søndre Strømfjord radar is typically used for incoherent scatter measurements in the ionosphere, but these are some of the first lower-atmospheric results, namely, 8.6–13.4 km, since the new data-taking system was implemented. At the time of the experiment, the steerability of the 32-m dish antenna was hampered because of a faulty elevation-scanning bearing. Therefore, the measurements were taken from an approximately vertical direction for the duration of the experiment. The spectra and the correlation functions obtained from the FDI data are compared to previous results at other frequencies. The data show the Søndre Strømfjord radar is providing reliable wind measurements in the lower atmosphere and that FDI can be implemented at L band.
Abstract
This paper describes an investigation of a thunderstorm that occurred in the summer of 1991 over the National Astronomy and Ionosphere Center in Arecibo, Puerto Rico. Observations were made using collinear dual-wavelength Doppler radars, which permit virtually simultaneous observation of the same pulse volume using transmission and reception of coherent UHF and VHF signals on alternate pulses. This made it possible to directly measure the vertical wind within the sampling volume using the VHF signal while using the UHF signal to study the nature of the precipitation. The observed storm showed strong similarities with systems observed in the Global Atmospheric Research Program's Atlantic Tropical Experiment study. Since this experiment can determine the various microphysical parameters, such as the vertical air velocity, the mean fall speeds of the precipitation, and the reflectivity, the relationships between these parameters that have been postulated in past studies can be tested. For example, in this paper, the method of using reflectivities to deduce the fall speeds of precipitation particles is studied. The method is found to be unreliable when used in turbulent environments.
Abstract
This paper describes an investigation of a thunderstorm that occurred in the summer of 1991 over the National Astronomy and Ionosphere Center in Arecibo, Puerto Rico. Observations were made using collinear dual-wavelength Doppler radars, which permit virtually simultaneous observation of the same pulse volume using transmission and reception of coherent UHF and VHF signals on alternate pulses. This made it possible to directly measure the vertical wind within the sampling volume using the VHF signal while using the UHF signal to study the nature of the precipitation. The observed storm showed strong similarities with systems observed in the Global Atmospheric Research Program's Atlantic Tropical Experiment study. Since this experiment can determine the various microphysical parameters, such as the vertical air velocity, the mean fall speeds of the precipitation, and the reflectivity, the relationships between these parameters that have been postulated in past studies can be tested. For example, in this paper, the method of using reflectivities to deduce the fall speeds of precipitation particles is studied. The method is found to be unreliable when used in turbulent environments.
Abstract
A prototype system for the measurement and computation of air–sea fluxes in realtime was tested in the Humidity Exchange Over the Sea (HEXOS) main experiment, HEXMAX. The system used a sonic anemometer/thermometer for wind speed, surface stress and sensible heat flux measurements and a Lyman-α fast hygrometer for latent beat flux. A small desktop computer combining both fast analog to digital (A/D) capabilities, external bus (IEEE-488) operation of a slow voltmeter/scanner unit, and a plug-in board for computation of turbulence spectra by Fast Fourier Transform was used for acquisition of 17 channels of data. At the end of a ten-minute averaging period, air–sea fluxes were computed from the velocity, temperature, and humidity variance spectra using the inertial-dissipation method. A second computer and data acquisition system was used for simultaneous computations of covariance fluxes for comparison.
The sonic anemometer/thermometer proved to be well suited for this application: the velocity data appear to be of good quality and the temperature data wore unaffected by salt contamination. We suggest an infrared hygrometer as a replacement for the Lyman-α. For the six week HEXMAX period the inertial-dissipation flux estimates agreed with covariances computed from the same instruments with a typical average root-mean-square difference of ± 10% for stress and ± 25% for sensible and latent heat.
Abstract
A prototype system for the measurement and computation of air–sea fluxes in realtime was tested in the Humidity Exchange Over the Sea (HEXOS) main experiment, HEXMAX. The system used a sonic anemometer/thermometer for wind speed, surface stress and sensible heat flux measurements and a Lyman-α fast hygrometer for latent beat flux. A small desktop computer combining both fast analog to digital (A/D) capabilities, external bus (IEEE-488) operation of a slow voltmeter/scanner unit, and a plug-in board for computation of turbulence spectra by Fast Fourier Transform was used for acquisition of 17 channels of data. At the end of a ten-minute averaging period, air–sea fluxes were computed from the velocity, temperature, and humidity variance spectra using the inertial-dissipation method. A second computer and data acquisition system was used for simultaneous computations of covariance fluxes for comparison.
The sonic anemometer/thermometer proved to be well suited for this application: the velocity data appear to be of good quality and the temperature data wore unaffected by salt contamination. We suggest an infrared hygrometer as a replacement for the Lyman-α. For the six week HEXMAX period the inertial-dissipation flux estimates agreed with covariances computed from the same instruments with a typical average root-mean-square difference of ± 10% for stress and ± 25% for sensible and latent heat.
Abstract
The response of a Kaijo Denki DAT 300 ultrasonic anemometer-thermometer is analyzed with respect to temperature and vertical velocity. The effects of the emitting-receiving cycle of the transducer array on w, T, and CowT spectra are considered. The resulting relations are used to interpret temperature spectra obtained during the HEXMAX (Humidity Exchange over the Sea, Main Experiment) measuring campaign.
Abstract
The response of a Kaijo Denki DAT 300 ultrasonic anemometer-thermometer is analyzed with respect to temperature and vertical velocity. The effects of the emitting-receiving cycle of the transducer array on w, T, and CowT spectra are considered. The resulting relations are used to interpret temperature spectra obtained during the HEXMAX (Humidity Exchange over the Sea, Main Experiment) measuring campaign.
