Search Results
Abstract
Observations are described of Langmuir circulation obtained using upward-pointing bottom-mounted sonars, and a methodology to use the data to estimate the dispersion of floating particles is suggested. Observations of linear bands of acoustic scatterers separated by 2–20 m and detected using side-scan sonar in Loch Ness, Scotland, and in the southern North Sea are ascribed to subsurface bubbles in the convergence zones produced by Langmuir circulation. Data from the two observation sites are compared. The sonar is able to monitor the variability of the patterns over many hours. When the currents are sufficiently small, as in Loch Ness, individual bubble clouds produced by breaking waves remain in the beam long enough for their speed to be resolved, and the rate of convergence into the bands can be estimated. It increases linearly with wind speed. The acoustic data and direct measurements using current meters are used to derive estimates of the response time of bubble bands to changes in wind, and their mean separation, length, and persistence time. The bands in Loch Ness are shorter, but persist longer, than those in similar wind conditions in the relatively shallow and well-mixed North Sea. It is suggested that these differences may be ascribed to the presence of turbulence generated by the shear stress of the strong tidal currents on the seabed in the North Sea, a factor absent in Loch Ness. Models are devised to simulate the dispersion of plumes of floating particles released from a fixed position in a field of Langmuir circulation advected by tidal currents, using the sonar data. The estimates of diffusivities show an increase with wind speed, but are sensitive to the choice of some underdetermined parameters. The resulting estimates of lateral dispersion of floating particles overlap the range of those of Faller and Auer.
Abstract
Observations are described of Langmuir circulation obtained using upward-pointing bottom-mounted sonars, and a methodology to use the data to estimate the dispersion of floating particles is suggested. Observations of linear bands of acoustic scatterers separated by 2–20 m and detected using side-scan sonar in Loch Ness, Scotland, and in the southern North Sea are ascribed to subsurface bubbles in the convergence zones produced by Langmuir circulation. Data from the two observation sites are compared. The sonar is able to monitor the variability of the patterns over many hours. When the currents are sufficiently small, as in Loch Ness, individual bubble clouds produced by breaking waves remain in the beam long enough for their speed to be resolved, and the rate of convergence into the bands can be estimated. It increases linearly with wind speed. The acoustic data and direct measurements using current meters are used to derive estimates of the response time of bubble bands to changes in wind, and their mean separation, length, and persistence time. The bands in Loch Ness are shorter, but persist longer, than those in similar wind conditions in the relatively shallow and well-mixed North Sea. It is suggested that these differences may be ascribed to the presence of turbulence generated by the shear stress of the strong tidal currents on the seabed in the North Sea, a factor absent in Loch Ness. Models are devised to simulate the dispersion of plumes of floating particles released from a fixed position in a field of Langmuir circulation advected by tidal currents, using the sonar data. The estimates of diffusivities show an increase with wind speed, but are sensitive to the choice of some underdetermined parameters. The resulting estimates of lateral dispersion of floating particles overlap the range of those of Faller and Auer.
Abstract
A self-contained instrument, the Autonomously Recording Inverted Echo Sounder (ARIES II), carrying two 250-kHz beam side-scan sonars and with the capacity to record sonar data sampled at 3.2 kHz for 168 h with the sonars operating with a pulse repetition rate of 2 Hz, has been constructed and tested in a mooring deployment that lasted for 25 days near the edge of the continental shelf west of Scotland. The mean water depth was 146 m. ARIES II was positioned at a mean transducer depth of 34.6 m with sonars directed upward at 20° to the horizontal to obtain acoustic returns from targets at or near the sea surface. The instrument was preprogrammed to record continuously over periods of 2, 4, and 13 h, the last to cover the M2 tidal cycle dominant in the area.
Sonographs are presented to illustrate observations of surface waves and wave groups, internal solitons, rain showers, and Langmuir circulation. An analysis is made of the effects of surface waves, currents, and internal waves on the instrument. The potential use of the instrument is demonstrated in providing estimates of the propagation direction and speed of internal waves, as well as in estimating the drift, orientation, and mean separation of Langmuir bands. The separation is found to increase with wind speed.
Abstract
A self-contained instrument, the Autonomously Recording Inverted Echo Sounder (ARIES II), carrying two 250-kHz beam side-scan sonars and with the capacity to record sonar data sampled at 3.2 kHz for 168 h with the sonars operating with a pulse repetition rate of 2 Hz, has been constructed and tested in a mooring deployment that lasted for 25 days near the edge of the continental shelf west of Scotland. The mean water depth was 146 m. ARIES II was positioned at a mean transducer depth of 34.6 m with sonars directed upward at 20° to the horizontal to obtain acoustic returns from targets at or near the sea surface. The instrument was preprogrammed to record continuously over periods of 2, 4, and 13 h, the last to cover the M2 tidal cycle dominant in the area.
Sonographs are presented to illustrate observations of surface waves and wave groups, internal solitons, rain showers, and Langmuir circulation. An analysis is made of the effects of surface waves, currents, and internal waves on the instrument. The potential use of the instrument is demonstrated in providing estimates of the propagation direction and speed of internal waves, as well as in estimating the drift, orientation, and mean separation of Langmuir bands. The separation is found to increase with wind speed.
Abstract
Two approaches are used to characterize how accurately the north Alabama Lightning Mapping Array (LMA) is able to locate lightning VHF sources in space and time. The first method uses a Monte Carlo computer simulation to estimate source retrieval errors. The simulation applies a VHF source retrieval algorithm that was recently developed at the NASA Marshall Space Flight Center (MSFC) and that is similar, but not identical to, the standard New Mexico Tech retrieval algorithm. The second method uses a purely theoretical technique (i.e., chi-squared Curvature Matrix Theory) to estimate retrieval errors. Both methods assume that the LMA system has an overall rms timing error of 50 ns, but all other possible errors (e.g., anomalous VHF noise sources) are neglected. The detailed spatial distributions of retrieval errors are provided. Even though the two methods are independent of one another, they nevertheless provide remarkably similar results. However, altitude error estimates derived from the two methods differ (the Monte Carlo result being taken as more accurate). Additionally, this study clarifies the mathematical retrieval process. In particular, the mathematical difference between the first-guess linear solution and the Marquardt-iterated solution is rigorously established thereby explaining why Marquardt iterations improve upon the linear solution.
Abstract
Two approaches are used to characterize how accurately the north Alabama Lightning Mapping Array (LMA) is able to locate lightning VHF sources in space and time. The first method uses a Monte Carlo computer simulation to estimate source retrieval errors. The simulation applies a VHF source retrieval algorithm that was recently developed at the NASA Marshall Space Flight Center (MSFC) and that is similar, but not identical to, the standard New Mexico Tech retrieval algorithm. The second method uses a purely theoretical technique (i.e., chi-squared Curvature Matrix Theory) to estimate retrieval errors. Both methods assume that the LMA system has an overall rms timing error of 50 ns, but all other possible errors (e.g., anomalous VHF noise sources) are neglected. The detailed spatial distributions of retrieval errors are provided. Even though the two methods are independent of one another, they nevertheless provide remarkably similar results. However, altitude error estimates derived from the two methods differ (the Monte Carlo result being taken as more accurate). Additionally, this study clarifies the mathematical retrieval process. In particular, the mathematical difference between the first-guess linear solution and the Marquardt-iterated solution is rigorously established thereby explaining why Marquardt iterations improve upon the linear solution.
Abstract
Detailed information on the four-channel Airborne Chromatograph for Atmospheric Trace Species (ACATS-IV), used to measure long-lived atmospheric trace gases, is presented. Since ACATS-IV was last described in the literature, the temporal resolution of some measurements was tripled during 1997–99, chromatography was significantly changed, and data processing improved. ACATS-IV presently measures CCl3F [chlorofluorocarbon (CFC)-11], CCl2FCClF2 (CFC-113), CH3CCl3 (methyl chloroform), CCl4 (carbon tetrachloride), CH4 (methane), H2 (hydrogen), and CHCl3 (chloroform) every 140 s, and N2O (nitrous oxide), CCl2F2 (CFC-12), CBrClF2 (halon-1211), and SF6 (sulfur hexafluoride) every 70 s. An in-depth description of the instrument operation, standardization, calibration, and data processing is provided, along with a discussion of precision and uncertainties of ambient air measurements for several airborne missions.
Abstract
Detailed information on the four-channel Airborne Chromatograph for Atmospheric Trace Species (ACATS-IV), used to measure long-lived atmospheric trace gases, is presented. Since ACATS-IV was last described in the literature, the temporal resolution of some measurements was tripled during 1997–99, chromatography was significantly changed, and data processing improved. ACATS-IV presently measures CCl3F [chlorofluorocarbon (CFC)-11], CCl2FCClF2 (CFC-113), CH3CCl3 (methyl chloroform), CCl4 (carbon tetrachloride), CH4 (methane), H2 (hydrogen), and CHCl3 (chloroform) every 140 s, and N2O (nitrous oxide), CCl2F2 (CFC-12), CBrClF2 (halon-1211), and SF6 (sulfur hexafluoride) every 70 s. An in-depth description of the instrument operation, standardization, calibration, and data processing is provided, along with a discussion of precision and uncertainties of ambient air measurements for several airborne missions.
