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Abstract
Side-scan sonars operating at 80–250 kHz have been deployed to produce narrow beams directed parallel and normal to shore on a gently sloping beach. These provide measurements of processes (such as wave propagation) seaward of the edge of the surf zone. Shoreward propagation of sound into the surf zone and hence useful information retrieval from this zone is prevented, however, by high bubble or suspended sediment absorption at its outer edge, as found in earlier Doppler sonar studies at 195 kHz by J.A. Smith. The Shoreward limit of acoustic propagation has a variable structure related to incident wave groups, the position at which waves break, and to dynamical processes within the surfzone determining the position of the bubble or suspended sediment boundary.
Abstract
Side-scan sonars operating at 80–250 kHz have been deployed to produce narrow beams directed parallel and normal to shore on a gently sloping beach. These provide measurements of processes (such as wave propagation) seaward of the edge of the surf zone. Shoreward propagation of sound into the surf zone and hence useful information retrieval from this zone is prevented, however, by high bubble or suspended sediment absorption at its outer edge, as found in earlier Doppler sonar studies at 195 kHz by J.A. Smith. The Shoreward limit of acoustic propagation has a variable structure related to incident wave groups, the position at which waves break, and to dynamical processes within the surfzone determining the position of the bubble or suspended sediment boundary.
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New dynamical systems techniques are used to analyze fluid particle paths in an eddy resolving, barotropic ocean model of the Gulf Stream. Specifically, the existence of finite-time invariant manifolds associated with transient, mesoscale events such as ring detachment and merger is proved based on computer-assisted analytic results. These “Lagrangian” invariant manifolds completely organize the dynamics and mark the pathways by which fluid parcels may be exchanged across stream. In this way, the Lagrangian flow geometry of a detaching ring or a ring–jet interaction event, as well as the exact associated particle flux, is obtained.
The detaching ring geometry indicates that a significant amount of the fluid entrained by the ring originates in a long thin region on the far side of the jet and that this region extends as far upstream as the western boundary current. In the ring–stream interaction case, particle transport occurs both to and from the ring and is concentrated in thin regions on the near side of the jet and around the perimeter of the ring.
Abstract
New dynamical systems techniques are used to analyze fluid particle paths in an eddy resolving, barotropic ocean model of the Gulf Stream. Specifically, the existence of finite-time invariant manifolds associated with transient, mesoscale events such as ring detachment and merger is proved based on computer-assisted analytic results. These “Lagrangian” invariant manifolds completely organize the dynamics and mark the pathways by which fluid parcels may be exchanged across stream. In this way, the Lagrangian flow geometry of a detaching ring or a ring–jet interaction event, as well as the exact associated particle flux, is obtained.
The detaching ring geometry indicates that a significant amount of the fluid entrained by the ring originates in a long thin region on the far side of the jet and that this region extends as far upstream as the western boundary current. In the ring–stream interaction case, particle transport occurs both to and from the ring and is concentrated in thin regions on the near side of the jet and around the perimeter of the ring.
Abstract
The M 2 internal tide in Monterey Submarine Canyon is simulated using a modified version of the Princeton Ocean Model. Most of the internal tide energy entering the canyon is generated to the south, on Sur Slope and at the head of Carmel Canyon. The internal tide is topographically steered around the large canyon meanders. Depth-integrated baroclinic energy fluxes are up canyon and largest near the canyon axis, up to 1.5 kW m−1 at the mouth of the upper canyon and increasing to over 4 kW m−1 around Monterey and San Gregorio Meanders. The up-canyon energy flux is bottom intensified, suggesting that topographic focusing occurs. Net along-canyon energy flux decreases almost monotonically from 9 MW at the canyon mouth to 1 MW at Gooseneck Meander, implying that high levels of internal tide dissipation occur. The depth-integrated energy flux across the 200-m isobath is order 10 W m−1 along the majority of the canyon rim but increases by over an order of magnitude near the canyon head, where internal tide energy escapes onto the shelf. Reducing the size of the model domain to exclude remote areas of high barotropic-to-baroclinic energy conversion decreases the depth-integrated energy flux in the upper canyon by 20%. However, quantifying the role of remote internal tide generation sites is complicated by a pressure perturbation feedback between baroclinic energy flux and barotropic-to-baroclinic energy conversion.
Abstract
The M 2 internal tide in Monterey Submarine Canyon is simulated using a modified version of the Princeton Ocean Model. Most of the internal tide energy entering the canyon is generated to the south, on Sur Slope and at the head of Carmel Canyon. The internal tide is topographically steered around the large canyon meanders. Depth-integrated baroclinic energy fluxes are up canyon and largest near the canyon axis, up to 1.5 kW m−1 at the mouth of the upper canyon and increasing to over 4 kW m−1 around Monterey and San Gregorio Meanders. The up-canyon energy flux is bottom intensified, suggesting that topographic focusing occurs. Net along-canyon energy flux decreases almost monotonically from 9 MW at the canyon mouth to 1 MW at Gooseneck Meander, implying that high levels of internal tide dissipation occur. The depth-integrated energy flux across the 200-m isobath is order 10 W m−1 along the majority of the canyon rim but increases by over an order of magnitude near the canyon head, where internal tide energy escapes onto the shelf. Reducing the size of the model domain to exclude remote areas of high barotropic-to-baroclinic energy conversion decreases the depth-integrated energy flux in the upper canyon by 20%. However, quantifying the role of remote internal tide generation sites is complicated by a pressure perturbation feedback between baroclinic energy flux and barotropic-to-baroclinic energy conversion.
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Abstract
The interannual and intraseasonal variability of the North American monsoon is of great interest because a large proportion of the annual precipitation for Arizona and New Mexico arrives during the summer monsoon. Forty-one years of daily monsoon season precipitation data for Arizona and New Mexico were studied using wavelet analysis. This time-localized spectral analysis method reveals that periodicities of less than 8 days are positively correlated with mean daily precipitation during the 1 July–15 September monsoon period. Roughly 17% of the years indicate no significant periodicity during the monsoon period for either region and are associated with low monsoon precipitation. High- and low-frequency modes explain an equivalent percentage of the variance in monsoon precipitation in both Arizona and New Mexico, and in many years concurrent multiple periodicities occur. Wavelet analysis was effective in identifying the contribution of high-frequency modes that had not been discerned in previous studies. These results suggest that precipitation processes during the monsoon season are modulated by phenomena operating at synoptic (2–8 days) and longer (>8 days) time scales and point to the need for further studies to better understand the associated atmospheric processes.
Abstract
The interannual and intraseasonal variability of the North American monsoon is of great interest because a large proportion of the annual precipitation for Arizona and New Mexico arrives during the summer monsoon. Forty-one years of daily monsoon season precipitation data for Arizona and New Mexico were studied using wavelet analysis. This time-localized spectral analysis method reveals that periodicities of less than 8 days are positively correlated with mean daily precipitation during the 1 July–15 September monsoon period. Roughly 17% of the years indicate no significant periodicity during the monsoon period for either region and are associated with low monsoon precipitation. High- and low-frequency modes explain an equivalent percentage of the variance in monsoon precipitation in both Arizona and New Mexico, and in many years concurrent multiple periodicities occur. Wavelet analysis was effective in identifying the contribution of high-frequency modes that had not been discerned in previous studies. These results suggest that precipitation processes during the monsoon season are modulated by phenomena operating at synoptic (2–8 days) and longer (>8 days) time scales and point to the need for further studies to better understand the associated atmospheric processes.
An acoustic echo sounder mounted on the NOAA ship Oceanographer during GATE proved to be a valuable tool for investigating the structure and dynamics of the tropical marine boundary layer up to 800 m in height. Under suppressed weather conditions the facsimile-recorded echo intensity returns depicted a mixed layer characterized by convective plumes rising from the surface of the water to 400 m. Disturbed weather events resulted in a substantial modification of the boundary layer; layered structures formed that at times limited the depth of the mixed layer to 100 m. The Doppler frequency shift of the acoustic returns made it possible to determine the vertical velocity field.
An acoustic echo sounder mounted on the NOAA ship Oceanographer during GATE proved to be a valuable tool for investigating the structure and dynamics of the tropical marine boundary layer up to 800 m in height. Under suppressed weather conditions the facsimile-recorded echo intensity returns depicted a mixed layer characterized by convective plumes rising from the surface of the water to 400 m. Disturbed weather events resulted in a substantial modification of the boundary layer; layered structures formed that at times limited the depth of the mixed layer to 100 m. The Doppler frequency shift of the acoustic returns made it possible to determine the vertical velocity field.
Abstract
The population of bubbles produced by breaking waves in (10 m) winds of up to 12 m s−1 is analyzed using calibrated images from a vertical pencil-beam sonar system placed on the seabed near the Dutch coast. The structure in the images is parameterized, and the volumetric bubble backscatter is inverted to yield bubble concentrations. Data were obtained at three acoustic frequencies, with inversion effected by prescribing a bubble spectrum with two free variables, leaving a redundant measurement to test the robustness of the model. Median concentrations may in this way be obtained up to the sea surface. Measurements are multiply regressed on wind and dominant-wave variables. Bubbles penetrate to a depth of about a factor of 6γ −1 times the significant wave height H s , where γ is the wave age, or ratio of dominant-wave phase speed to wind speed. The measured mean bubble radius decreases weakly with depth, unless waves are gently sloping, at about 5% m−1. At 0.4 m, the mean radius ranges from 30 to 80 μm and is typically about two-thirds of the radius contributing most to void fraction. The total, depth-integrated surface area of the bubbles and their upward displacement of the sea surface, or “void displacement,” increase as wind speed to the powers 7 ± 1 and 8 ± 1, respectively, dependences ascribed to the preferential breaking of short, steep wind waves. It is estimated, on extrapolating trends, that the total bubble surface area on average is equal to that of the sea surface above them, and the mean void displacement is equal to the mean bubble radius, at a wind speed of about 15 m s−1.
Abstract
The population of bubbles produced by breaking waves in (10 m) winds of up to 12 m s−1 is analyzed using calibrated images from a vertical pencil-beam sonar system placed on the seabed near the Dutch coast. The structure in the images is parameterized, and the volumetric bubble backscatter is inverted to yield bubble concentrations. Data were obtained at three acoustic frequencies, with inversion effected by prescribing a bubble spectrum with two free variables, leaving a redundant measurement to test the robustness of the model. Median concentrations may in this way be obtained up to the sea surface. Measurements are multiply regressed on wind and dominant-wave variables. Bubbles penetrate to a depth of about a factor of 6γ −1 times the significant wave height H s , where γ is the wave age, or ratio of dominant-wave phase speed to wind speed. The measured mean bubble radius decreases weakly with depth, unless waves are gently sloping, at about 5% m−1. At 0.4 m, the mean radius ranges from 30 to 80 μm and is typically about two-thirds of the radius contributing most to void fraction. The total, depth-integrated surface area of the bubbles and their upward displacement of the sea surface, or “void displacement,” increase as wind speed to the powers 7 ± 1 and 8 ± 1, respectively, dependences ascribed to the preferential breaking of short, steep wind waves. It is estimated, on extrapolating trends, that the total bubble surface area on average is equal to that of the sea surface above them, and the mean void displacement is equal to the mean bubble radius, at a wind speed of about 15 m s−1.
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
The development of convective cells within anvil precipitation, in a region of moderate convective activity that might be called a small mesoscale convective system, is described and discussed. The presence of precipitation-sized hydrometeors in the air as the convection develops makes early stages visible to radar that might not otherwise be seen. Two kinds of convective initiation are illustrated. In one, a vigorous cell is initiated over an outflow boundary, but within light precipitation. In the other, the initiation is evidently by an instability created by the melting layer, perhaps by a mechanism first discussed by Findeisen. In this latter type, the new convective elements are not severe but they generate supercooled cloud within the anvil, extend entirely through the anvil to altitudes above 12 km MSL, and produce graupel showers with rain at the ground exceeding 50 dBZ. The instability itself may be generated in large part by moistening and cooling the sounding by the falling precipitation.
Abstract
The development of convective cells within anvil precipitation, in a region of moderate convective activity that might be called a small mesoscale convective system, is described and discussed. The presence of precipitation-sized hydrometeors in the air as the convection develops makes early stages visible to radar that might not otherwise be seen. Two kinds of convective initiation are illustrated. In one, a vigorous cell is initiated over an outflow boundary, but within light precipitation. In the other, the initiation is evidently by an instability created by the melting layer, perhaps by a mechanism first discussed by Findeisen. In this latter type, the new convective elements are not severe but they generate supercooled cloud within the anvil, extend entirely through the anvil to altitudes above 12 km MSL, and produce graupel showers with rain at the ground exceeding 50 dBZ. The instability itself may be generated in large part by moistening and cooling the sounding by the falling precipitation.
