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S. A. Thorpe and A. J. Hall

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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J. S. Hall and L. A. Riley

Abstract

No abstract.

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A. Graham, D. K. Woolf, and A. J. Hall

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.

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Ferguson Hall, T. J. Henderson, and Stuart A. Cundiff

Cloud seeding operations using dry ice dispensed from aircraft have been carried out by the California Electric Power Company in the High Sierra near Bishop, Calif., since February 1948. Limited visual observations suggest that snow showers can be produced from non-precipitating orographic clouds by seeding, the snow reaching the ground at the higher elevations. Comparison of the annual runoff from the seeded watershed with that from adjacent areas for the three years 1948, 1949 and 1950 showed a positive departure in the 1948–49 season significant at the 1% level. Significant departures did not appear during the other two years, but the average for the three years was significant at the 5% level. From the same analysis it might be estimated that the average annual flow during the three-year period was augmented by 9% with the 90% confidence limits being zero and 18%. On the other hand the unusual circulation prevailing during the 1948–49 winter may have been responsible, at least in part, for the departure in flow during this season. Snowpack comparisons did not indicate a significant increase in the Bishop area, but it is shown that such a result is not incompatible with the possibility that additional precipitation was produced which was not reflected in the snow surveys.

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Melinda M. Hall, Michael McCartney, and J. A. Whitehead

Abstract

A moored array at the equator in the western basin of the Atlantic provides a 604-day time series of abyssal currents and temperatures spanning the full breadth of the Antarctic Bottom Water (AABW) flowing from the Brazil Basin to the Guiana Basin. Mean AABW transport is estimated to be 2.0 Sv (Sv ≡ 106 m3 s−1), comprising organized westward flow of 2.24 Sv and return flow of 0.24 Sv. The low-frequency variability is dominated by a quasi-annual transport cycle of amplitude 0.9 Sv and a 120-day period of amplitude 0.6 Sv. Maximum transports occur in September–October, while minimum transports occur in February–March. Allowing for this quasi-annual cycle and extrapolating the 604-day record to a full two years adds about 7% to the estimated mean AABW transport. The array also provides limited sampling in the overlying lower North Atlantic Deep Water (LNADW), where a southern boundary intensified flow of LNADW gives the strongest recorded mean speed through the array, 9.9 cm s−1 into the Brazil Basin. The LNADW records also have a quasi-annual cycle with strong LNADW flow episodes occurring in April–May. Time series of temperature indicate that the LNADW/AABW transition layer rises and falls in synchrony with the quasi-annual AABW transport cycle (uplifted transition layer during strong AABW transport periods). An observed overall warming trend appears to be accompanied by a decline in AABW transport.

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S. A. Thorpe, M. J. Ulloa, D. Baldwin, and A. J. Hall

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.

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S. A. Thorpe, T. R. Osborn, J. F. E. Jackson, A. J. Hall, and R. G. Lueck

Abstract

The rate of dissipation of turbulent kinetic energy has been measured with airfoil probes mounted on an autonomous vehicle, Autosub, on constant-depth legs at 2–10 m below the surface in winds up to 14 m s−1. The observations are mostly in an area limited by fetch to 26 km where the pycnocline depth is about 20 m. At the operational depths of 1.55–15.9 times the significant wave height H s, and in steady winds of about 11.6 m s−1 when the wave age is 11.7–17.2, dissipation is found to be lognormally distributed with a law-of-the-wall variation with depth and friction velocity. Breaking waves, leaving clouds of bubbles in the water, are detected ahead of the Autosub by a forward-pointing sidescan sonar, and the dissipation is measured when the clouds are subsequently reached. Bands of bubbles resulting from the presence of Langmuir circulation are identified by a semiobjective method that seeks continuity of band structure recognized by both forward- and sideways-pointing sidescan sonars. The times at which bands are crossed are determined and are used to relate dissipation rates and other measured parameters to the location of Langmuir bands. Shear-induced “temperature ramps” are identified with large horizontal temperature gradients. The turbulence measurements are consequently related to breaking waves, the bubble clouds, Langmuir circulation, and temperature ramps, and therefore to the principal processes of mixing in the near-surface layer of the ocean, all of which are found to have associated patterns of turbulent dissipation rates. A large proportion of the highest values of dissipation rate occur within bubble clouds. Dissipation is enhanced in the convergence region of Langmuir circulation at depths to about 10 m, and on the colder, bubble containing, side of temperature ramps associated with water advected downward from near the surface. Near the sea surface, turbulence is dominated by the breaking waves; below a depth of about 6H s the local vertical mixing in stronger Langmuir circulation cells exceeds that produced on average by the shear-induced eddies that form temperature ramps.

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S. A. Thorpe, M. S. Cure, A. Graham, and A. J. Hall

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.

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Richard J. Hall, Adam A. Scaife, Edward Hanna, Julie M. Jones, and Robert Erdélyi

Abstract

The variability of the North Atlantic Oscillation (NAO) is a key aspect of Northern Hemisphere atmospheric circulation and has a profound impact upon the weather of the surrounding landmasses. Recent success with dynamical forecasts predicting the winter NAO at lead times of a few months has the potential to deliver great socioeconomic impacts. Here, a linear regression model is found to provide skillful predictions of the winter NAO based on a limited number of statistical predictors. Identified predictors include El Niño, Arctic sea ice, Atlantic SSTs, and tropical rainfall. These statistical models can show significant skill when used to make out-of-sample forecasts, and the method is extended to produce probabilistic predictions of the winter NAO. The statistical hindcasts can achieve similar levels of skill to state-of-the-art dynamical forecast models, although out-of-sample predictions are less skillful, albeit over a small period. Forecasts over a longer out-of-sample period suggest there is true skill in the statistical models, comparable with that of dynamical forecasting models. They can be used both to help evaluate and to offer insight into the sources of predictability and limitations of dynamical models.

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Christopher A. Shuman, Dorothy K. Hall, Nicolo E. DiGirolamo, Thomas K. Mefford, and Michael J. Schnaubelt

Abstract

The stability of the Moderate Resolution Imaging Spectroradiometer (MODIS) ice-surface temperature (IST) product from Terra was investigated for use as a climate-quality data record. The availability of climate-quality air temperature data TA from a NOAA observatory at Greenland’s Summit Station has enabled this high-temporal-resolution study of MODIS ISTs. During a >5-yr period (July 2008–August 2013), more than 2500 IST values were compared with ±3-min-average TA values from NOAA’s primary 2-m temperature sensor. This enabled an expected small offset between air and ice-sheet surface temperatures (TA > IST) to be investigated over multiple annual cycles. The principal findings of this study show 1) that IST values are slightly colder than the TA values near freezing but that this offset increases as temperature decreases and 2) that there is a pattern in IST–TA differences as the solar zenith angle (SoZA) varies annually. This latter result largely explains the progressive offset from the in situ data at colder temperatures but also indicates that the MODIS cloud mask is less accurate approaching and during the polar night. The consistency of the results over each year in this study indicates that MODIS provides a platform for remotely deriving surface temperature data, with the resulting IST data being most compatible with in situ TA data when the sky is clear and the SoZA is less than ~85°. The ongoing development of the IST dataset should benefit from improved cloud filtering as well as algorithm modifications to account for the progressive offset from TA at colder temperatures.

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