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Revisiting Oceanic Acoustic Gravity Surface Waves

JOURNAL OF PHYSICAL OCEANOGRAPHY 2015年第12期45卷 2953-2958页

作者： Smith, Jerome A. Univ Calif San Diego Scripps Inst Oceanog La Jolla CA 92093 USA

The reintroduction of compressibility into the equations for surface gravity waves can permit mixed acoustic-gravity modes that are periodic in the vertical as well as horizontal directions. These modes interact with the bottom even in deep water, so bottom motion can excite them. Because they propagate rapidly, it has been suggested they may be useful as precursors of tsunamis. Here the equations are revisited, and, using some robust approximations, some physical understanding and interpretation of the phenomena are presented. It is posed that these new modes can alternatively be thought of as acoustic modes slightly modified by a gravity wave boundary condition at the surface, rather than as surface waves dramatically modified by compressibility. Their potential use is not diminished;indeed, this alternative perspective should help make implementation more practical.

关键词： Circulation Dynamics Wave properties Waves oceanic observational techniques and algorithms Acoustic measurements effects Infrasound Applications Emergency preparedness

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Field Measurements of Surface and Near-Surface Turbulence in the Presence of Breaking Waves

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2015年第4期45卷 943-965页

作者： Sutherland, Peter Melville, W. Kendall Univ Calif San Diego Scripps Inst Oceanog La Jolla CA 92093 USA

Wave breaking removes energy from the surface wave field and injects it into the upper ocean, where it is dissipated by viscosity. This paper presents an investigation of turbulent kinetic energy (TKE) dissipation beneath breaking waves. Wind, wave, and turbulence data were collected in the North Pacific Ocean aboard R/P FLIP, during the ONR-sponsored High Resolution Air-Sea Interaction (HiRes) and Radiance in a Dynamic Ocean (RaDyO) experiments. A new method for measuring TKE dissipation at the sea surface was combined with subsurface measurements to allow estimation of TKE dissipation over the entire wave-affected surface layer. Near the surface, dissipation decayed with depth as z(-1), and below approximately one significant wave height, it decayed more quickly, approaching z(-2). High levels of TKE dissipation very near the sea surface were consistent with the large fraction of wave energy dissipation attributed to non-air-entraining microbreakers. Comparison of measured profiles with large-eddy simulation results in the literature suggests that dissipation is concentrated closer to the surface than previously expected, largely because the simulations did not resolve microbreaking. Total integrated dissipation in the water column agreed well with dissipation by breaking for young waves, c(m)/u(*) <50 (where c(m) is the mean wave frequency and u(*) is the atmospheric friction velocity), implying that breaking was the dominant source of turbulence in those conditions. The results of these extensive measurements of near-surface dissipation over three field experiments are discussed in the context of observations and ocean boundary layer modeling efforts by other groups.

关键词： Circulation/ Dynamics Atmosphere-ocean interaction Turbulence Waves oceanic observational techniques and algorithms In situ oceanic observations

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Gravity Wave Diagnostics and Characteristics in Mesoscale Fields

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JOURNAL OF THE ATMOSPHERIC SCIENCES 2015年第11期72卷 4372-4392页

作者： Kruse, Christopher G. Smith, Ronald B. Yale Univ New Haven CT 06511 USA

As numerical models of complex atmospheric flows increase their quality and resolution, it becomes valuable to isolate and quantify the embedded resolved gravity waves. The authors propose a spatial filtering method combined with a selection of quadratic diagnostic quantities such as heat, momentum, and energy fluxes to do this. These covariant quantities were found to be insensitive to filter cutoff length scales between 300 and 700 km, suggesting the existence of a cospectral gap. The gravity waves identified with the proposed method display known properties from idealized studies, including vertical propagation, upwind propagation, the relationship between momentum and energy flux, and agreement with fluxes derived from an alternative method involving simulations with and without terrain. The proposed method is applied to 2- and 6-km-resolution realistic WRF simulations of orographic and nonorographic gravity waves over and around New Zealand within complex frontal cyclones. Deep mountain wave, shallow mountain wave, jet-generated gravity wave, and convection-generated gravity wave events were chosen for analysis. The four wave events shared the characteristics of positive vertical energy flux, negative zonal momentum flux, and upwind horizontal energy flux. Two of the gravity wave events were dissipated nonlinearly.

关键词： Circulation Dynamics Gravity waves Mesoscale processes Mountain waves observational techniques and algorithms algorithms Mathematical and statistical techniques Filtering techniques Fourier analysis

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Characteristics, Energetics, and Origins of Agulhas Current Meanders and Their Limited Influence on Ring Shedding

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2015年第9期45卷 2294-2314页

作者： Elipot, Shane Beal, Lisa M. Univ Miami Rosenstiel Sch Marine & Atmospher Sci Miami FL 33149 USA

The Agulhas Current intermittently undergoes dramatic offshore excursions from its mean path because of the downstream passage of mesoscale solitary meanders or Natal pulses. New observations and analyses are presented of the variability of the current and its meanders using mooring observations from the Agulhas Current Time-Series Experiment (ACT) near 34 degrees S. Using a new rotary EOF method, mesoscale meanders and smaller-scale meanders are differentiated and each captured in a single mode of variance. During mesoscale meanders, an onshore cyclonic circulation and an offshore anticyclonic circulation act together to displace the jet offshore, leading to sudden and strong positive conversion of kinetic energy from the mean flow to the meander via nonlinear interactions. Smaller meanders are principally represented by a single cyclonic circulation spanning the entire jet that acts to displace the jet without extracting kinetic energy from the mean flow. Synthesizing in situ observations with altimeter data leads to an account of the number of mesoscale meanders at 34 degrees S: 1.6 yr(-1) on average, in agreement with a recent analysis by Rouault and Penven (2011) and significantly less than previously understood. The links between meanders and the arrival of Mozambique Channel eddies or Madagascar dipoles at the western boundary upstream are found to be robust in the 20-yr altimeter record. Yet, only a small fraction of anomalies arriving at the western boundary result in meanders, and of those, two-thirds can be related to ring shedding. Most Agulhas rings are shed independently of meanders.

关键词： Geographic location/entity Indian Ocean Atm/Ocean Structure/ Phenomena Boundary currents Eddies observational techniques and algorithms Altimetry Buoy observations Mathematical and statistical techniques Empirical orthogonal functions

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Pressure Response of Aanderaa and Sea-Bird Oxygen Optodes

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2015年第12期32卷 2305-2317页

作者： Bittig, Henry C. Fiedler, Bjoern Fietzek, Peer Koertzinger, Arne GEOMAR Helmholtz Ctr Ocean Res Kiel Marine Biogeochem Dept D-24105 Kiel Germany Kongsberg Maritime Contros GmbH D-24105 Kiel Germany

This study investigated the effect of hydrostatic pressure of up to 6000 dbar on Aanderaa and Sea-Bird oxygen optodes both in the laboratory and in the field. The overall pressure response is a reduction in the O-2 reading by 3%-4% per 1000 dbar, which is closely linear with pressure and increases with temperature. Closer inspection reveals two superimposed processes with an opposite effect: an O-2-independent pressure response on the luminophore that increases optode O-2 readings and an O-2-dependent change in luminescence quenching that decreases optode O-2 readings. The latter process dominates and is mainly due to a shift in the equilibrium between the sensing membrane and seawater under elevated pressures. If only the dominant O-2-dependent process is considered, then the Aanderaa and Sea-Bird optodes differ in their pressure response. Compensation of the O-2-independent process, however, yields a uniform O-2 dependence for Aanderaa optodes with standard foil and fast-response foil as well as for Sea-Bird optodes. A new scheme to calculate optode O-2 from raw data is proposed to account for the two processes. The overall uncertainty of the optode pressure correction amounts to 0.3% per 1000 dbar, which is mainly due to variability between the sensors.

关键词： Physical Meteorology and Climatology Chemistry oceanic observational techniques and algorithms algorithms Data quality control In situ oceanic observations Instrumentation/sensors

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The Evaluation of a New Method to Detect Mixing Layer Heights Using Lidar Observations

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2015年第11期32卷 2041-2051页

作者： Hicks, Micheal Sakai, Ricardo Joseph, Everette Natl Weather Serv Sterling VA 20166 USA Howard Univ Washington DC 20059 USA

A new automatic mixing layer height detection method for lidar observations of aerosol backscatter profiles is presented and evaluated for robustness. The new detection method incorporates the strengths of Steyn et al.'s error function-ideal profile (ERF) method and Davis et al.'s wavelet covariance transform (WCT) method. These two methods are critical components of the new method, and their robustness is also evaluated and then contrasted to the new method. The new method is applied to aerosol backscatter observations in two ways: 1) by looking for the most realistic mixing height throughout the entire profile and 2) by searching for mixing height below significant elevated obscurations (e.g., clouds or aerosol layers). The first approach is referred to as the hybrid method and the second as the hybrid-lowest method. Coincident radiosounding observations of mixing heights are used to independently reference the lidar-based *** were 4030 cases examined over a 5-yr period for mixing heights. The efficacy of the lidar-based methods was determined based on diurnal, seasonal, stability, and sky obscuration conditions. Of these conditions, the hybrid method performed best for unstable and cloudy situations. It determined mixing heights reliably (less than +/- 0.30-km bias) for close to 70% of those cases. The hybrid-lowest method performed best in stable and clear-sky conditions;it determined mixing heights reliably for over 70% of those cases. The WCT method performed the best overall.

关键词： Atm Ocean Structure Phenomena Boundary layer Microscale processes variability observational techniques and algorithms algorithms

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Integration of Scientific Echo Sounders with an Adaptable Autonomous Vehicle to Extend Our Understanding of Animals from the Surface to the Bathypelagic

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2015年第11期32卷 2173-2186页

作者： Moline, Mark A. Benoit-Bird, Kelly O'Gorman, David Robbins, Ian C. Univ Delaware Sch Marine Sci & Policy Lewes DE 19958 USA Oregon State Univ Coll Earth Ocean & Atmospher Sci Corvallis OR 97331 USA Calif Polytech State Univ San Luis Obispo Ctr Coastal Marine Sci San Luis Obispo CA 93407 USA

Acoustic echo sounders designed to map and discriminate organisms in the water column have primarily been deployed on ships. Because of acoustic attenuation of higher frequencies used to detect and discriminate micronekton and nekton, this has effectively restricted the range of this information to the upper water column. In an effort to overcome these range limitations by reducing the distance between the transducer and the targets of interest, dual-frequency (38 and 120 kHz) split-beam echo sounders were integrated into a Remote Environmental Monitoring Units (REMUS) 600 autonomous underwater vehicle (AUV), effectively doubling the range of quantitative acoustic data into the mesopelagic zone (600-1200 m). Data from the first set of missions in a range of conditions revealed that the AUV provided a stable platform for the echo sounders and improved vertical and horizontal positional accuracy over echo sounders towed by ships. In comparison to hull-mounted echo sounders, elimination of ship noise and surface bubbles provided a 17- and 19-dBW decrease in the noise floor for the 38- and 120-kHz echo sounders, respectively, effectively increasing the sampling range by 30%-40%. The extended depth range also increased the resolution of the acoustic horizontal footprint from 37-40 to 0.6-3.7 m, enabling discrimination of individual targets at depth. Also developed here is novel onboard echo sounder data processing and autonomy to allow sampling not feasible in a surface ship or towed configuration. Taken together, these data demonstrate an effective new tool for examining the biology of animals in the mesopelagic zone (600-1200 m) in ways previously only possible in the upper ocean.

关键词： observational techniques and algorithms Acoustic measurements effects In situ oceanic observations Instrumentation sensors Sampling Applications Animal studies Ecosystem effects

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Correcting Daytime Thermal Offset in Unventilated Pyranometers

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JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY 2015年第11期32卷 2088-2099页

作者： Serrano, A. Sanchez, G. Cancillo, M. L. Univ Extremadura Dept Phys E-06071 Badajoz Spain

A main source of error in solar radiation measurements is the thermal offset inherent to pyranometers. Despite acknowledgment of its importance, its correction has been widely ignored for several decades. This neglect may have caused a generalized underestimation in solar radiation measurements. This study focuses on the correction of this error in solar irradiance measurements. For this aim a plethora of correction models built as a linear combination of several environmental variables related to the ambient temperature and to the incoming radiation were proposed. The models are fitted to experimental measurements obtained during capping events and, finally, their performance is evaluated and compared. The main results indicate that models with only one independent variable moderately correct the thermal offset error. These simple models are useful when no additional instrumentation other than the pyranometer is available. On the other hand, the more complex models show the best performance, with a coefficient of determination R-2 over 0.8, an RMSE under 2 W m(-2), and an absolute value of mean bias error (MBE) under 0.5 W m(-2). Additionally, these models are used to study the differences between nighttime and daytime correction, revealing the unsuitability of using nighttime-fitted models to correct the daytime thermal offset. The general validity of the models is tested by their application to two different pyranometers. Results indicate that, whereas the factors involved in the best-performing models are the same, the values of the loading coefficients differ and therefore must be specifically calculated for each pyranometer.

关键词： Physical Meteorology and Climatology Irradiance Shortwave radiation observational techniques and algorithms Instrumentation sensors

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Vertical Flow in the Southern Ocean Estimated from Individual Moorings

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JOURNAL OF PHYSICAL OCEANOGRAPHY 2015年第9期45卷 2209-2220页

作者： Sevellec, F. Garabato, A. C. Naveira Brearley, J. A. Sheen, K. L. Univ Southampton Natl Oceanog Ctr Southampton Ocean & Earth Sci Southampton SO14 3ZH Hants England

This study demonstrates that oceanic vertical velocities can be estimated from individual mooring measurements, even for nonstationary flow. This result is obtained under three assumptions: (i) weak diffusion (Peclet number >> 1), (ii) weak friction (Reynolds number >> 1), and (iii) small inertial terms (Rossby number << 1). The theoretical framework is applied to a set of four moorings located in the Southern Ocean. For this site, the diagnosed vertical velocities are highly variable in time, their standard deviation being one to two orders of magnitude greater than their mean. The time-averaged vertical velocities are demonstrated to be largely induced by geostrophic flow and can be estimated from the time-averaged density and horizontal velocities. This suggests that local time-mean vertical velocities are primarily forced by the time-mean ocean dynamics, rather than by, for example, transient eddies or internal waves. It is also shown that, in the context of these four moorings, the time-mean vertical flow is consistent with stratified Taylor column dynamics in the presence of a topographic obstacle.

关键词： Geographic location/entity Southern Ocean Circulation/ Dynamics Vertical motion observational techniques and algorithms Buoy observations

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The Role of the Selection Problem and Non-Gaussianity in Attribution of Single Events to Climate Change

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JOURNAL OF CLIMATE 2015年第24期28卷 9873-9891页

作者： Christiansen, Bo Danish Meteorol Inst Lyngbyvej 100 DK-2100 Copenhagen O Denmark

In attempts to attribute a single eventsuch as an observed heat wave or floodingto climate change, the probability distributions of the quantity under consideration for current and preindustrial conditions are compared to determine a possible changed occurrence rate. These distributions are typically calculated from large ensembles produced by climate models and require large computational resources. In this study, a simple alternative surrogate method together with analytical considerations will be used as a test bed to inform about methodological issues connected to the selection problem and deviations from Gaussianity that should be considered before comprehensive climate models are *** author will mainly study the influence of the selection problem, which in this context means that when an event has been observed it is not obvious how the probability distributions should be defined. Should similar events be looked for in the immediate neighborhood of the observation or in an extended area? It is shown that this choice will have serious consequences for the distributions of the events and the attribution to climate *** author also demonstrates that deviations from Gaussianity can have a large influence on the conclusions and that it is important that the ensembles adequately represent the features that contribute to the extreme events under consideration. In particular, it is shown that the fractional attributable risk has very different behavior for heavy-tailed distributions than for Gaussian distributions. In the example considered with the surrogate methodEuropean heat wavesimportant features also include the seasonal variation in skewness.

关键词： Physical Meteorology and Climatology Climate variability observational techniques and algorithms Climate records Mathematical and statistical techniques Risk assessment Statistical techniques Time series

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