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H. Melling and Roland List

Abstract

The scattering of a narrow-band acoustic signal from atmospheric turbulence generates an echo of randomly varying amplitude and frequency. The mean frequency of the power spectrum of this echo is of interest, as it is related by a Doppler shift to the mean motion of the turbulent air. A data processing technique based on the theory which was developed by Srivastava and Carbone (1969) for application to microwave radars is discussed, wherein this mean frequency may be deduced from measurements of the instantaneous frequency of the echo. The theory demonstrates that the probability density of instantaneous echo frequency is dependent on only the first two moments of the power spectrum and that these moments may be determined simply by averaging data in the time domain. Although a typical echo-sounder receiver generates as output only one Cartesian component of the complex-valued echo, an equation due to Rice (1945) is cited which demonstrates that for the narrow band random signals typical of atmospheric echoes, the mean rate of zero crossing of this component is a good estimate of the mean frequency of the signal spectrum. Thus, the addition of an interval timer to an echo sounder gives it radial wind sensing capability. Because only averaging of measurements is necessary to estimate this mean, this approach to Doppler velocity extraction is computationally more efficient than fast Fourier transform methods usually applied. In addition, the electronic hardware required is much less expensive.

The validity in echo sounding of the further step of equating the instantaneous rate of zero crossing to the instantaneous frequency of the signal is demonstrated by application of the technique to an electronically simulated echo. Use of an actual atmospheric echo in this demonstration was precluded by the marked statistical nonstationarity of such echoes. Statistical data for instantaneous zero crossing were found to be in good agreement with theoretical predictions for instantaneous frequency. Similar statistical analyses are presented of the echoes from snow, of those from atmospheric thermal turbulence and of the acoustic noise background.

On the basis of the demonstrated statistical behavior of echo sounder signals, a procedure is described for the estimation from digitized data of this kind of mean-scattering cross sections and mean velocities of turbulent air parcels, and for the suppression of the effects of background noise.

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H. Melling and Roland List

Abstract

Acoustic echoes received during snowfall have been investigated using a vertically directed echo sounder. Doppler shift analysis of the echoes indicates consistent downward motion in the range 1–2 m s−1 and thus demonstrates that the snowflakes are the cause of the observed backscattering. Consideration of the mean scattered power yields a radar reflectivity for the snow of 35–42 dBZ, and a precipitation rate of 1.2–2.7 mm h−1. The latter figure may be compared with the actual mean precipitation rate of 0.9 mm h−1 averaged over the duration of the showers, and intervening periods of little or no snow. The observed probability distribution of instantaneous echo power is log-normal. This is in contrast to the theoretically predicted exponential function. The standard deviation of the log-normal distribution is close to 6.1 dB, which is the saturation value expected for acoustic waves interacting strongly with the turbulent atmosphere. Multiple scattering may thus be significant in these observations and also generally in atmospheric echo sounding.

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M-L. Timmermans, H. Melling, and L. Rainville

Abstract

A 50-day time series of high-resolution temperature in the deepest layers of the Canada Basin in the Arctic Ocean indicates that the deep Canada Basin is a dynamically active environment, not the quiet, stable basin often assumed. Vertical motions at the near-inertial (tidal) frequency have amplitudes of 10– 20 m. These vertical displacements are surprisingly large considering the downward near-inertial internal wave energy flux typically observed in the Canada Basin. In addition to motion in the internal-wave frequency band, the measurements indicate distinctive subinertial temperature fluctuations, possibly due to intrusions of new water masses.

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Humfrey Melling, Paul H. Johnston, and David A. Riedel

Abstract

A practical technology based on moored subsea instrumentation has been developed to measure the draft of polar pack ice. The technology exploits the complementary capabilities of an ice-profiling sonar designed and built for the application and of a commercially available acoustic Doppler sonar. The former instrument observes the zenithal range of sea ice passing through its single narrow sonar beam, while the latter observes the radial motion of the ice along its four inclined beams. The sequence of ranges obtained by the ice-profiling sonar is combined with supplementary observations of hydrostatic pressure to yield a sequence of ice draft versus time; the sequence of Doppler speeds provides ice velocity that can be integrated to obtain displacement; by combining the draft and displacement sequences the profile of draft versus position is obtained. The foremost practical problem in calibration is establishing the temporal variation in the zero-draft reference. The technology is well suited to use in ice-congested waters, but difficulties remain for applications in marginal ice zones.

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E. Carmack, I. Polyakov, L. Padman, I. Fer, E. Hunke, J. Hutchings, J. Jackson, D. Kelley, R. Kwok, C. Layton, H. Melling, D. Perovich, O. Persson, B. Ruddick, M.-L. Timmermans, J. Toole, T. Ross, S. Vavrus, and P. Winsor

Abstract

The loss of Arctic sea ice has emerged as a leading signal of global warming. This, together with acknowledged impacts on other components of the Earth system, has led to the term “the new Arctic.” Global coupled climate models predict that ice loss will continue through the twenty-first century, with implications for governance, economics, security, and global weather. A wide range in model projections reflects the complex, highly coupled interactions between the polar atmosphere, ocean, and cryosphere, including teleconnections to lower latitudes. This paper summarizes our present understanding of how heat reaches the ice base from the original sources—inflows of Atlantic and Pacific Water, river discharge, and summer sensible heat and shortwave radiative fluxes at the ocean/ice surface—and speculates on how such processes may change in the new Arctic. The complexity of the coupled Arctic system, and the logistic and technological challenges of working in the Arctic Ocean, require a coordinated interdisciplinary and international program that will not only improve understanding of this critical component of global climate but will also provide opportunities to develop human resources with the skills required to tackle related problems in complex climate systems. We propose a research strategy with components that include 1) improved mapping of the upper- and middepth Arctic Ocean, 2) enhanced quantification of important process, 3) expanded long-term monitoring at key heat-flux locations, and 4) development of numerical capabilities that focus on parameterization of heat-flux mechanisms and their interactions.

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