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M. A. Srokosz

Abstract

A simple analytic formula for the probability of wave breaking at a given point is derived on the basis of Gaussian statistics. The result is found to depend on a nondimensionalized variance of the sea surface acceleration and a threshold parameter α. This parameter determines those values of the downward acceleration at the crest (values greater than αg, α positive) for which the wave is considered to be breaking. The results are related to other recent work on wave breaking and the choice of the parameter α is discussed.

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M. A. Srokosz

Abstract

A new statistical distribution for the surface elevation of weakly nonlinear water waves is derived using the Pearson System of distributions. The new distribution avoids some problems associated with previously proposed distributions. Namely, its probability density function is positive everywhere, unlike prior results obtained with Gram–Charlier series. Furthermore, it is derived without requiring the assumptions that the wavefield is unidirectional and narrow band, as made in some earlier studies. The distribution obtained is a form of the beta distribution and depends only on two parameters, the variance and the skewness of the sea surface elevation. The new distribution is compared to wave data, measured on a reservoir, and found to give a reasonable fit.

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G. D. Quartly and M. A. Srokosz

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In this paper seasonal variations in the Agulhas Retroflection region are studied by analyzing data from the radar altimeter, flown on the Geosat satellite, and output from the Fine Resolution Antarctic Model (FRAM). The observations of mesoscale variability from Geosat suggest that any seasonal variation that exists is confined to certain limited areas of the Agulhas Retroflection region. Results from FRAM appear to show no seasonal variability, despite the fact that the model is driven by a seasonally varying wind field. A by-product of this study is a comparison between Geosat observations and FRAM output. This shows that, apart from the issue of seasonal variations, there is reasonable agreement, and that such differences as do exist give insights into how the model might be improved.

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J. A. Ewing, M. S. Longuet-Higgins, and M. A. Srokosz

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Recent theoretical studies of the accelerations in regular gravity waves of finite steepness have shown striking differences between the Eulerian and the Lagrangian accelerations (those measured by fixed instruments or freely floating instruments, respectively). In the present paper, attention is directed to field observations of accelerations in random seas. Two sets of data are analyzed, representing Eulerian and Lagrangian measurements. The Eulerian accelerations are found to be notably asymmetric, with maximum downwards accelerators exceeding −1.6g. The Lagrangian acceleration histograms are narrower and more symmetric, in general. As might be expected, the acceleration variance is highly sensitive to the high-frequency cutoff, in both types of data.

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G. D. Quartly, M. A. Srokosz, and A. C. McMillan

Abstract

In this paper waveforms, that is, returns from the ocean surface, from a number of spaceborne radar altimeter instruments [European Remote-sensing Satellites (ERS-1 and -2), TOPEX, and Poseidon] are examined. This is the first paper to analyze waveform data from a number of altimeters in a consistent manner. Mean shapes and various statistical properties (bin-to-bin correlations, number of independent samples) were determined and the authors comment on their anomalies. The analyses were performed for data over the deep ocean, as that is the best understood surface. However, the determined functional characteristics of the individual altimeters are applicable to their operation over all surfaces. The implications of the existence of these anomalies for the retrieval of geophysical parameters from radar altimeter data are discussed. It is argued that the need for physically based theories, in order to understand radar altimeter returns from the ocean (or indeed any other) surface, implies a need for the engineering and software design of the instrument to be such as to avoid spurious anomalies in the waveforms.

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G. D. Quartly, T. H. Guymer, and M. A. Srokosz

Abstract

Rain has long been categorized as a contaminant of altimeter data, but little has been done previously to ascertain the magnitude and frequency of its effect or its geographical distribution. Proceeding from recent analysis of ERS-1 data, and the insight gained, this paper addresses all these issues, highlighting the particular advantages of Topex's dual-frequency capability.

The primary effect of rain is on the observed backscatter values σ0 through attenuation of the radar signal. An empirical relationship is derived linking the observed σ0 values at C band and Ku band. Through a series of case studies and global analyses, prominent departures from this relationship are shown to be caused by regions of significant precipitation. As it is predominantly the Ku-band returns that are affected, C-band σ0,s are likely to be of great use for the determination of wind speeds in regions or events of intense rain. The different attenuation rates give dual-frequency altimetry the potential to yield information on rain distribution and intensity that complement that achievable by infrared sensors alone.

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J. Gómez-Enri, C. P. Gommenginger, M. A. Srokosz, P. G. Challenor, and J. Benveniste

Abstract

For early satellite altimeters, the retrieval of geophysical information (e.g., range, significant wave height) from altimeter ocean waveforms was performed on board the satellite, but this was restricted by computational constraints that limited how much processing could be performed. Today, ground-based retracking of averaged waveforms transmitted to the earth is less restrictive, especially with respect to assumptions about the statistics of ocean waves. In this paper, a theoretical maximum likelihood estimation (MLE) ocean waveform retracker is applied tothe Envisat Radar Altimeter system (RA-2) 18-Hz averaged waveforms under both linear (Gaussian) and nonlinear ocean wave statistics assumptions, to determine whether ocean wave skewness can be sensibly retrieved from Envisat RA-2 waveforms. Results from the MLE retracker used in nonlinear mode provide the first estimates of global ocean wave skewness based on RA-2 Envisat averaged waveforms. These results show for the first time geographically coherent skewness fields and confirm the notion that large values of skewness occur primarily in regions of large significant wave height. Results from the MLE retracker run in linear and nonlinear modes are compared with each other and with the RA-2 Level 2 Sensor Geophysical Data Records (SGDR) products to evaluate the impact of retrieving skewness on other geophysical parameters. Good agreement is obtained between the linear and nonlinear MLE results for both significant wave height and epoch (range), except in areas of high-wave-height conditions.

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J. Gourrion, D. Vandemark, S. Bailey, B. Chapron, G. P. Gommenginger, P. G. Challenor, and M. A. Srokosz

Abstract

Globally distributed crossovers of altimeter and scatterometer observations clearly demonstrate that ocean altimeter backscatter correlates with both the near-surface wind speed and the sea state. Satellite data from TOPEX/Poseidon and NSCAT are used to develop an empirical altimeter wind speed model that attenuates the sea-state signature and improves upon the present operational altimeter wind model. The inversion is defined using a multilayer perceptron neural network with altimeter-derived backscatter and significant wave height as inputs. Comparisons between this new model and past single input routines indicates that the rms wind error is reduced by 10%–15% in tandem with the lowering of wind error residuals dependent on the sea state. Both model intercomparison and validation of the new routine are detailed, including the use of large independent data compilations that include the SeaWinds and ERS scatterometers, ECMWF wind fields, and buoy measurements. The model provides consistent improvement against these varied sources with a wind-independent bias below 0.3 m s−1. The continuous form of the defined function, along with the global data used in its derivation, suggest an algorithm suitable for operational application to Ku-band altimeters. Further model improvement through wave height inclusion is limited due to an inherent multivaluedness between any single realization of the altimeter measurement pair [σ o, H S] and observed near-surface winds. This ambiguity indicates that H S is a limited proxy for variable gravity wave properties that impact upon altimeter backscatter.

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Robin W. Pascal, Margaret J. Yelland, Meric A. Srokosz, Bengamin I. Moat,, Edward M. Waugh, Daniel H. Comben, Alex G. Cansdale, Mark C. Hartman, David G. H. Coles, Ping Chang Hsueh, and Timothy G. Leighton

Abstract

Waves and wave breaking play a significant role in the air–sea exchanges of momentum, sea spray aerosols, and trace gases such as CO2, but few direct measurements of wave breaking have been obtained in the open ocean (far from the coast). This paper describes the development and initial deployments on two research cruises of an autonomous spar buoy that was designed to obtain such open-ocean measurements. The buoy was equipped with capacitance wave wires and accelerometers to measure surface elevation and wave breaking, downward-looking still and video digital cameras to obtain images of the sea surface, and subsurface acoustic and optical sensors to detect bubble clouds from breaking waves. The buoy was free drifting and was designed to collect data autonomously for days at a time before being recovered. Therefore, on the two cruises during which the buoy was deployed, this allowed a variety of sea states to be sampled in mean wind speeds, which ranged from 5 to 18 m s−1.

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Ian M. Brooks, Margaret J. Yelland, Robert C. Upstill-Goddard, Philip D. Nightingale, Steve Archer, Eric d'Asaro, Rachael Beale, Cory Beatty, Byron Blomquist, A. Anthony Bloom, Barbara J. Brooks, John Cluderay, David Coles, John Dacey, Michael Degrandpre, Jo Dixon, William M. Drennan, Joseph Gabriele, Laura Goldson, Nick Hardman-Mountford, Martin K. Hill, Matt Horn, Ping-Chang Hsueh, Barry Huebert, Gerrit De Leeuw, Timothy G. Leighton, Malcolm Liddicoat, Justin J. N. Lingard, Craig Mcneil, James B. Mcquaid, Ben I. Moat, Gerald Moore, Craig Neill, Sarah J. Norris, Simon O'Doherty, Robin W. Pascal, John Prytherch, Mike Rebozo, Erik Sahlee, Matt Salter, Ute Schuster, Ingunn Skjelvan, Hans Slagter, Michael H. Smith, Paul D. Smith, Meric Srokosz, John A. Stephens, Peter K. Taylor, Maciej Telszewski, Roisin Walsh, Brian Ward, David K. Woolf, Dickon Young, and Henk Zemmelink

Abstract

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