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Gary T. Mitchum, David W. Hancock III, George S. Hayne, and Douglas C. Vandemark

Abstract

Data from satellite altimeters are often degraded by the occurrence of unrealistically high radar return cross sections, which indicate a breakdown of the rough surface scattering model used to interpret these measurements in terms of satellite to sea surface height ranges. The TOPEX altimetric data are examined and nearly 200 000 such events during the 7-yr period, 1993–99, inclusive, are identified. The primary purpose of this paper is to make a comprehensive description of where and when these events occur, which is important because many of the communities that make use of the TOPEX data are generally unaware of this phenomenon. It is shown that these events affect almost 6% of the over-ocean TOPEX data, but only approximately 60% of these events are rejected by the recommended TOPEX data flagging. A global description of these events is made, showing that the events are associated with regions of climatologically weak winds (e.g., the summer hemispheres and the western Pacific warm pool region), supporting the existing hypothesis that these events are due to returns from surfaces where centimeter-scale waves are suppressed. The TOPEX results are confirmed with a comparison to anomalous returns from the NASA Scatterometer (NSCAT), and the relationship to very low wind speeds is further examined using the Tropical Ocean Global Atmosphere–Tropical Atmosphere Ocean array (TOGA–TAO) moored buoys. Finally, it is shown that there is some evidence that not all of the events can be accounted for by very low wind speeds. This suggests that future work might exploit the occurrence of these events to study other phenomena, such as surface slicks, that may lead to additional geophysical applications of the altimetric data.

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Edward J. Walsh, David W. Hancock III, Donald E. Hines, Robert N. Swift, and John F. Scott

Abstract

The Surface Contour Radar (SCR) is a 36-GHz computer-controlled airborne system, which produces ocean directional wave spectra with much higher angular resolution than pitch-and-roll buoys. SCR observations of the evolution of the fetch-limited directional wave spectrum are presented which indicate the existence of a fully-developed sea state. The JONSWAP wave growth model for wave energy and frequency was in best agreement with the SCR measurements. The model of Donelan et al. correctly predicted the propagation direction of waves in the asymmetrical fetch situation nearshore. The Donelan et al. parameterization is generalized to permit other growth algorithms to predict the correct direction of propagation in asymmetrical fetch situations.

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