Accounting for Surface Wave Distortion of the Marine Wind Profile in Low-Level Ocean Storms Wind Measurements

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  • 1 National Center for Atmospheric Research, Boulder, Colorado
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Abstract

Marine wind measurements at three heights (3.0,4.5, and 5.0 m) from both moored and drifting buoys during the Ocean Storms Experiment are described. These winds are compared with each other, with winds from ships, from subsurface ambient acoustic noise, and from the analyses of three numerical weather prediction centers. In the mean, wind directions generally differ by only a small constant offset of a few degrees. No wave influence on the wind direction is evident, because the differences are not systematic and, with few exceptions, they are less than the expected error. After correcting for some apparent calibration and instrument bias, the Ocean Storms wind speeds display similar behavior when compared to the analyzed wind products. There is excellent agreement up to a transition wind speed between 7 and 10 m s−1, above which all the measured winds tend to be relatively low. The transition speed is found to increase with anemometer height, so this behavior is interpreted as being due to the distortion of the wind profile by surface waves. The wave effects are shown to be profound. By increasing the stress by 40% or more in high winds, the corrections are shown to be essential for numerical models to simulate the oceanic response to storm events. The Ocean Storms corrections are used to construct functions describing wave influence on both the vertical wind shear and the mean wind speed profile. These functions can only be regarded as crude approximations because the Ocean Storms data are far from ideal for determining them. However, they can be used to assess potential influences of surface waves on any low-level wind measurement.

Abstract

Marine wind measurements at three heights (3.0,4.5, and 5.0 m) from both moored and drifting buoys during the Ocean Storms Experiment are described. These winds are compared with each other, with winds from ships, from subsurface ambient acoustic noise, and from the analyses of three numerical weather prediction centers. In the mean, wind directions generally differ by only a small constant offset of a few degrees. No wave influence on the wind direction is evident, because the differences are not systematic and, with few exceptions, they are less than the expected error. After correcting for some apparent calibration and instrument bias, the Ocean Storms wind speeds display similar behavior when compared to the analyzed wind products. There is excellent agreement up to a transition wind speed between 7 and 10 m s−1, above which all the measured winds tend to be relatively low. The transition speed is found to increase with anemometer height, so this behavior is interpreted as being due to the distortion of the wind profile by surface waves. The wave effects are shown to be profound. By increasing the stress by 40% or more in high winds, the corrections are shown to be essential for numerical models to simulate the oceanic response to storm events. The Ocean Storms corrections are used to construct functions describing wave influence on both the vertical wind shear and the mean wind speed profile. These functions can only be regarded as crude approximations because the Ocean Storms data are far from ideal for determining them. However, they can be used to assess potential influences of surface waves on any low-level wind measurement.

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