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W. G. Large, J. Morzel, and G. B. Crawford

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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R. F. Milliff, P. P. Niiler, J. Morzel, A. E. Sybrandy, D. Nychka, and W. G. Large

Abstract

Observations of the surface wind speed and direction in the Labrador Sea for the period October 1996–May 1997 were obtained by the NASA scatterometer (NSCAT), and by 21 newly developed Minimet drifting buoys. Minimet wind speeds are inferred, hourly, from observations of acoustic pressure in the Wind-Speed Observation Through Ambient Noise (WOTAN) technology. Wind directions are inferred from a direction histogram, also accumulated hourly, as determined by the orientation of a wind vane attached to the surface floatation. Effective temporal averaging of acoustic pressure (20 min), and the interval over which the direction histogram is accumulated (160 s), are shown to be consistent with low-pass filtering to preserve mesoscale time- and space-scale signals in the surface wind. Minimet wind speed and direction retrievals in the Labrador Sea were calibrated with collocated NSCAT data. The NSCAT calibrations extend over the full field lifetimes of each Minimet (90 days on average). Wind speed variabilities of O(5 m s–1) and wind direction variabilities of O(40°) are evident on timescales of one to several hours in Minimet time series. Wind speed and direction rms differences versus spatial separation comparisons (from 0 to 400 km) for the NSCAT and Minimet records demonstrate similar rms differences in wind speed as a function of spatial separation, but O(20°) larger rms differences in Minimet direction. These differences are consistent with spatial smoothing effects in the median filter step for wind direction retrievals within the NSCAT swath. Zonal and meridional surface wind components are constructed from the calibrated Minimet wind speed and direction dataset. Rms differences versus spatial separation for these components are used to estimate mesoscale spatial correlation length scales of 250 and 290 km in the zonal and meridional directions, respectively.

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