Comparison of Low-Level Cloud Motion Vectors and Moored Buoy Winds

Abstract

Moored buoy wind measurements made at 3 m height at three sites (8°27′N, 150°45′W; 0°, 150°W; 0°, 125°W) in the central equatorial Pacific during 1976 and 1977 were compared with satellite-derived low-level (∼1 km) cloud motion vectors within a 5° square centered on each buoy site. The satellite winds were determined on a routine basis by NOAA's National Environmental Satellite Service and the buoy wind measurements were made by NOAA's Pacific Marine Environmental Laboratory. To compare satellite and buoy wind measurements, 3 h vector averages of the buoy data were computed at times of satellite wind observations. The number of pairs of buoy winds and winds derived from infrared satellite imagery was 265. Directions of the low-level cloud motion vectors were similar to the buoy wind directions. The average satellite wind speed was 8.0 m s−1, which was 3.3 m s−1 or 70% greater than the average buoy wind speed. At the 0°, 125°W site where 190 satellite-buoy wind pairs were obtained, the average wind speed difference was 3.8 m s−1 and 30% of the deviations were greater than 5 m s−1. The standard deviation of the satellite winds was nearly 80% larger than the wind recorder data. At the 0°, 125°W site the correlation between the satellite and buoy wind speeds was 0.25. Our results indicated that the mean and fluctuations of low-level cloud motion vectors were not in satisfactory enough agreement with the buoy wind data to be considered representative of the near-surface wind field. Although our results were presumably a priori expected because of the vertical separation between the measurements, this note provides an estimate of the difference between the two types of measurements.