FY 1980

Surface wind measurements and low-level cloud motion vectors near the intertropical convergence zone in the central Pacific Ocean from November 1977 to March 1978

Halpern, D.

Mon. Weather Rev., 107(11), 1525–1534 (1979)

Moored buoy wind measurements were made at 15-min intervals at 3.5 m height at 6, 7 and 8°N along 150°W in the central equatorial Pacific from November 1977 to March 1978. During November, December and January the Intertropical Convergence Zone occurred near 8, 7 and 6°N, respectively, indicating a southward movement of ~3 km day⁻¹. The east and north spectra of each record did not contain any significant (at the 95% confidence level) peaks and the spectral estimates decreased with increasing frequency with a slope of about –1.5. Wind fluctuations at frequencies <0.01 and 0.005 cycles per hour (cph) were coherent (at the 95% confidence level) with zero phase difference for horizontal distances of 110 and 220 km, respectively. The noise level of the coherent wind fluctuations was less than 50%. The mean value of a first-order approximation of the divergence between 6 and 7°N was (–1.0 ± 6.5) × 10⁻⁵ s⁻¹. On short time-scales, such as 28 h, the divergence between 6 and 7°N changed from zero to –1.3 × 10⁻⁴ s⁻¹. Spectral estimates of the divergence contained a small peak at ~6 days which was not expected because of the absence of a corresponding peak in the north spectrum. Satellite-derived low-level cloud motion vectors determined on a routine basis by NOAA's National Environmental Satellite Service at four locations within ~275 km of the buoys were compared with coincident 3 h vector-averaged buoy wind measurements. The average rms difference between all the satellite and buoy wind speeds was 4.4 m s⁻¹. Approximately 3 and 44% of all the buoy and satellite wind speeds, respectively, were greater than 10 m s⁻¹. For wind speeds the average orthogonal regression line was satellite wind speed (m s⁻¹) = –7.1 + 2.5 buoy wind speed (m s⁻¹). For wind directions the average regression line was satellite wind direction (deg) = 069 + 0.23 buoy wind direction (deg), indicating that the winds veered (clockwise turning) with height for northeast trade winds and backed (counterclockwise turning) with height for southeast trade winds. Because of the turning of the wind vector with height, the directions of the satellite winds were more easterly than the surface winds. Although the results of the comparison between buoy and satellite wind vectors were presumably a priori expected because of the vertical separation between the measurements, our results provide an estimate of the difference between the two types of measurements.