Analysis of aircraft measurements of momentum flux in the subcloud layer over the tropical atlantic ocean during gate

Abstract

The momentum flux data obtained by the gust probe aboard the NOAA DC-6 aircraft during GATE are analyzed. Vertical profiles are obtained for Phases I and III and correlated with vertical wind velocity profiles using the geostrophic departure method. Reasonable agreement is obtained using the horizontal equations of motion with negligible advective acceleration. The vertical profiles of momentum flux and wind speed variance compare well with the numerical model results of Deardorff (1972) and Wyngaard et al. (1974). Vertical distributions of power spectra for vertical eddy motion and cospectra corresponding to the momentum flux components are obtained along with the height variation of the dominant length scales of vertical eddy motion and the dissipation rate of turbulence kinetic energy. When normalized by mixed-layer similarity, these results agree well with previous determinations in the boundary layer over tropical oceans and over land.

Aircraft measurements of boundary-layer turbulence over the central Equatorial Pacific Ocean

“…Substituting this into (15) and taking the ratio CD(10)/C,(50) results in a factor 1.23, and a profile-derived drag coefficient of C&10) = 1.55 x 1o-3 which is somewhat lower than the velocity band derivation of the drag coefficients in the previous section but is in agreement with drag coefficients from GATE gust probe data (Greenhut, 1981).…”

“…GATE values range from 0.03 to 0.09 N me2 (Greenhut, 1981). The wind speeds during GATE were correspondingly lower, with a typical value of 5 m s-i, whereas the profile in Figure 6 gives a value of 8.5 m s-l at 50 m. Using (12), the calculated drag coefficient at 50 m from the vertical profiles is CD(50) = 1.26 x 1O-3 .…”

“…The average durations of flight legs ranged from 4.5 to 5.1 min with small standard deviations, except for the 91 m level in EPOCS I where longer flight legs were made necessary by the convergence boxes. Uniformity of the flight leg intervals eliminates the problem of the dependence of fluxes and variances on leg duration although it has been shown by Reinking (1980a) and Greenhut (1981) using GATE data that this dependence is weak. The raw data were digitally recorded aboard the aircraft at 80 samples/set with an 11.5 Hz low-pass filter applied to reduce aircraft noise.…”

Similarity functions in the tropical marine boundary layer

“…Previous studies (Wipperman, 1972;Clarke and Hess, 1974;Brook and Allen, 1978) have shown that the similarity functions A, B, C, and D are relatively insensitive to thermal wind shear effects which, in any case, are small in the tropics (Greenhut, 1981).…”

“…Meteorological conditions during the NORPAX flights are described by Ramage et al (1979;1981). The near-tropical convergence zone (CZ) was always south of 11' N and north of the equator so that the ascents and descents at 11' N were north of the CZ and those at or near the equator were south of the CZ in all cases.…”