ALBERT

Description: During the "Atlantic Trade Wind Experiment (ATEX)" from Feb. 6th to 23rd, 1969, an intensive aerological programme and detailed measurements near the sea surface were carried out simultaneously from three ships in the Atlantic northeast trade. From these data the kinematics, the dynamics and the kinetic energy budget of the trade wind flow in the lowest 3 km are investigated in this paper. The vertical shear of the horizontal wind vector is small in the mixed layer (lowest 600 m); a weak maximum is indicated at 400 m. Below about 1OOO m the large-scale horizontal flow is always divergent. Consequently, a large-scale sinldng motion of 300 to 500 m per day is observed through that level; sinking motion is also present at the trade wind inversion (1400 m). A considerable weakening of the divergence is noted during a period when the intertropical convergence zone (ITCZ) overrides the southern portion of the ATEX triangle. The northward displacement of the ITCZ is accompanied with a low pressure system approaching from the middle latitudes. The thermal wind vector is constant below the inversion and directed almost opposite to the surface geostrophic wind vector. Consequently, the magnitude of the geostrophic wind decreases rapidly with height while its direction remains nearly constant. The difference between the actual and the geostrophic wind vanishes at about 1200 m. Convective motions contribute to the small-scale vertical momentum transport. At the wind maximum the convective momentum transport amounts to about 30% of the surface stress. In a layer extending 300 m above the wind maximum the momentum is transported downward against the vertical gradient of the mean motion. The kinetic energy of the mean motion generated by the large-scale pressure gradient within the lowest 1OOO m primarily serves to balance the loss of mean kinetic energy due to conversion into turbulent kinetic energy in the surface boundary layer (lowest 20 m). In order to provide a reasonable description of the turbulent kinetic energy budget below the inversion the effects of condensation and evaporation within the cloud layer have to be taken into account.