On the low‐level wind structure in the Atlantic trade

From ship‐borne radiosonde and radar wind observations during the Atlantic Trade Wind Experiment 1969 (ATEX) the relation between actual and geostrophic wind in the subcloud layer is determined. The frictional force and the divergence of vertical transport of momentum between the sea surface and 500‐m height are computed by the ageostrophic method. Along the surface wind direction, balance between pressure gradient and frictional force is closely established. Perpendicular to the surface wind the expected balance of pressure gradient and Coriolis force is not obtained but a substantial frictional force results from this component. The boundary condition that the stress vanishes at the lowest wind maximum leads to an underestimation of surface stress by about 35 per cent. It is suggested that convective transport of momentum must take place in the subcloud layer.