AbstractThis study examines the influence of weak horizontal advections of momentum on a well‐mixed planetary boundary layer. In the simple flow system examined, the direction and magnitude of flow modifications depend on the Rossby number, the nondimensional surface drag coefficient and the orientation of the geostrophic wind gradient field. It is found that advective modification of the boundary layer vertical motion field involves dependencies on the square of the local vorticity and dependencies on vorticity gradients. The mixed‐layer assumptions are relaxed for a special flow case to show that these results do not depend crucially on the mixed‐layer cond