Structure of offshore flow

The horizontal and vertical structure of the mean flow and turbulent fluxes are examined using aircraft observations taken near a barrier island on th e east coast of the United States during offshore flow periods. The spatial structure is strongly influenced by the surface roughness and surface temp erature discontinuities at the coast. With offshore flow of warm air over c ool water, the sea surface momentum flux is large near the coast and decrea ses rapidly with increasing offshore distance or travel time. The decrease is attributed to advection and decay of turbulence from land. The rate of d ecrease is dependent on the characteristic timescale of the eddies in the u pstream land-based boundary layer that are advected over the ocean. As a co nsequence, the air-sea momentum exchange near the coast is influenced by up stream conditions and similarity theory is not adequate to predict the flux . The vertical structure reveals an elevated layer of downward momentum flux and turbulence energy maxima over the ocean. This increase in the momentum flux with height contributes to acceleration of the low-level mean wind. In the momentum budget, the vertical advection term, vertical flux divergence term, and the horizontal pressure gradient term are all of comparable magn itude and all act to balance large horizontal advection. An interpolation t echnique is applied to the aircraft data to develop fetch-height cross sect ions of the mean flow and momentum flux that are suitable for future verifi cation of numerical models.