We present aircraft observations of the response of the 3-dimensional turbulence structure of the stable atmospheric boundary layer to the interaction of the mesoscale flow and coastal topography. The measurements were made by the NCAR C-130 off the coast of California, around Cape Mendocino during the Coastal Waves 96 field program. Northwesterly winds bring the marine air-mass over cool, upwelling coastal waters, resulting in the formation of a stable internal boundary layer (IBL). Turbulence is largely confined to the IBL, and is shear driven. Upwind of the Cape and far offshore, turbulence within the IBL can be scaled using local similarity scaling. As the flow rounds the Cape, diverging and accelerating to form an expansion fan, this scaling breaks down. Bulk estimates of the surface fluxes derived from near-surface measurements of mean quantities differ significantly from direct eddy-correlation measurements. Modelling result are presented in a companion paper.