A simple diffusion model of cross-shore mixing employing an inhomogeneous diffusivity is applied to near-surface particle dispersion within a high-resolution numerical model of the central California coastal ocean. The theoretical model employs a linear diffusivity, Ky = νy, where y is the cross-shore coordinate and ν a constant, as traditionally assumed for wall-layer flows. A realistic implementation of the Regional Ocean Modeling System (ROMS) is used to generate hundreds of thousands of three-dimensional, passive particle trajectories. Particles were released daily along the inner shelf for one model year, 2002, tracked for 16 days and treated statistically as an instantaneous point-source release. Application of the theoretical model to the observed moments of particle position yielded estimates for the offshore slope of the diffusivity, ν ∼ 3 – 4 cm s–1. Using this value, the cross-shore particle density was well described over the outer continental shelf and slope by an analytical solution to the theoretical model. Unlike wall turbulence, the implied mixing length was not equal to the distance from the coast, but was found to be equal to the local first mode internal Rossby radius of deformation. The successful theoretical model has many potential applications, such as quantifying the amount of offshore larval loss from the shelf and/or carbon loss from the coastal upwelling zone.