Turbulent diffusion from a patchy surface into the boundary layer

Turbulent diffusion from a small surface source into the boundary layer is studied by means of a wind-tunnel experiment. On the tunnel floor, a strip-shaped heater was placed perpendicularly to the mean flow. We regarded the heat as a passive scalar, and measured the air temperature in the boundary layer over the tunnel floor. The passive scalar, advected on the mean flow, diffuses in the floor-normal direction. For the diffusion of the passive scalar throughout the boundary layer, we find a characteristic length scale in the mean-flow direction. This length scale, which is always twice the boundary-layer thickness regardless of the other experimental parameters, would be useful when we consider the atmospheric boundary layer over a patchy surface.