THE SOURCE AREA INFLUENCING A MEASUREMENT IN THE PLANETARY BOUNDARY-LAYER - THE FOOTPRINT AND THE DISTRIBUTION OF CONTACT DISTANCE

This paper considers the ground area which affects the properties of fluid parcels observed at a given spot in the Planetary Boundary Layer (PBL). We examine two source-are functions: the "footprint," giving the source area for a measurement of vertical flux; and the distribution of "contact distance", the distance since a particle observed aloft last made contact with the surface. We explain why the distribution of contact distance extends vastly farther upwind than the footprint, and suggest for the extent of the footprint the inequalities: [GRAPHICS] where U is the mean streamwise (x) velocity, h is the observation height, tau-L is the Lagrangian timescale, sigma-nu and sigma-w are the standard deviations of the cross-stream horizontal (y) and vertical (z) velocity fluctuations, and z is the Lagrangian Similarity prediction for the rate of rise of the centre of gravity of a puff released at ground. Simple analytical solutions for the contact-time and the footprint are derived, by treating the PBL as consisting of two sub-layers. The contact-time solutions agree very well with the predictions of a Lagrangian stochastic model, which we adopt in the absence of measurements as our best estimate of reality, but the footprint solution offers no improvement over the above inequality.