Larger concentrations tend to move slower and live longer than smaller ones.
The transition between the low and large diffusion coefficients is explained with a model and simulations of the motions of test particles, subject to random displacements on both the granular and supergranular scales, simultaneously. In this model, the supergranular flow acts as a negligible drift on short timescale, but dominates the granular diffusion on longer timescales. We also investigate the possibility that concentrations are temporarily confined, as if they were caught in supergranular vertices, that form short-lived, relatively stable environments. The best agreement of model and data is found for step lengths of 0.5 and 8.5 Mm, associated evolution times of 14 minutes and 24 hr, and a confinement time of no more than a few hours. One possible explanation for the difference is a bias in our measurements to the longest lived, and therefore slower concentrations in our data sets.