  We study the clustering properties of advected, non-interacting,passive
scalar particles in a Burgers fluid with noise, a problem which maps to that of
passive sliding particles moving under gravity on a surface evolving through
the Kardar-Parisi-Zhang equation. Numerical simulations show that both the
density-density correlation function and the single-site mass distribution
scale with system size. The scaling functions diverge at small argument,
indicating strong clustering of particles. We analytically evaluate the scaling
functions for the two-point correlation and mass distribution of noninteracting
particles in thermal equilibrium in a random landscape, and find that the
results are remarkably similar to those for nonequilibrium advection.
