  Lagrangian studies of the local temperature mixing and heat transport in
turbulent Rayleigh-Benard convection are presented, based on three-dimensional
direct numerical simulations. Contrary to vertical pair distances, the temporal
growth of lateral pair distances agrees with the Richardson law, but yields a
smaller Richardson constant due to correlated pair motion in plumes. Our
results thus imply that Richardson dispersion is also found in anisotropic
turbulence. We find that extremely large vertical accelerations appear less
frequently than lateral ones and are not connected with rising or falling
thermal plumes. The height-dependent joint Lagrangian statistics of vertical
acceleration and local heat transfer allows us to identify a zone which is
dominated by thermal plume mixing.
