Molecular dynamics simulations of the ice Ih surface between 180 and 210 K showed that the dynamics of the water molecules in the top two to three bilayers is substantially faster than that of the molecules in the lower (bulk) bilayers. Within the simulation time of tens of nanoseconds. there is rapid exchange of molecules between these upper bilayers, but not between these and the lower bilayers. In-plane translation of the molecules in the top surface bilayer leads to rapid surface reconstruction, and a structure consisting primarily of water heptagons, hexagons, and pentagons replaces the crystal geometry. An Arrhenius analysis of the diffusion of molecules in the top bilayer yields a barrier for in-plane diffusion E-0 = 23.2 +/- 2.9 kJ mol(-1) and a preexponential factor D-0 = 0.77 cm(2) s(-1) (ln(D-0) = -0.26 +/- 1.99). The activation barrier, which is the same as the experimentally measured value, is similar to the energy required to break a hydrogen bond and supports a diffusion mechanism where water molecules move by repeated breaking and formation of hydrogen bonds. The similarity between the simulated and experimental barrier heights supports an in-plane diffusion mechanism where molecules must move to the upper bilayers before diffusion can occur.