ARPS is a 3-D prognostic mesoscale atmospheric model based on non-hydrostatic, compressible dynamics in a terrain-following vertical co-ordinate. The model contains 2nd and 4th order advection options, and flux-corrected transport is applied for tracer concentrations. Leapfrog time-differencing is used with a time-split treatment of acoustic modes. Sub-grid turbulence is described with the 1.5 order TKE formulation. ARPS contains detailed parameterisations for radiation transfer, cloud microphysics, and surface-atmosphere interactions.