In this paper we introduce a simulation method for modelling clear-air atmospheric turbulence effects for long horizontal stratospheric paths. Divergence angles of several hundred microradians in combination with link distances up to 800 km require to adapt the appropriate resolution of the transverse optical field along the path. For this purpose, we implemented a propagation method in Cartesian coordinates. We choose two reference scenarios for high-altitude platform crosslinks and discuss the influence of simulation parameters to the derived results. Finally a method for computation of temporal IM/DD-time signals form simulated intensity matrices is presented.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.