A contrail studied by surface radiometers and lidar is simulated with use of a 2D cloud microphysical model to understand the evolution in the contrail meso- and microstructure along with its optical and radiative properties. The model inputs are based on sounding data, and constrained by the measured perturbations in the surface radiation field due to the passage of the contrail. Microphysically, after persisting for 0.5-h the contrail contained high (~1--10 cm-3) numbers of minute (~3 &mgr;m radius) ice particles, very unlike natural cirrus. The surface radiative fluxes computed under these conditions agree well with the observations. When analyzing the resultant cloud forcing at the surface and top of atmosphere, it is found (at least for this case) that contrail cirrus clouds may act to cool the surface and warm the upper troposphere. ¿ 1998 American Geophysical Union