The radiative impact of subvisible cirrus ice clouds at and just below the tropical tropopause has been studied using a zonally averaged interactive chemistry-radiation-dynamics model. Model runs have been performed with and without the inclusion of the radiative heating of these thin ice clouds, and with and without sedimentation. Near-infrared optical depths of 0.005--0.08 were computed for assumed log-normal size distributions of spherical particles having mode radii of 2--10 &mgr;m. Particles with 6 &mgr;m mode radii have computed scattering ratios of 3--15 at 603 nm, in good agreement with lidar observations. The increased radiative heating of these clouds, 0.1--0.2 K/day, results in temperature increases of 1--2 K and vertical velocity increases of 0.02--0.04 mm/s. As a consequence of the warmer tropopause, lower stratosphere water vapor increases by as much as 1 ppmv. The dehydration resulting from sedimentation was found to be a much smaller effect. ¿ 1998 American Geophysical Union