The impact of subvisible cirrus clouds near the tropical tropopause on stratospheric water vapor

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 mu m. Particles with 6 mu 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.