Tropical cirrus cloud properties derived from TOGA/COARE airborne polarization lidar

During the January–February 1993 TOGA/COARE intensive field program, airborne polarization lidar data were collected from cirrus clouds generated by thunderstorm anvils over the western Pacific Ocean. The cirrus cloud tops typically extended to near the 16.85 km average tropical tropopause. Average lidar linear depolarization ratios δ show a consistent increase with height, with the maximum δ ≈ 0.52 values found at the tropopause. This finding can be interpreted in terms of a fundamental dependence of ice crystal habit or axis ratio on temperature. Although similar δ trends have been observed elsewhere, a complication in this case is that the cirrus may have been affected by the Pinatubo volcanic aerosol, which midlatitude research indicates could alter ice crystal shapes.