Formation of large ('100µm) ice crystals near the tropical tropopause

Recent high-altitude aircraft measurements with in situ imaging instruments indicated the presence of rela- tively large ('100µm length), thin (aspect ratios of '6:1 or larger) hexagonal plate ice crystals near the tropical tropopause in very low concentrations (<0.01 L 1 ). These crystals were not produced by deep convection or aggrega- tion. We use simple growth-sedimentation calculations as well as detailed cloud simulations to evaluate the conditions required to grow the large crystals. Uncertainties in crys- tal aspect ratio leave a range of possibilities, which could be constrained by knowledge of the water vapor concentra- tion in the air where the crystal growth occurred. Unfor- tunately, water vapor measurements made in the cloud for- mation region near the tropopause with different instruments ranged from <2 ppmv to '3.5 ppmv. The higher water va- por concentrations correspond to very large ice supersatura- tions (relative humidities with respect to ice of about 200%). If the aspect ratios of the hexagonal plate crystals are as small as the image analysis suggests (6:1, see companion pa- per (Lawson et al., 2008)) then growth of the large crystals before they sediment out of the supersaturated layer would only be possible if the water vapor concentration were on the high end of the range indicated by the different measure- ments (>3 ppmv). On the other hand, if the crystal aspect ratios are quite a bit larger ('10:1), then H2O concentra- tions toward the low end of the measurement range ('2- 2.5 ppmv) would suffice to grow the large crystals. Gravity- wave driven temperature and vertical wind perturbations only slightly modify the H2O concentrations needed to grow the crystals. We find that it would not be possible to grow the