Ice Crystal Step Growth in a Laboratory Simulated Upper Tropical Troposphere

The presence of sub-visual cirrus clouds below the tropical tropopause, defined as those clouds having optical depths on the order of 0.01 with pressures down to 100 mb and temperatures down to -85 °C, is well established (Immler et al 2007, Jensen et al 1996). Less known are the details of ice crystal growth under these temperature and humidity conditions as found in and just below the tropical tropopause. Total uptake of water vapor on ice crystals has been characterized using the deposition coefficient, defined as the fraction of water vapor molecules, relative to the total number of water vapor molecules, incorporated into an ice particle (Magee 2006). The preferential deposition of water vapor on ice crystals at -60 °C and 250 mb, and at -70 °C and 150 mb is examined. Water vapor is deposited preferentially on some facets of ice crystals but not others, leading to layered growth on one facet and a zero deposition coefficient and zero mass growth on the others. A critical range of supersaturation conditions favor stepped growth, above which ice crystals grow in either the a-axis or c-axis direction and below which no ice crystal growth occurs in either direction. While previous studies have only observed steps growing in the basal plane, this study observed steps growing in the prism plane. Lower ice supersaturations favor diffuse steps, while higher ice supersaturations favor sharp steps, both of which lead to growth in the c-axis direction. Total growth in the a-axis direction is determined by the thickness of each new step and the rate at which new steps form. Images of ice crystals may therefore be used as a measure of saturation, to be compared with in-situ aircraft measurements of ice supersaturation at known temperatures. Additionally, the presence of elevated concentrations of nitric oxide (NO) leads to crystal growth, with preferential growth in the prism plane.