The Life Cycles of Ice Crystals Detrained From the Tops of Deep Convection

Extensive anvil cirrus clouds generated by deep convection have important impacts on the Earth's radiation budget and climate. We use growth-sedimentation trajectory calculations to investigate the life cycles of anvil ice crystals as they are advected downwind from their convective source. Temperature, water vapor, and wind fields from a cloud-resolving model simulation of an isolated cumulonimbus cloud are used to drive the calculations. Ice crystals are initialized in the main upper-level detrainment zone with a size distribution based on in situ measurements made in a convective core at about 12km. Advection, deposition growth, and sedimentation of thousands of sample ice crystals are tracked over about 2.5hr; neither aggregation of ice crystals nor radiative effects are included. Results support the importance of deposition growth and gravitational size sorting in the evolution of the anvil cirrus. Most ice crystals initialized with maximum dimensions larger than about 200m fall out of the anvil and sublimate in subsaturated air below within about 2hr. Few small ice crystals are present in the lower part of the mature anvil. Vapor deposition growth accounts for about 50% of the ice mass remaining after about 2hr. Ice crystals larger than about 50m in the mature anvil have grown substantially by deposition of vapor. This result is consistent with the predominance of bullet rosette habits observed in mature anvils. Variations in ice crystal fall speeds and growth rates associated with ice crystal habit assumptions have little impact on the ice crystal life cycles. Plain Language Summary Numerical simulations of anvil cirrus ice crystal life cycles demonstrate the importance of deposition growth and sedimentation. Ice crystals with maximum dimensions larger than about 200m fall out of anvil cirrus within a few hours. Most ice crystals persisting in the anvils grow by deposition of vapor, and ice crystal growth is important for maintenance of the anvil ice mass. These results are consistent with the predominance of bullet rosette habits observed in mature anvil cirrus.