The Lifecycles of Anvil Cirrus Ice Crystals

Anvil cirrus generated by deep convection covers large tropical areas and has important impacts on the earth's radiation budget and climate. Images of ice crystals from recent high-altitude aircraft flights show a predominance of bullet rosettes in anvil cirrus with ages more than about an hour since detrainment from deep convection, suggesting either fresh nucleation of ice crystals in the anvil followed by deposition growth or growth of small crystals detrained from the convective updrafts. We use growth-sedimentation trajectory calculations to investigate the lifecycles of anvil ice crystals as they are advected downwind from the 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. Hundreds of thousands of simulated ice particles (SIPs) are tracked to represent the anvil evolution. The initial size distribution of SIPs is based on measurements made during an aircraft transit through a convective core at 13 km. Gravitational size sorting rapidly segregates the ice crystals, with smaller (< 200 μm) crystals remaining in the upper part of the anvil and larger crystals accumulating at cloud base. Most of the SIPs initialized with maximum dimensions larger than about 200 μm fall into subsaturated air below the anvil and sublimate within about two hours. Consistent with the predominance of bullet rosettes in aged anvil cirrus, we find that vapor-deposition growth increases the maximum dimensions of most surviving ice crystal by factors of 2-3, and growth of ice crystals is largely responsible for maintaining the aging anvil ice mass.