A Test of Ice Self-Collection Kernels Using Aircraft Data

Abstract

Abstract Aircraft observations from the Cirrus Regional Study of Tropical Anvils and Cirrus Layers (CRYSTAL)–Florida Area Cirrus Experiment (FACE) campaign obtained in the anvil of a large convective storm from 26 July 2002 are presented. During this flight a Lagrangian spiral descent was made, allowing the evolution of the ice particle size distribution to be followed. Relative humidities during ∼1 km (from −11° to −3°C) of the descent were within 4% of ice saturation. It was assumed that the ice particle size distribution was evolving through the process of aggregation alone. Three idealized ice–ice collection kernels were used in a model of ice aggregation and compared to the observed ice particle size distribution evolution: a geometric sweep-out kernel, a Golovin (sum of particle masses) kernel, and a modified-Golovin kernel (sum of particle masses raised to a power). The Golovin kernel performed worst. The sweep-out kernel produced good agreement with the observations when a constant aggregation efficiency of 0.09 was used. The modified-Golovin kernel performed the best and implied that the aggregation efficiency of sub-300-μm particles was greater than unity when compared with a geometric sweep-out kernel.