Microphysical Characteristics of Three Anvils Sampled during the Central Equatorial Pacific Experiment

Abstract

In situ microphysical measurements of three anvils were made 17 March, 1 April, and 4 April 1993 during the Central Equatorial Pacific Experiment for several constant altitude penetrations, in the same direction or opposite the ambient wind, from 7 to 14 km. Ice water contents (IWCs), derived from an optical array probe, increased downward in the anvil; concentrations also increased, but not as quickly. Median mass dimensions, defined as the ice crystal maximum dimension below which half of the mass is contained, increased from averages of less than 0.1 mm near the tops to averages of about 0.5 mm near the bases. Substantial variations in cloud parameters occurred in the horizontal direction as well, with larger IWCs and concentrations closer to the convective core and its remnants. Small crystals were measured and categorized according to their shapes with the Video Ice Particle Sampler, which detects particles with dimensions greater than 5 µm. For small IWCs, especially prevalent at cloud tops, small crystals make substantial, but variable, contributions to the total IWC and cross-sectional area Ac. Since Ac, closely related to cloud radiative properties, is at least one order of magnitude larger near the base of the cloud than near the top, it is suggested that larger particles near the bottom of thick anvils or near intense convective cores may be as important in determining radiative properties as the smaller crystals near the top. A wide variety of particle habits was seen. Many smaller crystals had unidentifiable circular images that differed from midlatitude cirrus, where columns dominate the small crystal population. Large crystals observed with the optical probes were typically complex and compact spatial crystals or aggregates in high IWC regions; this contrasts with the bullet rosettes commonly found in midlatitude cirrus produced by widespread ascent.