The 27–28 October 1986 FIRE IFO Cirrus Case Study: Cloud Microstructure

Abstract

The temperature and windfield structure and hydrometeor composition of cirrus clouds sampled by the NCAR King Air and Sabreliner aircraft on 28 October 1986 near Madison, Wisconsin are described as part of a case study that examines cirrus cloud radiative and microphysical properties. Two cloud layers were sampled from top to base. The upper layer was found at altitudes between 8.5 and 11.5 km and the lower between 6.0 and 8.5 km. Vertical velocities calculated from the increase in ice mass flux with height were typical of synoptic scale lifting. Stronger vertical velocities were measured in convective cells at the top of the lower layer. The total ice particle concentration was dominated by particles <200 ?m. Mean particle size and ice water content increased with decreasing altitude. The largest particles, exceeding 1000 ?m in the upper layer and 1500 ?m in the lower layer, probably resulted from aggregation, even at cold temperatures. Cloud emissivity and optical depth were calculated from the ice particle size spectra. The distribution of ice mass was narrow at cloud top and broadened with decreasing altitude. At the highest levels of the upper cloud, half the mass was in particles <150 ?m. In this region, we underestimate the mass by a significant fraction presumably contained in particles too small to detect. In the lower levels, half the mass was in particles <200?400 ?m. In the cloud sampled by the King Air, half the mass was in particles <400?600 ?m. Up to 10% of the mass in the higher cloud and up to 30% in the lower cloud was contained in particles >500 ?m. We relate the microstructure of a shallow liquid water layer associated with an altocumulus to lidar observations. Thirteen separate episodes of liquid water were sampled at about ?30°C. Mean droplet dimensions were <9 ?m, and the liquid water contents were low. Virtually no ice particles were detected within and below the layer. We surmised that under such conditions these liquid water clouds remained colloidally stable. Kelvin-Helmholz waves may have produced the undulations observed at cloud top.