Properties of Tropical and Midlatitude Ice Cloud Particle Ensembles. Part I: Median Mass Diameters and Terminal Velocities

Abstract

This is Part I of a study that characterizes several bulk properties of ice particle populations sampled in midlatitude and tropical cirrus and deep stratiform ice clouds, for the purpose of developing an understanding of how particles evolve in ice clouds and to derive empirical and analytical relationships that describe microphysical properties for use in cloud and climate models. The effort focuses on describing the microphysical properties of ice cloud layers in the vertical. The size distribution data and particle imagery were obtained during Lagrangian spiral descents and balloon-borne ascents through cloud layers that formed in association with synoptic-scale lifting (midlatitude) and deep convection (Tropics). Temperatures ranged between ?20° and ?63°C for the midlatitude clouds and between 0° and ?50°C for the tropical clouds. Optical depths spanned the range 0.4?7 for the midlatitude clouds and 20?30 for the tropical clouds. This part of the study characterizes median mass diameter (Dm) and median fall velocities (Vm) for the more than 2000 ensembles or particle size distributions (PSDs) examined. The Dm and Vm increase downward from cloud top to base, with the smallest Dm and Vm values found in the coldest (midlatitude) clouds and the largest values found in the warmest (tropical) clouds. The range of sizes that dominate the ice water content, and the associated range of particle fall speeds, are characterized in terms of Dm and Vm. The Vm are represented in terms of Dm and the slopes (?) of gamma distributions fitted to the particle size distributions. The Vm values increase with Dm and decrease with ? in a predictable manner. The magnitudes of the changes in Vm that result from differing ambient pressures between 250 and 1000 hPa are quantified. The observations are generalized so that the results can be extended to different pressure levels and other particle size distributions. The coefficients ? and ? in the power-law relationship Vt = ?D? fitted to the individual spectra are found to be inversely related to Dm. Many earlier studies have derived these coefficients from measurements at the surface. The wide variability noted in these coefficients may partially be attributed to variations in the Dm values of the populations considered. The relationship of the ? and ? coefficients found for particle ensembles at the surface to those at the pressure levels of ice clouds are derived.