A General Approach for Deriving the Properties of Cirrus and Stratiform Ice Cloud Particles

Abstract

A new approach is described for calculating the mass (m) and terminal velocity (Vt) of ice particles from airborne and balloon-borne imaging probe data as well as its applications for remote sensing and modeling studies. Unlike past studies that derived these parameters from the maximum (projected) dimension (D) and habit alone, the ?two-parameter approach? uses D and the particle's projected cross-sectional area (A). Expressions were developed that relate the area ratio (Ar; the projected area of an ice particle normalized by the area of a circle with diameter D) to its effective density (?e) and to Vt. Habit-dependent, power-law relationships between ?e and Ar were developed using analytic representations of the geometry of various types of planar and spatial ice crystals. Relationships were also derived from new or reanalyzed data for single ice particles and aggregates observed in clouds and at the ground. The mass relationships were evaluated by comparing calculations to direct measurements of ice water content (IWC). The calculations were from Particle Measuring Systems (PMS) 2D-C and 2D-P probes of particle size distributions in ice cloud layers on 3 days during an Atmospheric Radiation Measurement (ARM) field campaign in Oklahoma; the direct measurements were from counterflow virtual impactor (CVI) observations in ice cloud layers during the field campaign. Agreement was generally to within 20%, whereas using previous mass?dimension relationship approaches usually produced larger differences. Comparison of ground-based measurements of radar reflectivity with calculations from collocated balloon-borne ice crystal measurements also showed that the new method accurately captured the vertical reflectivity structure. Improvements in the accuracy of the estimates from the earlier mass?dimension relationships were achieved by converting them to the new form. A new, more accurate mass?dimension relationship for spatial, cirrus-type crystals was deduced from the comparison. The relationship between Vt and Ar was derived from a combination of theory and observations. A new expression accounting for the drag coefficients of large aggregates was developed from observational data. Explicit relationships for calculating Vt as a function of D for aggregates with a variety of component crystals were developed.