The Characteristics of Graupel Particles in Northeastern Colorado Cumulus Congestus Clouds

Abstract

The physical characteristics of graupel particles were investigated from in in situ collections of ice particles sampled in first-echo summertime cumulus congestus clouds in northeastern Colorado with the NCAR/NOAA sailplane operating during the National Hail Research Experiment. Ice particles were collected in vials containing silicone oil. Each particle was photographed at several different orientations and then melted to determine its equivalent diameter. This permitted the mass and axial dimensions to be determined and the density, axial ratios and terminal velocity to be estimated for each of 125 particles. The mass and terminal velocity of graupel particles were found to be considerably lower than those of equivalent diameter ice spheres. Best-fit equations to the mass-diameter and terminal velocity-diameter data were computed. Graupel densities were typically lower than 0.5 g cm?3 systematic differences were noted between the densities of lump and conical graupel. Calculations indicated that the results reported in this paper may not he applicable to thunderstorms with warm cloud-base temperatures. Measurements of the microstructure of the penetrated cumulus congestus clouds permitted several generalizations to he made on the growth of conical graupel and enabled previous theories of conical graupel formation to be examined. Conical graupel formation through riming of small single crystals appears to be the dominant mechanism operating in the clouds investigated.