MEASUREMENTS OF THE HEAT AND MASS-TRANSFER PARAMETERS CHARACTERIZING CONICAL GRAUPEL GROWTH

Rigidly suspended conical graupel were grown in a wind tunnel, startin g from 1-mm hexagonal plates, with liquid water content varied from 0. 5 to 3.0 g m-3, velocity from 1.1 to 3.0 m s-1, ambient temperature f rom -4.4 to -20.9-degrees-C, cloud droplet median volume radius from 1 2 to 21 mum, and ambient pressure from 100 to 60 kPa. Growth condition s were chosen to simulate natural conditions in which conical graupel grow and serve as embryos for hail. Final graupel diameters ranged fro m 1.5 to 6 mm, with Reynolds numbers between 300 and 1500. Measurement s of the mass, volume, growth height, geometric shape, and surface tem perature with time were used to calculate the Nusselt and Sherwood num bers (representing the convective heat and mass transfers), bulk colle ction efficiency, and accretion density. The bulk collection efficienc y and Nusselt number were parameterized in terms of the Stokes paramet er and Reynolds number, respectively. The density and cone angle were parameterized in terms of the relative graupel-airstream velocity, the cloud-droplet median volume radius, and the surface temperature. The surface temperatures were measured remotely with an infrared radiomete r to within +/-0.2-degrees-C and are the first ever of growing graupel . The bulk collection efficiency was found to be 25% lower than that f or ideal smooth spheres, while the Nusselt numbers were approximately 50% higher than those of smooth cones. The enhanced heat convection an d mass deposition or sublimation is attributed to the roughness of the ice surface. The parameterizations of bulk collection efficiency, Nus selt number, density, cone angle, and geometric shape obtained represe nt solutions to the heat and mass transfer equations for the laborator y-grown conical graupel and can be used to improve graupel growth calc ulations in cloud dynamical models.