| The heat and mass transfer of graupel growing in a simulated cloud environment has been investigated experimentally. Rigidly suspended graupel were grown in a wind tunnel airstream under liquid water contents from 0.5 to 3.0 g m{dollar}sp{lcub}-3{rcub}{dollar}, velocities from 1.1 to 3.0 m s{dollar}sp{lcub}-1{rcub}{dollar}, ambient temperatures from {dollar}-{dollar}4.4 to {dollar}-{dollar}20.9{dollar}spcirc{dollar}C, cloud droplet median volume radii from 12 to 21 {dollar}mu{dollar}m and ambient pressures from 100 to 60 kPa. Measurements of the mass, volume, growth height, geometric shape and surface temperature with time were used to calculate the bulk collision efficiency, Nusselt and Sherwood numbers and accretion density. The bulk collision efficiency and Nusselt number were parametrized in terms of the Stokes parameter and Reynolds number respectively. The density and cone angle were parametrized in terms of the relative graupel-air stream velocity, the cloud droplet median volume radius and the surface temperature. The surface temperature measurements were made remotely with an infrared radiometer to within {dollar}pm{dollar}0.2{dollar}spcirc{dollar}C, and represent the first measurements of the surface temperature of growing graupel.; The bulk collision efficiency was found to be approximately 30% lower than that for ideal spheres as calculated by Langmuir and Blodgett (1946). The Nusselt number was found to be approximately 50% higher than that for smooth cones. The enhanced heat convection and mass deposition or sublimation is attributed to the roughness of the ice surface. Densities ranged between 0.16 and 0.70 g cm{dollar}sp{lcub}-3{rcub}{dollar} and were between 10 and 30% lower than those predicted by Heymsfield and Pflaum (1983) for densities lower than 0.4 g cm{dollar}sp{lcub}-3{rcub}{dollar}.; The parametrizations of density, cone angle, Nusselt number and bulk collision efficiency represent complete solutions to the heat and mass transfer equations for graupel. Hence the graupel stage of the cold rain process can be fully and accurately incorporated into cloud dynamical models. Such a complete numerical description has not been previously reported. |
