Measurements of the Heat and Mass Transfer Parameters Characterizing Conical Graupel Growth

Abstract

Rigidly suspended conical graupel were grown in a wind tunnel, starting from 1-mm hexagonal plates, with liquid water content varied from 0.5 to 3.0 g m?1, velocity from 1.1 to 3.0 m s?1, ambient temperature from ?4.4 to ?20.9°C, cloud droplet median volume radius from 12 to 21 ?m, and ambient pressure from 100 to 60 kPa. Growth conditions were chosen to simulate natural conditions in which conical graupel grow and serve as embryos for hail. Final graupel diameters ranged from 1.5 to 6 mm, with Reynolds numbers between 300 and 1500. Measurements of the mass, volume, growth height, geometric shape, and surface temperature with time were used to calculate the Nusselt and Sherwood numbers (representing the convective heat and mass transfers), bulk collection efficiency, and accretion density. The bulk collection efficiency and Nusselt number were parameterized in terms of the Stokes parameter 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 radiometer to within ±0.2°C and are the first ever of growing graupel. The bulk collection efficiency was found to be 25% lower than that for ideal smooth spheres, while the Nusselt numbers were approximately 50% higher than those of smooth cones. The enhanced heat convection and mass deposition or sublimation is attributed to the roughness of the ice surface. The parameterizations of bulk collection efficiency, Nusselt number, density, cone angle, and geometric shape obtained represent solutions to the heat and mass transfer equations for the laboratory-grown conical graupel and can be used to improve graupel growth calculations in cloud dynamical models.