Vertical Wind Tunnel Experiments and a Theoretical Study on the Microphysics of Melting Low-Density Graupel

Abstract

Vertical wind tunnel experiments were carried out to investigate the melting of low-density lump graupel while floating at their terminal velocities. The graupel characteristics such as maximum dimension, density, and axis ratio were 0.39 ± 0.06 cm, 0.41 ± 0.07 g cm−3, and 0.89 ± 0.06. The airstream of the wind tunnel was gradually heated simulating lapse rates between 4.5 and 3.21 K km−1. Each experimental run was performed at a constant relative humidity that was varied between 12% and 92% from one experiment to the other. From the image processing of video recordings, variations in minimum and maximum dimension, volume, aspect ratio, density, equivalent radius, and ice core radius were obtained. New parameterizations of the terminal velocity prior to melting and during melting were developed. It was found that mass and heat transfer in the dry stage is 2 times as high as that of liquid drops at the same Reynolds number. Based on the experimental results, a model was developed from which the external and internal convective enhancement factors during melting due to surface irregularities and internal motions inside the meltwater were derived using a Monte Carlo approach. The modeled total melting times and distances deviated by 10% from the experimental results. Sensitivity tests with the developed model revealed strong dependencies of the melting process on relative humidity, lapse rate, initial graupel density, and graupel size. In dependence on these parameters, the total melting distance varied between 600 and 1200 m for typical conditions of a falling graupel.