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

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, volume 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. SIGNIFICANCE STATEMENT: The accuracy of weather forecast models to predict precipitation depends strongly on the representation of cloud processes in those models. Heavy rain events are mostly the result of melting ice particles. Furthermore, melting affects the storm characteristics and its destructive potential. In this study, we investigated the melting of low-density graupel, which constitute an important class of precipitation particles. Our experiments in a vertical wind tunnel under close to atmospheric conditions indicated an increased melting rate of graupel due to surface irregularities. We provided experimentally derived coefficients that supplement present theoretical concepts describing melting in forecast models. In this way, our study contributes to the improvement of current weather forecasts.