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Wind tunnel studies on the melting of ice hydrometeors: the effect of collision and turbulence on melting (HydroCOMET)

Subject Area Atmospheric Science
Term from 2017 to 2020
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 336453817
 
Final Report Year 2021

Final Report Abstract

In this project, we investigated the melting of hailstones and graupel. This is an important topic because heavy rain precipitating from deep convective or thunderstorm clouds of squall lines is mostly initiated via the ice phase through melting. Thus far, its understanding and quantitative descriptions are deficient. With the Mainz vertical wind tunnel, we have the facility to investigate atmospheric processes like melting under conditions that are close to that in real atmosphere. Melting ice particles were supported in the observation section of the wind tunnel with their terminal velocities while the ambient air was warmed at a desired rate. The ice particles in our experiments were solid ice spheres with high densities, which represent hailstones, and lump graupel generated from rime ice with low densities. Additionally, we used artificial hailstones and graupel that we produced from synthetic material by a 3D printing technology. This was a novel method for such experiments. We monitored and recorded the melting processes with a high-speed video camera, an infrared camera, and with an in-line holographic imaging system. Based on our results, we have some suggestions to improve numerical cloud simulations and radar retrievals. An important finding was that surface properties have a significant effect on the terminal velocity. Lobes on the surface of hailstones reduce the kinetic energy and the destructive potential on the ground. We provided a relationship to calculate terminal velocities of lobed hailstones for different sizes and densities. Its application in cloud resolving models will improve the microphysical description of hailstones, and thereby achieve a better prediction of the destructive potential of a hailstorm. For low-density graupel, we developed parameterizations for their mass, terminal velocity, Reynolds number, and drag coefficient before the onset of melting. An important parameterization describes the terminal velocity of the melting graupel as a function of the melt water fraction. All these descriptions are directly applicable in cloud microphysical models to investigate special cases with low-density graupel. In contrast, the critical melting temperature of graupel, which is dependent on the relative humidity, can be calculated by the theory for ice spheres in cloud models. From the temporal evolution of melting hailstones observed during the experiments, we defined five melting stages. They describe the changes in terminal velocity, shape, the transition from the rough solid ice surface to a smooth moving liquid surface, how the melt water is accumulated on the ice surface, and how shedding of droplets from the melt water occurs. A typical shedding event was that a filament of water was teared off and burst into 1 mm drops. In later melting stages, the filaments contained high water masses so that they burst into larger drops. An interesting observation was that these fell often back towards the hailstone, but before they hit the surface, they were blown up like a bag and burst into many small 100 µm drops. Another type of a shedding event happened when the bursting of a filament with high liquid water mass produced a large 3.5 mm drop. Before being carried upward it re-collided with the melt water attached to the hailstone surface and induced high intrinsic instability in it, which led to an explosion-like shedding event. The melting behavior of graupel is different from that of hailstones. We defined four melting stages that describe the transition from the lump graupel with surface irregularities over an ice particle with a smooth surface to a drop. Due to the loose structure of graupel, no drop shedding occurred. First, the melt water was soaked into the graupel and filled the voids of the rime ice; afterwards, it accumulated on the graupel surface. We showed that the heat transfer of a lump graupel was enhanced in comparison to an ice sphere; during the soaking stages due to surface irregularities, and during the fully soaked stages due to internal mixing of the melt water. An important result was that the density of the graupel has a strong effect on melting: increasing the density from 0.2 g/cm3 to 0.6 g/cm3 increases the melting distance by a factor of two. It is important to consider this parameter in cloud model simulations. Our activities attracted attention even in the public media. Dr. Alexander Theis participated in the broadcast “alle wetter!” of the Hessische Rundfunk, Germany: https://www.hr-fernsehen.de/sendungen-az/alle-wetter/sendungen/alle-wetter-vom-29092020,video-133206.html. Furthermore, the following newspaper articles reported about our research on melting hasilstones: “Moleküle zum Anfassen“ (Allgemeine Zeitung Mainz, 11.09.2017); "Die Wolkenforscher“ (Allgemeine Zeitung Mainz, 02.12.2017) and “Mikroskop in den Wolken - einzigartiger Windkanal in Mainz“ (dpa, 15.01.2019).

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