Wind tunnel investigations into the melting of snowflakes and hailstones

• Applicant: Privatdozent Dr. Miklós Szakáll
• Subject Area: Atmospheric Science
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 553699895

The goal of this project is to gain a comprehensive understanding of the melting process of hailstones and snowflakes of nature-like shapes. Due to climate change, extreme precipitation events are very likely to increase in rate and frequency in the next decades. In the initiation of such events the phase change from ice to liquid water plays a key role, even in the tropics. Hailstones that may reach sizes from 5 mm to more than 15 cm are the largest particles in the global precipitation system. Another important form of ice hydrometeors are snowflakes, which sizes, densities, and shapes show the highest variability as they develop by aggregation of various types of ice crystals. In the proposed project, experimental studies will be conducted at the Mainz vertical wind tunnel, a worldwide unique facility for investigating individual particles under simulated atmospheric conditions in a vertical air flow. The project is planned as a complementation to earlier investigations by other authors and by our own research group. In this project, we address the following two questions that remained still open: 1) In contrast to spherical ice particles as proxies for hailstones, nature-like hailstones have an irregular non-spherical shape. This has an impact especially on the fall speed that is lower for a nature-like hailstone. Thus, it is exposed to warm ambient air for longer time periods and would melt faster than a spherical one. Furthermore, the size distribution of droplets shed from the ice surface will be changed, which we have shown in a feasibility study. These effects have to be characterized to modify parameterizations for numerical models accordingly. 2) There is some evidence in previous studies that shedding of drops and/or breakup of ice particles may occur during the melting of snowflakes, too. The probability is higher for aggregates consisting of dendritic ice crystals. Important parameters are the relative humidity and the warming rate. The drop size spectrum from shedding impacts the development of cloud and raindrop sizes and may enhance the relative humidity, affecting both the microphysics of precipitation particles and the dynamics of clouds. The breakup of ice branches might be an important secondary ice process, either in a dry case, or after melting of the broken branches and subsequent lifting and refreezing. Thus, the data obtained will be invaluable for the accurate representation of the melting process in cloud models and for the enhancement of future projections of extreme weather events in a changing climate.

• DFG Programme: Research Grants