PROM-IMPRINT (2019-2022)
Description
Many aspects of precipitation formation remain poorly understood, leading to significant uncertainties in the prediction of clouds and precipitation. In particular, microphysical processes, describing the nucleation of cloud particles and their subsequent growth into precipitation, are still not fully understood. Since approximately 63% of global precipitation originates from the ice phase, improving our understanding of ice microphysics is essential for enhancing precipitation forecasts.
A key region in this context is the dendritic growth layer (DGL), located at temperatures between −20 and −10°C, which plays a crucial role in ice particle growth and precipitation formation. Previous studies have identified increases in both particle size and number concentration within the DGL, associated with depositional growth, aggregation, and secondary ice production processes. This project focused on investigating these ice microphysical processes by combining polarimetric and multi-frequency Doppler cloud radar observations with Monte Carlo Lagrangian particle modeling.
In von Terzi et al. 2023, we presented a statistical analysis of a three-month dataset from polarimetric and multi-frequency Doppler radar observations. This unique combination allows for a detailed examination of ice particle evolution: polarimetric measurements serve as indicators of depositional growth and possible secondary ice processes, while the multi-frequency radar data provide insights into particle size changes due to aggregation and riming. The analysis revealed a notable increase in aggregate size near −15°C, with the mean size of aggregates correlating with an updraft peaking at approximately 0.1 m/s at −14°C. The data also suggest the growth of plate-like ice crystals at this temperature.
Interestingly, the analysis showed that aggregation increases in the DGL concurrently with an increase in ice particle number concentration. Since aggregation typically reduces the number of particles, this points to the presence of a secondary source of ice particles. Several mechanisms could explain this observation:
- Secondary ice production, such as ice–ice collisional fragmentation;
- Sedimentation of small ice particles from above, colder layers into the DGL;
- Local activation of ice nucleating particles (INPs), possibly enhanced by the observed updraft, which could increase supersaturation with respect to ice around −15°C and lead to the nucleation and growth of new plate-like crystals.
Radar observations do not observe microphysical processes, such as nucleation directly, making it difficult to pinpoint the origin of the observed polarimetric signatures and potential increase in ice particle concentration.
To further investigate these processes, we used the Monte Carlo Lagrangian particle model McSnow (Welss et al. 2024,Brdar and Seifert 2017), constrained by our observational dataset. Simulations showed that the ice particles responsible for the observed polarimetric features and increase in number concentration must be nucleated locally near −15°C, suggesting that sedimentation alone cannot account for the observed signatures.
Furthermore, McSnow simulations indicated that neither collisional fragmentation nor INP activation alone can fully reproduce the observed radar signals. However, a combination of both processes could potentially explain the findings. This highlights the value of integrating radar observations with detailed modeling to advance our understanding of cloud microphysics.
Nevertheless, further laboratory studies are necessary to better constrain and validate the processes occurring in the DGL. The findings of this project are documented in my doctoral dissertation and in von Terzi et al. 2023.
