Cloud physics research group

Modeling and observing secondary ice production

Clouds and precipitation are important elements of the climate system. They affect our daily weather and, in the longer term, the climate. Understanding how ice forms in clouds is key to accurate weather forecasting and climate modeling. Ice particles play an important role in the formation of surface precipitation and also affect the propagation of radiation from the Sun and the Earth.

Ice formation in clouds occurs over a wide temperature range (0°C to -40°C) during primary and secondary ice formation. In the majority of clouds, primary ice formation (PIP) is generated by ice-forming aerosol particles (INP) through heterogeneous nucleation. Observations show that the concentration of ice crystals often exceeds the concentration of available active ice nuclei. This suggests that ice crystals can be generated by secondary processes (SIP). In contrast to PIP, the range of possible processes for SIP is not yet fully understood. Although many review articles mention the potentially important role of SIP in precipitation formation, relatively little theoretical work has been done on the subject so far. Models used for research and operational forecasting purposes either do not take this process into account or use a parameterized formula that has been shown in recent years by laboratory measurements to be based on questionable assumptions.

Measuring the impact of secondary ice formation is complicated by several factors. For aircraft measurements, it is not easy to identify the processes that contribute to the growth of ice crystals, and for laboratory measurements it is impossible to reproduce the dynamic and microphysical environment of clouds. In this research, we mainly use numerical simulations to investigate the impact of SIP. The aim is to identify the dominant SIP mechanisms and estimate their effects under different environmental conditions, such as cloud type, temperature, liquid water content, primary ice formation, and different INP concentrations. In an international collaboration, laboratory experiments are planned to clarify how snow sublimation and snow-snow collisions affect secondary ice crystal formation.

In the first part of the project, we will further develop an existing microphysical scheme (University of Pecs and NCAR bin scheme), which is used for a variety of purposes, to model a wide range of processes leading to secondary ice formation. Sensitivity tests will be performed to determine which SIP may be dominant depending on the dynamic, thermodynamic, and microphysical characteristics of the cloud.

In a collaboration with Johannes Gutenberg University Mainz, we will investigate how fragmentation due to snow sublimation and snowflake-snowflake collisions affect secondary ice formation. Currently, only a qualitative description of these processes exists. Using laboratory measurements, we aim to quantify how environmental conditions affect secondary ice crystal formation. This quantification will allow the incorporation of the process into the numerical model. In addition, we intend to perform case studies by modeling clouds that have been intensively observed by aircraft measurements.

Compared to existing research, this project is a unique combination of numerical modeling and state-of-the-art laboratory observations:(i) it brings together experts in numerical modeling, data analysis and experimental physics, facilitating a broad investigation of SIP; (ii) we use a 3D model to investigate the interaction between the dynamical and microphysical properties of clouds and SIP processes;(iii) our collaboration with the University of Mainz will allow the study of a secondary ice formation process for which few results have been obtained so far; (iv) the integration of laboratory data and the application of real-world observations will ensure that our descriptions are based on real physical processes.

The complex methodology of this project is relatively rarely applied in the field of cloud science. Few research initiatives, both domestically and abroad, offer such a synergistic combination of modeling and experimental work. The aim of this research is to better understand how secondary ice formation affects precipitation processes and how it influences cloud lifetimes and optical properties.

Reference:

Kolorev, A. and Leisner, T., 2020: Review of experimental studies of secondary ice production. Atmospheric Chemistry and Physics, 20, pp. 11767–11797. https://doi.org/10.5194/acp-20-11767-2020