Pfeifer et al. (2025) Efficient ice multiplication from freezing raindrop fragmentation

Identification

• Journal: Communications Earth & Environment
• Year: 2025
• Date: 2025-11-21
• Authors: Nils Pfeifer, Bernd Mom, Dmitri Moisseev, Susan M. Hartmann, Julian Meusel, Corinna Hoose, Maximilian Maahn
• DOI: 10.1038/s43247-025-02953-3

Research Groups

• Leipzig Institute for Meteorology (LIM), Leipzig University, Leipzig, Germany
• Institute for Atmospheric and Earth System Research/Physics, University of Helsinki, Helsinki, Finland
• Finnish Meteorological Institute, Helsinki, Finland
• Department of Atmospheric Microphysics, Leibniz Institute for Tropospheric Research, Leipzig, Germany
• Institute of Meteorology and Climate Research Troposphere Research, Karlsruhe Institute of Technology, Karlsruhe, Germany

Short Summary

This study quantifies secondary ice production (SIP) from freezing raindrop fragmentation (DFF) during a refreezing rain event using combined in situ and remote sensing observations, estimating 1.2 to 6.1 secondary ice crystals produced per drop, with drops of 5.0 x 10^-4 m to 1.0 x 10^-3 m diameter being most prone to breakup.

Objective

• To quantify the effectiveness of secondary ice production (SIP) through drop fragmentation upon freezing (DFF) during a refreezing rain event, specifically by estimating the number of secondary ice crystals produced per freezing drop and identifying the characteristics of fragmenting drops.

Study Configuration

• Spatial Scale: Ground-based measurements at Hyytiälä, Finland (61.8°N, 24.29°E, 147 m above mean sea level). The Video In Situ Snowfall Sensor (VISSS) observed a volume of 4.71 x 10^-2 m x 4.71 x 10^-2 m x 5.89 x 10^-2 m. Radar observations covered a range of 102 m to 996 m with a resolution of 25.5 m.
• Temporal Scale: A specific refreezing rain event on 16 February 2024, with four distinct phases of precipitation occurring between approximately 14:00 UTC and 17:00 UTC. Data collection for January and February 2024.

Methodology and Data

• Models used: None explicitly used for analysis; observational classification methods applied.
• Data sources:
  • Ground-based in situ measurements: Video In Situ Snowfall Sensor (VISSS) (third generation), capturing videos at 220 Hz with a resolution of 4.6 x 10^-5 m per pixel.
  • Remote sensing: 94 GHz dual-polarization frequency-modulated continuous-wave Doppler cloud radar, providing spectral moments, linear depolarization ratio (LDR), and dual-polarization Doppler spectra.
  • Atmospheric profiles: GRAW DFM-17 radiosondes for temperature and humidity profiles.
  • Surface meteorological data: Local weather station providing measurements of temperature, humidity, wind, and pressure.

Main Results

• Drop fragmentation upon freezing (DFF) was identified as the primary secondary ice production (SIP) mechanism during the refreezing rain event.
• The study estimated that between 1.2 and 6.1 secondary ice crystals are produced per freezing drop. The lower limit (1.2) is based on classified particles larger than 4.1 x 10^-4 m, while the upper limit (6.1) includes smaller particles (less than 4.1 x 10^-4 m) assumed to be produced by DFF.
• Raindrops with diameters between 5.0 x 10^-4 m and 1.0 x 10^-3 m were found to be particularly susceptible to breakup, with a maximum breakup fraction of approximately 30% around 5.0 x 10^-4 m.
• Observed deformation types included spicules and bulges (characteristic of bubble bursting) and hemispheres (resulting from complete breakup), with hemispheres being the most prevalent.
• Significant SIP activity was concentrated within a transient period of approximately 1.5 hours (Phase III of the event).
• The process was observed to be active at relatively high temperatures (close to 273.15 K, above 270.15 K), suggesting its importance near the melting layer.

Contributions

• Provides the first observational quantification of secondary ice production (SIP) by drop fragmentation upon freezing (DFF) during a refreezing rain event using a combination of in situ and remote sensing data.
• Offers a direct, observation-based estimate of the number of secondary ice crystals produced per freezing drop (1.2 to 6.1), addressing a key uncertainty in cloud microphysics.
• Identifies the specific size range of raindrops (5.0 x 10^-4 m to 1.0 x 10^-3 m diameter) most prone to fragmentation, providing crucial insights for improving atmospheric model parameterizations.
• Confirms the activity of DFF at relatively high temperatures (close to 273.15 K), highlighting its potential relevance for ice multiplication near the melting layer.
• Introduces a novel ground-based methodology for quantifying SIP, with potential for extension to other microphysical processes like rime-splintering.
• The consistency of results with previous airborne studies in different geographical locations validates the methodology and suggests broader applicability of the findings.

Funding

• Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) for the “Evaluating Microphysical Pathways Of midlatitude Snow formation” (Project 516261703).
• Deutsche Forschungsgemeinschaft (DFG) (Project 386137169).
• Horizon Europe program under Grant Agreement No. 101137680 via project CERTAINTY (Cloud-aERosol inTeractions & their impActs IN The earth sYstem).
• ATMO-ACCESS (ATMO-TNA-4–0000000049).
• CleanCloud (Clouds and climate transitioning to post-fossil aerosol regime), Horizon Europe grant agreement 101137639.
• Open Access Publishing Fund of Leipzig University.

Citation

@article{Pfeifer2025Efficient,
  author = {Pfeifer, Nils and Mom, Bernd and Moisseev, Dmitri and Hartmann, Susan M. and Meusel, Julian and Hoose, Corinna and Maahn, Maximilian},
  title = {Efficient ice multiplication from freezing raindrop fragmentation},
  journal = {Communications Earth & Environment},
  year = {2025},
  doi = {10.1038/s43247-025-02953-3},
  url = {https://doi.org/10.1038/s43247-025-02953-3}
}