Researcher: Paul Ockenfuß (Ludwig-Maximilians-University of Munich (LMU))
Place: University of Helsinki, Finland
Time Period: 21.09.2025 - 19.10.2025
Funded by PROM Network Funds

From 21 September to 19 October, 2025, Paul Ockenfuß visited the group of Prof. Dr. Dmitri Moisseev at the University of Helsinki. The goal was to reactivate a method to derive the rime mass fraction of snowflakes from a combination of video in situ observations of snowflakes using the PIP instrument and measurements from a mass gauge next to the video imager. This method was presented in Moisseev, Lerber, and Tiira 2017 and forms the basis of the rime mass-mean Doppler velocity relation presented by Kneifel and Moisseev 2020. Since the relation by Kneifel and Moisseev 2020 is based on data from only two winter periods between 2013 to 2015, heavy near-surface riming cases were scarce. A longer observation timeseries would improve statistical significance in the heavy-riming region.

Because the PIP at the Hyytiälä field station is out of operation and will likely no longer be maintained in the future, it was decided that a reactivation of the existing data processing pipeline is not ideal. Instead, we worked towards a new data processing pipeline, which uses observations from the VISSS. Similar to the PIP, the VISSS is a video in situ snowflake imager. The most recent version VISSS3 is currently installed at the Hyytiälä field site and subject to active research and further development. Compared to the PIP, the VISSS provides higher resolution images. Furthermore, its combination of two cameras allows to observe particles from two orthogonal perspectives and precisely constrains the measurement volume.

During the research stay, the riming retrieval by Ockenfuß et al. 2025 was applied to W-band cloud radar observations during the winter months between January 2024 and February 2025, in order to detect (near) surface riming cases. Prof. Dr. Maximilian Maahn (University of Leipzig) provided VISSS level 2 data for these cases. In addition, data from the Pluuvio rain gauge located next to the VISSS was acquired from the Cloudnet database. By comparison of the gauge derived mass flux with the volume flux, the ensemble mean snow density was derived. For the calculation of the volume flux, a gamma function particle size distribution is assumed. The method of Brawn and Upton 2008 was implemented for the VISSS, which allows to estimate the most likely gamma parameters and moments from data. It was found that for the selected example cases, the derived snow density is often lower than predicted by the unrimed mass-size relation from Moisseev, Lerber, and Tiira 2017. This will subsequently lead to unphysical, negative rime mass fractions in the retrieval. In the next steps, we will focus on the underlying assumptions in the calculations. Especially, the aspect ratio and unrimed mass size relation deserve greater investigation. With the now implemented data processing, Fig. 3 by Tiira et al. 2016 can be recreated with VISSS data. Subsequently, Fig. 1 by Moisseev, Lerber, and Tiira 2017, showing the distribution of snow densitiy with respect to size, can be recreated as well. This way, an adapted, unrimed mass-size relation can be derived, which will form the basis to apply the rime mass fraction retrieval to the new instrument and time period.

References:

• Brawn, D., and U. Graham, 2008: Estimation of an atmospheric gamma drop size distribution using disdrometer data. Atmospheric Research 87.1, 66–79, doi: 10.1016/j.atmosres.2007.07.006.
  
• Kneifel, S., and D. Moisseev, 2020: Long-Term Statistics of Riming in Nonconvective Clouds Derived from Ground-Based Doppler Cloud Radar Observations. Journal of the Atmospheric Sciences 77.10, 3495–3508, doi: 10.1175/JAS-D-20-0007.1, https://journals.ametsoc.org/view/journals/atsc/77/10/jasD200007.xml.
  
• Moisseev, D., A. von Lerber, and J. Tiira, 2017: Quantifying the effect of riming on snowfall using ground-based observations. Journal of Geophysical Research: Atmospheres 122.7, 4019–4037, doi: https://doi.org/10.1002/2016JD026272, https://agupubs.onlinelibrary.wiley.com/doi/abs/10.1002/2016JD026272.
• Ockenfuß, P., et al., 2025: Spatial and Temporal Scales of Riming Events in Nonconvective Clouds Derived From Long-Term Cloud Radar Observations in Germany. Journal of Geophysical Research: Atmospheres 130.4., doi: 10.1029/2024jd042180.
  
• Tiira, J., et al., 2016: Ensemble mean density and its connection to other microphysical properties of falling snow as observed in Southern Finland. Atmospheric Measurement Techniques 9.9, 4825–4841, doi: 10.5194/amt-9-4825-2016.