Employing a shadowgraph imaging technique for cloud microphysical measurements on a mountain observatory
- 1Institute of Geophysics, Faculty of Physics, University of Warsaw, Warsaw, Poland (moein.mohammadi@fuw.edu.pl)
- 2Max Planck Institute for Dynamics and Self-Organization, Goettingen, Germany
Microphysical properties of cloud droplets, such as droplet size distribution and droplet
number concentration have been studied after performing a series of field experiments in
summer 2019 at Umweltforschungsstation Schneefernerhaus (UFS), an environmental
research station located just below the peak of Zugspitze in the German Alps.
“VisiSize D30” manufactured by Oxford Laser Ltd. is a shadowgraph imaging instrument
utilized for the first time to measure the size and velocity of cloud droplets during this
campaign. It applies a method called “Particle/Droplet Image Analysis” (PDIA) which
involves illuminating the region of interest from behind with an infrared pulse laser whilst
collecting shadow images of droplets passing through the measurement volume with a
high-resolution camera. Droplets detected inside the depth of field are then measured
based on their shadow images, and size distribution is built by analyzing a series of
images. Furthermore, while turbulent orographic clouds passing our measurement site
at UFS observatory during the campaign, a Phase Doppler Interferometer (PDI) device,
manufactured by Artium Tech. Inc., was also constantly measuring droplets passing
through its probe volume.
Analysis of simultaneously collected data from the two instruments, and applying
modifications to the original algorithms illustrate a reasonable agreement regarding the
droplet sizing and velocimetry between VisiSize D30 and PDI, at least for diameters
larger than 13 μm. Moreover, discrepancies have been observed concerning the
droplet number concentration results, especially in smaller sizes. Further investigation
by applying appropriate filters on data has allowed the attribution of discrepancies to
the different optical performance of the sensors regarding small droplets, and to high
turbulent velocity fluctuations relative to the mean flow resulting in an uncertain estimate
of the volume of air passing through the PDI probe volume.
How to cite: Mohammadi, M., Nowak, J., Bertens, A., Molacek, J., Kumala, W., and Malinowski, S.: Employing a shadowgraph imaging technique for cloud microphysical measurements on a mountain observatory, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9965, https://doi.org/10.5194/egusphere-egu21-9965, 2021.
