Intercomparison of holographic imaging and single-particle forward light scattering in-situ measurements of liquid clouds in changing atmospheric conditions

Abstract

Abstract. Upon a new measurement technique, it is possible to sharpen the determination of microphysical properties of cloud droplets using high resolving power imaging. The shape, size, and position of each particle inside a well-defined, three-dimensional sample volume can be measured with holographic methods without assumptions of particle properties. In-situ cloud measurements were carried out at the Puijo station in Kuopio, Finland, focusing on intercomparisons between cloud droplet analysers over the two months on September–November 2020. The novel holographic imaging instrument (ICEMET) was adapted to measure microphysical properties of liquid clouds and these values were compared to parallel measurements of cloud droplet spectrometer (FM-120) and particle measurements using a twin-inlet system. When the intercomparison was carried out during isoaxial sampling, our results showed good agreement in terms of variability between the instruments with the averaged ratios between ICEMET and FM-120 were 0.6 ± 0.2, 1.0 ± 0.5, and 1.2 ± 0.2 for total number concentration (Nd) of droplets, liquid water content (LWC), and median volume diameter (MVD), respectively. This agreement during isoaxial sampling was also confirmed by mutual information and Pearson correlation coefficients. The ICEMET observed liquid water content (LWC) was more reliably than FM-120 (without swivel-head mount), which was verified by comparing the estimated LWC to measured values whereas the twin-inlet DMPS system and FM-120 observations of Nd showed good agreement both in variability and amplitude. Field data revealed that ICEMET can detect small cloud droplets down to 5 μm via geometrical magnification.

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“…Droplet number concentrations and size distributions were measured using the forward-scattering optical spectrometer (fog monitor) described by Spiegel et al (2012) (FM-120, Droplet Measurement Technologies Inc., USA) with an observation range of 30 bins from 2 to 50 µm. Additionally, the number concentration and size distributions of large droplets and ice particles were measured with the holographic imaging system (icing condition evaluation method, ICEMET) described by Kaikkonen et al (2020) with an observational range from 5 to 200 µm (Tiitta et al, 2022).…”

“…To assess the modelling closure for droplet microphysics, we compared model-based droplet size distributions to observations carried out by the FM-120 and the ICEMET instruments. A more detailed analysis of the sources of interinstrument variability related to differences in time and bin size resolution, observational range, and sampling conditions was presented in Tiitta et al (2022). Model-based size distributions for hydrometeors were obtained as horizontally averaged values for 1 h-long intervals.…”