Abstract
Abstract. In situ observations of cloud properties in complex alpine terrain where research aircraft cannot sample are commonly conducted at mountain-top research stations and limited to single-point measurements. The HoloGondel platform overcomes this limitation by using a cable car to obtain vertical profiles of the microphysical and meteorological cloud parameters. The main component of the HoloGondel platform is the HOLographic Imager for Microscopic Objects (HOLIMO 3G), which uses digital in-line holography to image cloud particles. Based on two-dimensional images the microphysical cloud parameters for the size range from small cloud particles to large precipitation particles are obtained for the liquid and ice phase. The low traveling velocity of a cable car on the order of 10 m s−1 allows measurements with high spatial resolution; however, at the same time it leads to an unstable air speed towards the HoloGondel platform. Holographic cloud imagers, which have a sample volume that is independent of the air speed, are therefore well suited for measurements on a cable car. Example measurements of the vertical profiles observed in a liquid cloud and a mixed-phase cloud at the Eggishorn in the Swiss Alps in the winters 2015 and 2016 are presented. The HoloGondel platform reliably observes cloud droplets larger than 6.5 µm, partitions between cloud droplets and ice crystals for a size larger than 25 µm and obtains a statistically significantly size distribution for every 5 m in vertical ascent.
Highlights
The role of clouds remains a large source of uncertainty in climate and weather models (Flato et al, 2013) because our level of understanding of fundamental details of microphysical processes particular to mixed-phase clouds is low (Boucher et al, 2013)
Manual inspection by eye of cloud particles classified as cloud droplets by the HOLOSuite software has shown that HOLIMO 3G is capable of reliably observing cloud droplets starting at a size of 6.2 μm for the run at 13:51 UTC on 23 February 2016, when the cloud droplet number concentration (CDNC) is close to 400 cm−3
The HoloGondel platform has shown proof of concept that is can measure many vertical profiles of cloud microphysical parameters along with meteorological conditions unattended for days at a time
Summary
The role of clouds remains a large source of uncertainty in climate and weather models (Flato et al, 2013) because our level of understanding of fundamental details of microphysical processes particular to mixed-phase clouds is low (Boucher et al, 2013). Holography benefits from its well-defined sample volume, which is independent of particle size and air speed This is of particular importance for the HoloGondel platform, because the air speed through the instrument depends on the varying wind speed and the travel velocity of the cable car. It provides the spatial distribution of the particles in the sample volume on a millimeter scale (Beals et al, 2015). The HoloGondel platform can be mounted on cable cars at other locations This approach offers the opportunity to obtain a vast set of vertical cloud profiles at low operational and organizational costs.
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