Abstract
Abstract. Conventional techniques to measure boundary layer clouds such as research aircraft are unable to sample in orographically diverse or densely populated areas. In this paper, we present a newly developed measurement platform on a tethered balloon system (HoloBalloon) to measure in situ vertical profiles of microphysical and meteorological cloud properties up to 1 km above ground. The main component of the HoloBalloon platform is a holographic imager, which uses digital in-line holography to image an ensemble of cloud particles in the size range from small cloud droplets to precipitation-sized particles in a three-dimensional volume. Based on a set of two-dimensional images, information about the phase-resolved particle size distribution, shape and spatial distribution can be obtained. The velocity-independent sample volume makes holographic imagers particularly well suited for measurements on a balloon. The unique combination of holography and balloon-borne measurements allows for observations with high spatial resolution, covering cloud structures from the kilometer down to the millimeter scale. The potential of the measurement technique in studying boundary layer clouds is demonstrated on the basis of a case study. We present observations of a supercooled low stratus cloud during a Bise situation over the Swiss Plateau in February 2018. In situ microphysical profiles up to 700 m altitude above the ground were performed at temperatures down to −8 ∘C and wind speeds up to 15 m s−1. We were able to capture unique microphysical signatures in stratus clouds, in the form of inhomogeneities in the cloud droplet number concentration and in cloud droplet size, from the kilometer down to the meter scale.
Highlights
Boundary layer clouds play a key role in regulating the Earth’s climate and controlling its weather systems and are important for many aspects of our daily life
HOLographic Imager for Microscopic Objects (HOLIMO) 3B works on the principle of digital in-line holography (Fig. 2), which consists of a two-step process requiring a coherent light source and a digital camera
A database of stratus clouds (Miles et al, 2000) showed that the cloud droplet number concentration (CDNC) in continental clouds was more variable with height than in marine clouds where CDNC was determined near cloud base
Summary
Boundary layer clouds play a key role in regulating the Earth’s climate and controlling its weather systems and are important for many aspects of our daily life. Digital holographic cloud imagers have been used in previous field campaigns on ground-based (e.g., Thompson, 1974; Kozikowska et al, 1984; Borrmann et al, 1993; Raupach et al, 2006; Henneberger et al, 2013; Schlenczek et al, 2017), airborne (e.g., Conway et al, 1982; Fugal and Shaw, 2009; Beals et al, 2015; Glienke et al, 2017; Desai et al, 2019) and cable car (Beck et al, 2017) platforms, but have not yet been deployed on TBSs. The HoloBalloon platform merges the advantages of holography (well-defined sampling volume, spatial distribution of cloud particles) with the benefits of a TBS (highresolution measurements) with the aim to observe the cloud structure on different scales.
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