In-line holography of cloud volumes applied to the measurement of raindrops and snowflakes

The ground-based HODAR (HOlographic Droplet and Aerosol Recording) of the University of Mainz has been operational since 1989 on the Kleiner Feldberg Mountain Observatory near Frankfurt/Main, Germany, in a `small volumeʹ configuration. With this setup an air volume of approximately 1 l can be in situ recorded, which allows it to image all aerosol particles and droplets with diameter larger than 6 μm contained in the sample volume in situ. Recently the beam diameter and optical path length of the inline system have been enlarged such that large cloud volumes (approximately 500 l) can be recorded on single holograms. During the holographic image reconstruction step in the laboratory, particle and hydrometeor images can be retrieved from these holograms for objects with sizes between 20 μm and 20 mm diameter. In both versions it is possible to distinguish between droplets and ice crystals, or snowflakes. This paper describes the optical layout suitable for ground-based large volume holographic recording in the atmosphere and presents examples from measurements in rain and snow events