Abstract
Abstract. Accurate predictions of snowfall require good knowledge of the microphysical properties of the snow ice crystals and particles. Shape is an important parameter as it strongly influences the scattering properties of the ice particles, and thus their response to remote sensing techniques such as radar measurements. The fall speed of ice particles is another important parameter for both numerical forecast models as well as representation of ice clouds and snow in climate models, as it is responsible for the rate of removal of ice from these models. We describe a new ground-based in situ instrument, the Dual Ice Crystal Imager (D-ICI), to determine snow ice crystal properties and fall speed simultaneously. The instrument takes two high-resolution pictures of the same falling ice particle from two different viewing directions. Both cameras use a microscope-like setup resulting in an image pixel resolution of approximately 4 µm pixel−1. One viewing direction is horizontal and is used to determine fall speed by means of a double exposure. For this purpose, two bright flashes of a light-emitting diode behind the camera illuminate the falling ice particle and create this double exposure, and the vertical displacement of the particle provides its fall speed. The other viewing direction is close-to-vertical and is used to provide size and shape information from single-exposure images. This viewing geometry is chosen instead of a horizontal one because shape and size of ice particles as viewed in the vertical direction are more relevant than these properties viewed horizontally, as the vertical fall speed is more strongly influenced by the vertically viewed properties. In addition, a comparison with remote sensing instruments that mostly have a vertical or close-to-vertical viewing geometry is favoured when the particle properties are measured in the same direction. The instrument has been tested in Kiruna, northern Sweden (67.8∘ N, 20.4∘ E). Measurements are demonstrated with images from different snow events, and the determined snow ice crystal properties are presented.
Highlights
Accurate knowledge of atmospheric ice crystals and snowflakes, or snow particles, is needed for meteorological forecast and climate models
The field of view (FOV) is equal to the exposed sensor area, i.e. 4.8 mm × 3.6 mm. Both imaging systems use bright-field illumination from the back. This is achieved by a light-emitting diode (LED) with a simple focusing lens optics allowing for an even illumination of the FOV
The heavy riming on that day indicates the presence of cloud droplets, and the imaged drizzle droplets originate from such cloud or fog droplets that have grown large enough to precipitate and fall into the inlet of Dual Ice Crystal Imager (D-ice crystal imaging (ICI))
Summary
Accurate knowledge of atmospheric ice crystals and snowflakes, or snow particles, is needed for meteorological forecast and climate models (see, e.g. Tao et al, 2003; Stoelinga et al, 2003). Disdrometers, generally, are designed for snowflakes with larger dimensions and their size limit (pixel size) is as large as 200 μm Another category of instruments uses camera systems for optical imaging of snow particles. Different pixel resolutions may be used by the cameras, and the version described by Garrett et al (2012) used pixel resolutions between 9 and 32 μm Such multi-imagers provide more detail about the 3-D structure of the snow particle that adds valuable information to the microphysical data collected by imaging instruments. This work presents a novel instrument that uses two cameras for simultaneous particle imaging and fall speed measurement It is called the Dual Ice Crystal Imager (D-ICI) and is a development of ICI (Kuhn and Gultepe, 2016).
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