Microphysical properties and fall speed measurements of snow ice crystals using the Dual Ice Crystal Imager (D-ICI)

摘要

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 modelsas well as representation of ice clouds and snow in climate models, asit 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 andcreate 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 informationfrom single-exposure images.This viewing geometry is chosen instead of a horizontal one because shape and size of ice particlesas 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 withimages from different snow events, andthe determined snow ice crystal properties are presented.