Ice Fog and Light Snow Measurements Using a High-Resolution Camera System

Abstract

Abstract-Ice fog, diamond dust, and light snow usually form over extremely cold weather conditions, and they affect both visibility and Earth's radiative energy budget. Prediction of these hydrometeors using models is difficult because of limited knowledge of the microphysical properties at the small size ranges due to measurement issues. These phenomena need to be better represented in forecast and climate models; therefore, in addition to remote sensing accurate measurements using ground-based instrumentation are required. An imaging instrument, aimed at measuring ice fog and light snow particles, has been built and is presented here. The ice crystal imaging (ICI) probe samples ice particles into a vertical, tapered inlet with an inlet flow rate of 11 L min -1 . A laser beam across the vertical air flow containing the ice crystals allows for their detection by a photodetector collecting the scattered light. Detected particles are then imaged with high optical resolution. An illuminating LED flash and image capturing are triggered by the photodetector. In this work, ICI measurements collected during the fog remote sensing and modeling (FRAM) project, which took place during Winter of 2010-2011 in Yellowknife, NWT, Canada, are summarized and challenges related to measuring small ice particles are described. The majority of ice particles during the 2-month-long campaign had sizes between 300 and 800 lm. During ice fog events the size distribution measured had a lower mode diameter of 300 lm compared to the overall campaign average with mode at 500 lm.

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

“…Similarly to the ICI probe (Kuhn and Gultepe, 2016), also D-ICI has a funnel-shaped inlet, wider at the top, with a sharp upper edge (see Figs. 1 and 3).…”

“…2.4), particles are optically imaged by two imaging systems, each using a monochromatic CCD camera (Chameleon 1.3 MP Mono USB 2.0, Point Grey, now FLIR) having a 1280×960 pixel sensor chip with pixels that are 3.75 µm × 3.75 µm in size. These camera systems are similar to the microscope optics used in ice crystal imaging setups with single imaging systems (Kuhn et al, 2012;Kuhn and Gultepe, 2016). They consist of a microscope objective followed by a tube lens, as indicated in Fig.…”

“…The higher frame rate (380 s −1 ) of PIP allows determination of the fall speed with image analysis software that follows particles over several frames. The ice crystal imaging (ICI) probe uses a highresolution charge-coupled device (CCD) camera system with a pixel resolution of 4.2 µm (Kuhn and Gultepe, 2016). It has also been used to measure fall speed by double exposing snow particles using two flashes of illuminating light triggered at a known time difference.…”

“…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). The D-ICI has two cameras; the first camera uses a horizontal viewing direction (side view), whereas the second camera views the falling snow particle vertically (top view).…”

Shape Dependence of Falling Snow Crystals’ Microphysical Properties Using an Updated Shape Classification

“…The cross-sectional area versus particle size (A vs. D max ) and area ratio versus particle size (A r vs. D max ) relationships for the shape groups are analyzed in this section. The A vs. D max relationship is fitted to a power law given by Kuhn and Gultepe [26]:…”