Microphysical Properties of Slow-Falling and Fast-Falling Ice Pellets Formed by Freezing Associated with Evaporative Cooling

Abstract

This paper describes a numerical and observational study focused on ice-pellet formation and microphysical properties near 08C from an ice-pellet-dominated storm associated with an unusually warm and dry atmosphere on 10 April 2005, in Sapporo, Japan. A one-dimensional numerical model simulation indicated that precipitation particle temperatures were sensitive to environmental temperature and relative humidity and close to the wet-bulb temperature. The simulation demonstrated that completely melted snowflakes could refreeze by evaporative cooling. Moreover, initial freezing could be explained by contact ice nucleation at the height of the minimum wet-bulb temperature.Observations using a 2D video distrometer (2DVD) indicated that ice pellets exhibited two modes of fall velocities at surface temperatures near 08C during the same time period: slow falling and fast falling. The slowfalling ice pellets exhibited a velocity similar to the average terminal velocity of hail, whereas the velocities of the fast-falling ice pellets were closer to those of raindrops. Surface roundness and fracturing characteristics of ice pellets suggest that slow-falling ice pellets froze rapidly and uniformly in a relatively cold dry layer with a wet-bulb temperature near 248C. In contrast, the fast-falling ice pellets exhibited the properties of ice particles with a wet smooth surface, suggesting that they froze slowly in a relatively warm layer by contacting ice crystals or splinters generated by preceding slow-falling ice pellets.

Geometrical Properties of Hydrometeors during the Refreezing Process and Their Effects on Dual-Polarized Radar Signals

“…8b and 8d, which are the periods of high Z DR (Figs. 5f and 5h), showed a bimodal distribution of falling velocities (fast-and slow-falling) consistent with the previous study of Nagumo and Fujiyoshi (2015). The fast-falling group was close to the terminal velocity of raindrops.…”

“…The 2DVD provided the particle equivolumetric diameter (hereafter, diameter), fall velocity, and axis ratio from bidirectional images within the 10 cm 3 10 cm observable area. The particle shapes with a diameter over 1.5 mm were visually classified in the same manner as that described in Nagumo and Fujiyoshi (2015). The fall velocity was calculated from the time lag of bidirectional detections due to a slight vertical gap.…”

“…Boldface font indicates the top two categories in each case. The method of categories and computation are the same as those in Nagumo and Fujiyoshi (2015). The particles were categorized visually, and the total particle number in each period is less than the sum of the numbers in each category because some particles were placed in two or more categories.…”

“…However, there is only a fragmentary understanding of the spatiotemporal characteristics of the actual freezing process in the atmosphere. Nagumo and Fujiyoshi (2015) found that there were dual modes of falling velocities of ice pellets at the ground level and suggested that wet ice pellets and dry, low density ice pellets coexisted at the same time. It was also suggested that many dry, low density ice pellets occurred almost simultaneously within a narrow range of altitudes in subfreezing layer.…”

Fitting Precipitation Particle Size–Velocity Data to Mixed Joint Probability Density Function with an Expectation Maximization Algorithm

“…We used the precipitation diameter-velocity data obtained by a 2DVD installed at a site of the Institute of Low Temperature Science, Hokkaido University, Sapporo, Japan, located at 43.0838N, 141.3398E (Nagumo and Fujiyoshi 2015;Campbell et al 2018). The 2DVD (Kruger and Krajewski 2002), widely used in the scientific community (Huang et al 2010;Bernauer et al 2015Bernauer et al , 2016, measures precipitation particle diameter and velocity by matching two images captured by double line sensors installed at an upper and a lower position of the instrument.…”