Observations of the Falling Motion of Plate-Like Snow Crystals Part I: The Free-Fall Patterns and Velocity

Abstract

Free-fall patterns and the variations in the vertical and horizontal velocities of unrimed plate-like snow crystals were analyzed by means of a stereo-photogrammetric method.Whether the crystals exhibit a stable falling motion or not mainly depends on the Best number, which involves the mean vertical velocity (fall velocity), and the non-dimensional moment of inertia of the crystals.Unstable fall patterns were roughly classified into three types: nonrotation, swing, and rotation or spiral. For dendritic shaped crystals, the distinction among these types is approximately determined by the combination of the non-dimensional moment of inertia and the Reynolds number, which involves the mean vertical velocity.Although the standard deviation of the vertical velocity for dendritic shaped crystals was very small (1 to 3% of the mean vertical velocity), that of the horizontal velocity was considerably larger, measuring 5 to 20% of the mean vertical velocity. Accordingly, it appears likely that the variation in the horizontal velocity plays an important role in the random aggregation of plate-like snow crystals having almost the same shape and size.

Doppler lidar measurements of oriented planar ice crystals falling from supercooled and glaciated layer clouds

“…In contrast to this, the velocity distribution for specularly reflecting crystals shows a rather weaker variation with temperature, with average crystal fall speeds increasing from 0.25 m s −1 at −35 • C to 0.3 m s −1 at −15 • C; there appears to be a slight broadening of the distribution for the warmest clouds, with an average velocity of 0.4 m s −1 at −5 • C. Possibly this corresponds to planar crystals formed at the mid-levels, which thicken as they fall into air temperatures characteristic of columnar growth, increasing their weight relative to their drag and allowing them to sediment more quickly for a given Reynolds number. As in section 4, the mean Doppler velocities and implied crystal Reynolds numbers are lower than the maximum stable values found by Kajikawa (1992) for all assumed crystal types.…”

“…If the crystal is inclined at an angle to the horizontal plane, the asymmetrical drag acts to reorient the plate into the horizontal position: this reorientation is stabilised by viscosity, so oscillations around the horizontal are damped, and the particle falls stably and steadily (Willmarth et al, 1964). Measurements of natural ice crystals by Kajikawa (1992) confirm that horizontal orientation occurs for a variety of planar crystal types (all of the P1 and P2 types defined by Magono and Lee, 1966). As the crystals grow even larger, the inertial forces are too strong to be damped, and the horizontal orientation becomes unstable: this initially affects the wake downstream, producing a slow pitching motion, whilst at larger Reynolds number the vortices begin to shed fluid behind the falling crystal, leading to more irregular tumbling motions (Willmarth et al, 1964;Kajikawa, 1992).…”

“…Measurements of natural ice crystals by Kajikawa (1992) confirm that horizontal orientation occurs for a variety of planar crystal types (all of the P1 and P2 types defined by Magono and Lee, 1966). As the crystals grow even larger, the inertial forces are too strong to be damped, and the horizontal orientation becomes unstable: this initially affects the wake downstream, producing a slow pitching motion, whilst at larger Reynolds number the vortices begin to shed fluid behind the falling crystal, leading to more irregular tumbling motions (Willmarth et al, 1964;Kajikawa, 1992). Circular discs fall steadily between 1 < Re < 100; for the natural ice crystals measured by Kajikawa the maximum Reynolds number for stable orientation was in the range Re 50-100, and depended on the crystal habit.…”

Ice clouds microphysical retrieval using 94‐GHz Doppler radar observations: Basic relations within the retrieval framework

“…The polarimetric radar studies of Matrosov et al [2005b] show that the single dendrite shape snowflakes are horizontally aligned in the absence of strong wind and wind shear. According to the study of Sassen [1980], Liou [1986], Kajikawa [1992] and Prigerit et al [2001], the major dimension of the falling cirrus cloud particles is mainly horizontally oriented. Chepfer et al [1999] deduced from measurements of solar reflectance that horizontal ice particle alignment was noticeable in 40% of cirrus clouds.…”

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

“…Parameterizations of ice particle fall speeds were determined for specific ice particles and can be found for example in the work of Locatelli and Hobbs (1974) and Kajikawa and Okuhara (1997). Although improvements of parameterizations based on field and laboratory measurements have been reported (Heymsfield and Westbrook 2010), more direct measurements of terminal velocities for a variety of particle types are needed to develop better parameterizations, so that they can be used to improve climate models and the prediction of mass and heat budget based on numerical forecasting models.…”