Matching crystal habits and radiosonde profiles in Northern Finland

<p>Crystal habits encode atmospheric conditions. Temperature and relative humidity with respect to ice and liquid water are the microphysical drivers of the growth of snow crystals in terms of shape, size and degree of riming, while cloud thickness and the related growth time of crystals are the dynamical drivers. According to current versions of Nakaya’s habit diagram, rather large and eventually rimed crystals are formed above supersaturation. Below supersaturation compact and unrimed snow crystals are to be expected. In this study, we combine radiosonde profiles with snowflake images captured at the surface by a multi-angle snowflake camera during two-and-a-half winter seasons in Northern Finland (67.367 °N, 26.629 °E). Our objective is to quantify how well crystal habits correspond with what would be expected from radiosonde profiles at this continental site in the Arctic.</p>

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

We present ground-based in situ snow measurements in Kiruna, Sweden, using the ground-based in situ instrument Dual Ice Crystal Imager (D-ICI). D-ICI records dual high-resolution images from above and from the side of falling natural snow crystals and other hydrometeors with particle sizes ranging from 50 μ m to 4 mm. The images are from multiple snowfall seasons during the winters of 2014/2015 to 2018/2019, which span from the beginning of November to the middle of May. From our images, the microphysical properties of individual particles, such as particle size, cross-sectional area, area ratio, aspect ratio, and shape, can be determined. We present an updated classification scheme, which comprises a total of 135 unique shapes, including 34 new snow crystal shapes. This is useful for other studies that are using previous shape classification schemes, in particular the widely used Magono–Lee classification. To facilitate the study of the shape dependence of the microphysical properties, we further sort these individual particle shapes into 15 different shape groups. Relationships between the microphysical properties are determined for each of these shape groups.

December: Saving the Dammed

At the nadir of the year, this is how morning comes to the beaver meadow. Just as the sun rises above the eastern horizon, a flush of pale rose lights the snow newly fallen on the highest peaks. The beaver meadow remains in shadow, silent but for the creek flowing quietly between its rims of ice. The air temperature is well below freezing and frost whitens the pine needles like a dark-haired person starting to go gray. Wisps and sheets of snow flag off the summits in the steady wind. Over the course of a few minutes, the summit snow warms from pale rose to faint orange and then a rich, warm gold that also lights the rock outcrops at lower elevations. The wind reaches the beaver meadow before the sunlight, coming in abrupt blasts that shake loose the little tufts of snow remaining on the pine boughs. The wind sends the snow crystals slaloming across the ice on the creek with a dry, skittering sound like that of blowing sand. Before long, the meadow is submerged in a continual rushing sound created by wind gusting through the pines up slope, along the valley walls. The lateral moraine to the south keeps the beaver meadow in shadow until 9:30 a.m. Nothing is so slow as waiting for the warmth of sunlight on a cold winter morning. When the sunlight does reach the meadow, it brings out the colors of water, ice, grasses, and willows. Flowing portions of the creek change from gray to orange brown. The snow reflects the light in a painfully intense glare broken by the deep, long shadows that everything casts. With the sunlight comes a steady wind that blasts the crystalline snow onto my face like grit. Not much snow has fallen yet, but North St. Vrain Creek is completely frozen in places and covered with snow. The ice records the movements of water, freezing the pulses and turbulence in ice ripples and ledges, motionless swirls and bands. It seems a miracle that any water still flows in this gray and white world of ice and snow.

Air pockets and secondary habits in ice from lateral-type growth

Often overlooked in studies of ice growth is how the crystal faces grow laterally. This paper explores the implications of such lateral-type growth and how it may explain air pockets and other secondary features of vapor-grown ice in air. For example, using a new crystal-growth chamber, we observed air pockets forming at crystal corners when a sublimated crystal is regrown. This and other observations support the idea that the lateral spreading of a face, and its (in some cases) extension as a thin overhang over the adjoining region, is driven by a flux of surface-mobile admolecules across the face to the lateral-growth front. Inspired by recent work on this topic by Prof. A. Yamashita of Osaka Kyoiku University, we call this flux adjoining surface transport (AST) and the extension overgrowth protruding growth, then apply the concepts to observed ice and snow crystals, including some from a cloud chamber and others from our experiments. We also suggest that such lateral-type growth may explain other air pockets, droxtal centers in dendrites, hollow terracing and banding, multiple-capped columns, scrolls, trigonals, and sheath clusters. For dendrites and sheaths, AST may increase their maximum dimensions and round their tips.

Direct Lagrangian tracking simulation of droplet growth in vertically developing cloud

We present a direct Lagrangian simulation that computes key warm-rain processes in a vertically developing cloud, including cloud condensation nuclei (CCN) activation, condensational growth, collisional growth, and droplet gravitational settling. This simulation, which tracks the motion and growth of individual particles, is applied to a kinematic simulation of an extremely vertically elongated quasi-one-dimensional domain, after which the results are compared with those obtained from a spectral-bin model, which adopts the conventional Eulerian framework. The comparison results, which confirm good bulk statistical agreement between the Lagrangian and conventional spectral-bin simulations, also show that the Lagrangian simulation is free from the numerical diffusion found in the spectral-bin simulation. After analyzing the Lagrangian statistics of the surface raindrops that reach the ground surface, back-trajectory scrutiny reveals that the Lagrangian statistics of surface raindrops contains the information about the sky where the raindrops grow like the shape does for snow crystals.