Shape Of Snow Crystals Research Articles

Mass of different snow crystal shapes derived from fall speed measurements

Abstract. Meteorological forecast and climate models require good knowledge of the microphysical properties of hydrometeors and the atmospheric snow and ice crystals in clouds, for instance, their size, cross-sectional area, shape, mass, and fall speed. Especially shape is an important parameter in that it strongly affects the scattering properties of ice particles and consequently their response to remote sensing techniques. The fall speed and mass of ice particles are other important parameters for both numerical forecast models and the representation of snow and ice clouds in climate models. In the case of fall speed, it is responsible for the rate of removal of ice from these models. The particle mass is a key quantity that connects the cloud microphysical properties to radiative properties. Using an empirical relationship between the dimensionless Reynolds and Best numbers, fall speed and mass can be derived from each other if particle size and cross-sectional area are also known. In this study, ground-based in situ measurements of snow particle microphysical properties are used to analyse mass as a function of shape and the other properties particle size, cross-sectional area, and fall speed. The measurements for this study were done in Kiruna, Sweden, during snowfall seasons of 2014 to 2019 and using the ground-based in situ Dual Ice Crystal Imager (D-ICI) instrument, which takes high-resolution side- and top-view images of natural hydrometeors. From these images, particle size (maximum dimension), cross-sectional area, and fall speed of individual particles are determined. The particles are shape-classified according to the scheme presented in our previous study, in which particles sort into 15 different shape groups depending on their shape and morphology. Particle masses of individual ice particles are estimated from measured particle size, cross-sectional area, and fall speed. The selected dataset covers sizes from about 0.1 to 3.2 mm, fall speeds from 0.1 to 1.6 m s−1, and masses from 0.2 to 450 µg. In our previous study, the fall speed relationships between particle size and cross-sectional area were studied. In this study, the same dataset is used to determine the particle mass, and consequently, the mass relationships between particle size, cross-sectional area, and fall speed are studied for these 15 shape groups. Furthermore, the mass relationships presented in this study are compared with the previous studies. For certain crystal habits, in particular columnar shapes, the maximum dimension is unsuitable for determining Reynolds number. Using a selection of columns, for which the simple geometry allows the verification of an empirical Best-number-to-Reynolds-number relationship, we show that Reynolds number and fall speed are more closely related to the diameter of the basal facet than the maximum dimension. The agreement with the empirical relationship is further improved using a modified Best number, a function of an area ratio based on the falling particle seen in the vertical direction.

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.

Ice Spheres as Model Snow: Tumbling, Sintering, and Mechanical Tests

The snow microstructure is crucial for the mechanical behavior of snow, but is usually simplified in numerical models. In Discrete Element Models (DEM), often used in snow mechanics, the microstructure is typically represented by sphere assemblies. This renders a model validation difficult, as the real snow crystal shapes affect the actual contacts and consequently the macromechanical behavior. To approach this problem from the experimental side, we created simplified microstructures comprising spherical ice particles and examined this model snow under structural and mechanical aspects. We developed a new method to create uniform ice spheres and performed a detailed particle characterization using 3D computed tomography (CT). We examined sintered sphere assemblies to relate microstructural and macromechanical properties in a geometrically well-defined system. Microstructural variation was created by varying the sample density and sintering time, to study the role of the number and size of contacts. 3D CT scans were taken of sintered sphere samples, which were then tested in unconfined compression experiments together with a nature identical snow type as a reference. The scans were evaluated regarding the number of particles and contacts, and were used to compute the elastic modulus, as additional mechanical property. The results of the particle characterization showed the spherical shape and sharp size-distribution of the particles, which are the main prerequisites for model validation. The CT image analysis showed the reproducibility of the sphere assemblies and can be applied for efficient reconstruction of the experimental samples as initial condition for simulations. The mechanical properties conform with the reference snow and the expectations from literature, however, the CT scans revealed a complex geometry of the sintered contacts with measurable impact on the mechanical properties.

A global classification of snow crystals, ice crystals, and solid precipitation based on observations from middle latitudes to polar regions

This paper presents an extensive revision of Magono and Lee's (1966) classification of natural snow crystals, which has been widely used in snow and ice studies to describe snow crystal shapes. The new classification catalogs snow crystals and other solid precipitation particles into 121 categories, in contrast to Magono and Lee's 80 categories. Of these, 28 categories were created to classify new types of snow crystals that have been discovered in polar regions since 1968, seven were created after reconsidering the original categories, and six categories were created to classify solid precipitation particles such as frozen cloud particles and small raindrops. Because our observational area extended from middle latitudes (Japan) to polar regions, we refer to our new classification scheme as ‘global-scale classification’ or ‘global classification’. The global classification consists of three levels – general, intermediate, and elementary – which are composed of 8, 39, and 121 categories, respectively. This paper describes the characteristics of each type of snow crystal, ice crystal, and solid precipitation particle.

Snow metamorphism under alternating temperature gradients: Morphology and recrystallization in surface snow

The composition of the lower atmospheric boundary layer can be strongly influenced by snow photochemistry. Surface snow, where air‐snow interactions take place, is viewed as slowly changing quasi‐isothermal snow, implied by the observation of mainly rounded snow crystals. The role of snow for photochemistry is therefore expected to be purely geometric. However, large temperature gradients are often observed in these layers, which would imply high recrystallization and faceting. In controlled laboratory experiments we showed that temperature gradients on the order of 100 K m−1 do not lead necessarily to faceting if their sign changes with a daily cycle. The shape of snow crystals does not reflect necessarily the metamorphic process. With time‐lapse X‐ray tomography, recrystallization rates as high as 60% of the total ice mass were observed during 12 hours. The high recrystallization rates in apparently isothermal snow not only contradict the current understanding of snow metamorphism, they also might influence chemical air‐snow interactions.

Correlation between the specific surface area and the short wave infrared (SWIR) reflectance of snow

Abstract The albedo of snow is determined in part by the size and shape of snow crystals, especially in the short wave infrared (SWIR). Many models of snow albedo represent snow crystals by spheres of surface/volume (S/V) ratio equal to that of snow crystals. However, the actual S/V ratio of snow has never been measured simultaneously with the albedo, for a thorough test of models. Using CH4 adsorption at 77 K, we have measured the specific surface area (SSA) of snow samples, i.e. its ratio S/(V · ρ), where ρ is the density of ice, together with the snow spectral albedo using a field radiometer with nadir viewing, at Ny-Alesund, Svalbard. Tests are performed at 1310, 1629, 1740 and 2260 nm, and we find a good correlation between the SSA and the snow spectral albedo in the SWIR (linear correlation coefficient R2 > 0.98 for the last 3 wavelengths). Snow samples having varied crystals shapes such as rounded crystals in windpacks and hollow faceted crystals in depth hoar were studied and crystal shape did not affect the correlation in a detectable manner. An interest in using SSA rather than crystal size to predict SWIR albedo is that the reflectance of large hollow crystals such as depth hoar or surface hoar will be correctly predicted from their SSA, while considering their large dimensions would underestimate reflectance. We compare these correlations to those predicted by commonly used optical models. The best agreement is found when we compare our data to the modeled hemispheric reflectance, corrected by an adjustable factor that shows a small wavelength dependence. We propose that, once these results have been confirmed by more studies, it may be possible to design a rapid and simple optical method to measure snow SSA in the field. Our results may also allow a more detailed use of remote sensing data to study snow metamorphism, air–snow exchanges of gases, and climate.

Infrared Radiative Properties of the Antarctic Plateau from AVHRR Data. Part I: Effect of the Snow Surface

The effective scene temperature, or “brightness temperature,” measured in channel 3 (3.5–3.9 μm) of the Advanced Very High Resolution Radiometer (AVHRR) is shown to be sensitive, in principle, to the effective particle size of snow grains on the Antarctic plateau, over the range of snow grain sizes reported in field studies. In conjunction with a discrete ordinate method radiative transfer model that couples the polar atmosphere with a scattering and absorbing snowpack, the thermal infrared channels of the AVHRR instrument can, therefore, be used to estimate effective grain size at the snow surface over Antarctica. This is subject to uncertainties related to the modeled top-of-atmosphere bidirectional reflectance distribution function resulting from the possible presence of sastrugi and to lack of complete knowledge of snow crystal shapes and habits as they influence the scattering phase function. However, when applied to NOAA-11 and NOAA-12 AVHRR data from 1992, the snow grain effective radii of order 50 μm are retrieved, consistent with field observations, with no apparent discontinuity between two spacecraft having different viewing geometries. Retrieved snow grain effective radii are 10–20-μm larger when the snow grains are modeled as hexagonal solid columns rather than as spheres with a Henyey–Greenstein phase function. Despite the above-mentioned uncertainties, the retrievals are consistent enough that one should be able to monitor climatically significant changes in surface snow grain size due to major precipitation events. It is also shown that a realistic representation of the surface snow grain size is critical when retrieving the optical depth and effective particle radius of clouds for the optically thin clouds most frequently encountered over the Antarctic plateau.

Snow metamorphism as revealed by scanning electron microscopy

Current theories of snow metamorphism indicate that sublimating snow crystals have rounded shapes, while growing crystals have shapes that depend on growth rates. At slow growth rates, crystals are rounded. At moderate rates, they have flat faces with rounded edges. At fast growth rates, crystals have flat faces with sharp edges, and they have hollow faces at very fast growth rates. The main growth/sublimation mechanism is thought to be by the homogeneous nucleation of new layers at or near crystal edges. It was also suggested that the equilibrium shape of snow crystals would be temperature dependent: rounded above -10.5 degrees C, and faceted below. To test these paradigms, we have performed SEM investigations of snow samples having undergone metamorphism under natural conditions, and of snow samples subjected to isothermal metamorphism at -4 degrees and -15 degrees C in the laboratory. In general, current theories predicting crystal shapes as a function of growth rates, and of whether crystals are growing or sublimating, are verified. However, the transition in equilibrium shapes from rounded to faceted at -10.5 degrees C is not observed in our isothermal experiments that reveal a predominance of rounded shapes after more than a month of metamorphism at -4 and -15 degrees C. Some small crystals with flat faces that also have sharp angles at -15 degrees C, are observed in our isothermal experiments. These faces are newly formed, and contradict current theory. Several hypotheses are proposed to explain their occurrence. One is that they are due to sublimation at emerging dislocations.

Sensitivity studies of the effect of increased aerosol concentrations and snow crystal shape on the snowfall rate in the Arctic

The mesoscale model GESIMA is used to simulate microphysical properties of Arctic clouds and their effect on radiation. Different case studies during the FIRE ACE/SHEBA project show that GESIMA is able to simulate the cloud boundaries, ice and liquid water content, and effective radii in good agreement with observations. For two different aerosol scenarios, the simulation results show that the anthropogenic aerosol can alter microphysical properties of Arctic clouds, and consequently modify surface precipitation. Borys et al. [2000] proposed that anthropogenically induced decreases in cloud droplet size inhibit the riming process. On the contrary, we find that the accretion of snow crystals with cloud droplets is increased in the polluted cloud owing to its higher cloud droplet number concentration. Instead, the autoconversion rate of cloud droplets and accretion of drizzle by snow decreases caused by the shutdown of the collision‐coalescence process in the polluted cloud. The amount of precipitation reaching the surface as snow depends crucially on the crystal shape. If aggregates are assumed, then a tenfold increase in aerosol concentration leads to an increase in accumulated snow by 40% after 7 hours of simulation whereas the snow amount decreases by 30% when planar crystals are assumed because of the larger accretion efficiency of snow crystals with cloud droplets in case of aggregates.

Surface temperature adjustments to improve weather radar representation of multi-temporal winter precipitation accumulations

Hydrologists and water resources managers who work in areas that receive a significant portion of the annual precipitation in the form of snowfall rely on good approximates of snow accumulation in order to assess snowpack volumes for snowmelt streamflow estimation. Weather radar rainfall estimation has been used for hydrological modelling and radar has been used for the estimation of snowfall from individual events, yet radar has rarely been used to measure snowfall accumulation over time periods longer. Snowfall estimates for weekly, monthly, and seasonal accumulation periods have been compared to measured Nipher-shielded Belfort precipitation gauge quantities. A local scaling issue that caused overestimates is discussed. To enhance the accumulation estimates, the conventional scan radar images were adjusted using the near surface air temperatures. The adjustment for mixed precipitation improved the accumulation estimates, while the subsequent particle shape adjustment for snow crystal shape did not further enhance the radar estimates.

Specific surface area of snow samples determined by CH4 adsorption at 77 K and estimated by optical microscopy and scanning electron microscopy.

Snow is a divided medium that can adsorb atmospheric trace gases. Evaluating the impact of the snow cover on atmospheric chemistry therefore requires the knowledge of the specific surface area (SSA) of snow. This paper compares the results of three methods used to measure or estimate the SSA of four snow samples: CH4 adsorption at 77 K, optical microscopy (OM), and scanning electron microscopy (SEM, used only on two samples). Within error bars, CH4 adsorption and OM yield similar results on three of the four snow samples. Values for the 4th sample are within a factor of 2. For both samples where CH4 adsorption, OM, and SEM are used, all three methods yield similar results, but CH4 adsorption always has a better accuracy and a much better precision. Thus, despite its ease of use, estimates from OM images are often not accurate enough to monitor the evolution of snow SSA. The main sources of error in the OM method are the difficulty to determine snow crystal thicknesses and to take into account the topography of the snow crystal surface. The combination of CH4 adsorption and OM or SEM can provide useful information on the evolution of both the SSA and the shape of snow crystals. This will be useful to evaluate the respective contributions of adsorption/desorption and sublimation/condensation processes to the impact of the snow cover on atmospheric chemistry.

Wintertime precipitation behavior in the western Canadian Arctic region

To investigate the precipitation formation process in the two major types of weather systems (“Pacific origin” and “storm track” types) in the western Canadian Arctic region, an X‐band vertical pointing Doppler radar, microwave radiometer were installed at Inuvik, Northwest Territories, Canada, during the midwinter of 1995/1996. Precise precipitation observations specially for ice crystals with replicator, microscope, and close‐up photos were also conducted at the same place. Precipitation formation mechanisms were quite different between these two major weather systems. For the Pacific‐origin‐type disturbance, warm and moist air was advected from the Pacific Ocean between 1.5 and 3.5 km msl. Convective echo patterns appeared by radar, and a large amount of precipitable water vapor and liquid water path were estimated by a microwave radiometer. As the air temperature was below the freezing point, the liquid water was identified as being in the supercooled state. Densely rimed dendrites and graupel particles were observed predominantly on the ground. Collision and coalescence processes of supercooled cloud droplets were dominant. On the contrary, for the storm track disturbance the moisture came from the Arctic Ocean, and strong winds were observed on the ground. The air mass was colder than −20°C throughout the layer. Stratiform echo patterns were observed by radar, a smaller amount of precipitable water vapor, and only a small liquid water path were observed by a microwave radiometer. It was suggested that the condensation growth was predominant. Snow crystal shapes of plates, columns, and bullet rosettes were observed predominantly on the ground.

The specific surface area of fresh dendritic snow crystals

The surface area to mass ratio or specific surface area (SSA) is an often neglected characteristic of the snowpack that varies substantially with time, and with the shape of the individual snow crystal for fresh snow. The SSA for the dendritic shape of snow crystals was computed using a series of images photographed by W. A. Bentley. The specific images were dendritic crystals (P1d, P1e, P1f) and crystals that take a partial dendritic form and have ends or extensions (P2a, P2b, P2d, P2e, P2f, P2g) according to the Magono and Lee snow crystal classification. Image analysis, using known geometric relationships between length and width, and particle size distributions, examined the spatial properties of 50 sample snow crystals. Probability distribution functions were derived for SSA and these compared well with measured and other computed estimates of fresh snow SSA. For the non-rimed condition, the average SSA was 0.182 m 2 /g with a range from 0.09 to 0.33 m 2 /g. The presence of rime is discussed, and depending on the shape of the rime particles and the degree of surface coverage, the SSA can be doubled (20% coverage for needle or plate shaped rime). Fractal analysis was performed to determine various geometric relationships that characterize the dendritic form of snow crystal.

Effects of snow crystal shape on the scattering of passive microwave radiation

A discrete dipole scattering model is used to measure the passive microwave radiation scattered by snow particles having different shapes and sizes. The model results demonstrate that the shape of the snow crystal is insignificant in scattering microwave energy in the 37-GHz region of the spectrum.

The shapes of snow crystals—Fitness for purpose?

AbstractSnow crystals exhibit six quite sharp changes of habit between 0 and −25°C as between hexagonal plates, columns/needles and stellar dendrites. The questions arise as to why, and do these changes, unique in crystal physics, serve a useful purpose?.Calculations of the growth rates of the various forms, and also of ice spheres, as they fall through atmospheres with specified profiles of temperature and supersaturation, reveal that the transitions from plates to stellar dendrites that grow only between −12 and −16°C, and from plates to columns below −25°C, permit the more effective release of precipitation from layer clouds. The interruption of the plate regime by the growth of columns and needles between −8 and −3°C leads to the formation of larger precipitation elements that better survive evaporation below cloud base than would plate crystals, by virtue of having much higher collection efficiencies for supercooled cloud droplets.

Formation of patterns during growth of snow crystals

We discuss recent progress in the understanding of the formation of patterns of snow crystals. In particular, we concentrate on the transition from hexagonal to dendritic patterns that occurs with an increase in the degree of supersaturation. Our numerical results demonstrate that the anisotropy of surface kinetics plays a very important role in the formation of faceted shapes of snow crystals in the form of hexagonal prisms and of facets seen at the tips of dendrites. Furthermore, it is shown that the conditions for the transition from a hexagonal to a dendritic pattern are given by the supersaturation and the dimensionless crystal size relative to the mean free path of a water molecule in air.

Prediction of snow crystal shape and riming by vertical Doppler radar

The knowledge of snow crystal shape and riming is important in understanding various physical and chemical processes in the atmosphere. A method is presented to obtain information about shape and riming of snow crystals above the ground by use of vertical Doppler radar. An empirical relation between the observed degree of riming and the mean Doppler velocity of snow crystals is established. It is found that Doppler velocities of less than 0.5 m/s are a sign of weakly rimed or unrimed particles whereas 1.5 m/s and 2.5 m/s are expected for rimed crystals and graupel, respectively.

Snow crystals, natural and man made

Abstract A short history of observations on the shapes of snow crystals starts in ancient China, summarizes the remarkable but little-known essay by Kepler in 1611, includes contributions by Descartes and Hooke, and ends with the beautiful collections of microphotographs by Bentley and Humphreys (1931) and Nakaya (1954). Modern observations from aircraft and mountain observatories reveal that the dominant crystal habits, hexagonal plates, columns, needles and dendritic stellar forms, and transition from one to another, are strongly correlated with air temperature, with the supersaturation of the vapour having only a secondary influence. There follows a review of laboratory experiments to determine the nature and origin of natural ice-forming nuclei and the properties of artificial nuclei, such as silver iodide, through the study of epitaxial growth of ice crystals on single crystalline substrates. Growth of snow crystals under carefully controlled conditions in laboratory cloud chambers has established that the crystal habit changes sharply six times over the temperature range 0 to—25°C. The author argues that the basic habit is determined in the very early stages of growth by the relative rates of surface diffusion of water molecules across the basal and prism faces. He deduces the average migration distance, xs, for molecules on the basal face from measurements on the velocity of growth layers and of that on the prism faces from measurements on the axial ratios of small crystals. In both cases, xs, varies rapidly between 1 and 6 μm with temperature, and the two curves intersect at three temperatures that correspond to transitions between crystal habits. A theory is developed to explain the transition from regular plates to dendritic stellar crystals; the corners sprout when material from the vapour phase arrives at corners more rapidly than it can be carried away by surface diffusion and occurs only when the diameter of the plate exceeds a critical value dc, ∝ Ds, /Dv, = αx2 p, /Dv, where Ds, Dv, are the coefficients of surface and volume diffusion and xp, is the surface migration distance on the (prism) edge of the plate. The predictions of the theory are compared with observations on natural snow crystals. Equations are derived for the growth rates of snow crystals as they fall through the atmosphere. The predicted maximum attainable diameters of the various shapes are in good agreement with observations.

On the facet-skeletal transition of snow crystals: Experiments in high and low gravity

A Laboratory investigation of the influence of air velocity on the growth of columnar ice crystals from the vapor over the range -3 to -5°C shows that the linear growth velocity increases and that columns transform to sheath crystals or needles as air velocity increases from a few cm s -1 to 40 cm s -1. Comparison with a similar transition of plates to dendrites shows that, macroscopically, in both cases the facets sprout rounded tips at a critical velocity which is lower for higher ambient supersaturation. Studies in low gravity (20 s KC 135 aircraft parabola) show that chamber scale convection under normal gravity may have significant influence on growth even in the absence of an imposed air velocity. Falling snow crystals, both plate like and column like, become more skeletal in shape as they grow and fall with increasing velocity. This development depends critically on temperature (±0.5°C) and demonstrates that snow crystal shape is even more dependent on environmental growth conditions than previously thought.

Mathematical Description of the Shape of Plane Hexagonal Snow Crystals

Abstract Simple mathematical expressions are presented for describing the shapes of some plane hexagonal snow crystals. These expressions provide convenient means for cloud physical calculations and can also serve as a method for quantitative classification of snow crystal shapes. A few examples am worked out to illustrate the use of these expressions. They can be further developed for describing more complicated shapes.