Laura Levi

Experimental Study of Non-Basal Dislocations in Ice Crystals

AnSTRACT. Structures produced by chemical etching on the basal plane of ice c rystals have been studied a nd a re discussed in terms of non-basal g lide features. Slip bands, revealed by pa rallel rows of e tch pits, generally result from a rapidly applied stress. Etch channels were observed relative ly rarely in these exp eriments ; they can be inte rpreted as trails of dislocations moving slowly under the act ion of local st resses, sometimes to stresses produced during the e tching process. Features of the channels indicate that the dislocations emerging on the basal plane a re screw dislocations ; their Burgers vecto r was considered by previous authors to b e in the direction < I 123 ) , g liding on {I 122} and {101 o} slip p la nes. This assumption is inconsistent with the changes of channel direc tion we observed , for wh ich the {Olll } a nd {2421} slip plan es would have to be considered. As a simpler hypothesis the g lide system <000 1) , { IOIO} and ( 0001 ) , { l 120} is proposed.

Analysis of hailstones from a severe storm and their simulated evolution

Abstract A growth model is applied to the results of hailstone structure analysis with the purpose of obtaining information about the conditions existing in the cloud region where growth has occurred. The simulated hail trajectories indicate that the process, initialized at the stage where embryos of about 0.5-mm radius are assumed to grow by droplet collection, had lasted 40–50 min. During the first 20–25 min, the stones were found to grow in a cloud region where the updraft speed was W ⪅15 m s −1 and LWC∼1 gm −3 . Such relatively low LWC (liquid water concentration) had to be assumed in order to explain the existence of an opaque rime layer about the frozen drops that formed nearly 40% of the hail embryos. During this initial growth period, where the hail size remained below 1 cm, the evolution of the drag coefficient C D should be considered dependent on the embryo shape and fall mode. This behaviour was taken into account to explain the formation, in the same updraft, of stones of different crystallographic structure. Actually, most of them should have descended to low levels, where a large-crystal layer was formed, while less numerous ones would have travelled the whole time at high levels, thus being completely formed by small crystals. After the first growth period, W had to be increased up to ∼ 35 m s −1 in order to simulate the upward motion of the particles, occurring near the end of the process and determining the formation of a peripheral small-crystal layer. During this period, the evolution of hailstones with the observed structure had to be simulated by increasing the LWC to values depending on the droplet radius, r = 5, 7 or 10 μ m. The comparison of the results with those obtained by the application of a cloud model to meteorological data indicates that r = 7 μ m with maximum LWC ∼ 4 g m −3 would the most suitable choice. Actually, this condition could have existed near the storm core where W ∼ 35 m s −1 . The proposed trajectories are compared with those proposed by previous authors, on the basis of meteorological data provided for other hailstorms.

Effects of Growth Temperatures and Surface Roughness on Crystal Orientation of Ice Accreted in a Dry Regime

Abstract The dependence of crystal orientation on air and deposit temperatures in ice accreted in the dry growth regime has been re-examined by producing accretions over a wide range of these temperatures and critically examining the structural aspects of the deposits, such as surface roughness. It is shown that the variation in the preferred c-axis orientation with the air and deposit temperatures during growth is an intrinsic effect of she freezing mode of ice, rather than an effect of surface roughness. The Fφ distributions of the angle φ between the accretion radius and the crystal c-axis, and those of its component angles F(η) and F(), have been obtained. It is shown that for cloud temperature Ta > −18°C, the F(φ) distributions have a main maximum at small angles. For −18°C ≥ Ta ≥ −23°C, the location of the maximum depends strongly on the deposit temperature Td reaching values near 45° for Td < −10°C. For Ta < −23°C the distributions show a large disorder at Td ≥ −5°C, but a maximum about 45° when Td...

Study of a convective storm series and of precipitated hail in south Argentina

Abstract A study of the period 10–19 February 1990, when frequent hailstorms occurred in the High Valley of Rio Negro and Neuquen (40°S, 65–70°W), is performed. The synoptic situation responsible for the observed convective events is characterized by the presence of a blocking high on the Atlantic Ocean (40°S, 50°W) that results in prevailing easterly winds and consequent high surface humidity. Information about precipitated hail is obtained from a network of 110 hailpad stations and from the crystal structure of hailstone samples. It is shown that, though kinetic energies up to about 1000 Jm −2 were evaluated in some stations, rather small hail with diameter D ∼ 10 mm prevailed in the hail size distributions while particles with 25–30 mm diameter were relatively few. Crop damage estimations are related to kinetic energy and stone number density on the ground, derived from hailpad analysis. It is found that, due to the type of fruit cultivation, an energy of about 100 Jm −2 was enough to determine total crop destruction. A one dimensional time independent (1DTI) model is applied to morning radiosonde data to stimulate cloud development. It is shown that the atmosphere instability, though moderate, was favorable to convection on most days of the period. For days when hail falls occurred, an accretion growth model is used to simulate the vertical motion and the development of ice particles growing in the environmental conditions characterizing the modeled cloud. It is found, that the size attained by particles displaying an up-and-down trajectory in the model could be favorably compared with the largest sizes registered on the ground and that most simulated hailstone growth occurred at rather low cloud levels, in agreement with the observed hailstone structure. Relations between the hailstone size and the simulated updraft intensity at middle levels, usually located below the updraft maximum, are suggested. The importance of the ice phase on the cloud depth and on the updraft profile shape is discussed.

Surface temperature distribution for ice accreted on a cylindrical collector

Atmospheric icing has been studied by several authors due to its importance in many phenomena where a solid body grows by accretion of supercooled cloud or fog droplets. An important parameter, as for the deposit evolution is the surface temperature, Ts, which determines the deposit density and morphology in dry regime and the transition for dry to wet regime. In the present work an algebraic solution is given for the differential equation representing the equilibrium surface temperature of an ice accretion growing on a fixed cylinder. This solution differs from previous ones because it takes into account the heat flux occurring in a cylinder, due to the temperature gradient created at the surface. It is shown that, for an ice cylinder growing in normal atmospheric conditions, this flux is of the same order of magnitude of that representing the heat exchange with the environment. The proposed mathematical treatment requires the representation of the different heat fluxes in the form of Fourier series, the terms of which are functions of cos (nθ), where θ is the angular distance from the radius through the stagnation point. Special attention is given to the Nusselt number Nu(θ), which affects all heat fluxes related with ventilation. An expression for Nu(θ), valid for an heat conducting cylinder, in the range 4 x 103<Re<2x104, is proposed. Several Ts(θ) curves are given for different environmental conditions and the effects of varying the Stokes and the Reynolds numbers are considered. The results obtained for an isolating, an ice and a Cu collector are compared.

A discussion of mechanisms proposed to explain habit changes of vapor-grown ice crystals

Abstract In the present work, surface kinetics processes that can contribute to the growth behavior of ice crystals from the vapor phase are revised and proposed interpretations of crystal habit changes are discussed. Following the main initial papers on this subject by Hallet, Mason et al. and Kobayashi, relationships are considered between linear growth rate and step velocity. More recent results obtained by Sei and Gonda (SG2) for molecular steps naturally formed on basal and prism surfaces are shown to confirm Hallets interpretation of previous curves obtained for the velocity of giant steps that were artificially formed on basal surfaces only. The different behavior of the condensation coefficient α(T) characterizing growth in pure water vapor, observed by Lamb and Scott for surfaces intersecting a substrate and by Sei and Gonda for free surfaces, is discussed by considering that α is the product of the adsorption and accommodation coefficients β and γ, respectively. It is noted that, as in previous works, β=1 was assumed, the variations of α discussed to interpret crystal habit changes were made to coincide with variations of γ. However, Sei and Gondas results show that in the temperature range where crystal habit changes are observed, values of β(T)≪1 are found. As these depend on surface orientation, they should play an important role in the phenomenon. The dependence of crystal habits on two-dimensional nucleation is also discussed on the basis of measurements carried out by Nelson and Knight of the critical supersaturation σcr on the basal and prism surfaces. A possible relationship between the curves for σcr(T) and those of β(T) derived from Sei and Gondas results is suggested. The mechanisms determining the large anisotropy exhibited by crystals grown in atmospheric conditions are discussed by taking into account that the growth rate curves R(T) on the basal and prism surfaces show a correlation between maximum and minimum values, which are not observed for crystals grown in pure vapor.

Requirements for low density riming and two stage growth on atmospheric particles

Abstract A theoretical study is carried out of the conditions that can be expected to determine low density riming on atmospheric ice particles. Using a growth simulation model, critical liquid water contents L wc and air temperatures T a are calculated, which correspond to a density ρ =0.5 g/cm 3 for rime deposit on ice particles with radii varying from 1 to 10 mm. Their dependence on the used laws for the ice density as a function of Macklins parameter and for the drag coefficient as a function of Reynolds number, is discussed. The evolution of the density and related parameters for free falling particles growing by accretion from initial values of the radius R and density ρ is studied in different environmental conditions. It is shown that the temperature of the deposit T s increases with R , up to the transition to wet growth, represented by T s =0°C. Only for L wc ≥2 g/m 3 the transition from low density ice to wet growth is found to occur rapidly, at a distance from the center R ≤1 cm. This distance is considered to represent the maximum radius of regions where two-stage growth, due to water penetration and freezing into pores of low-density layers, can be responsible for rapid variations of the particle density and consequently of its free-fall speed, which would characterize the effect of hail growth via microphysical recycling.

Modelled spongy growth and shedding process for spheroidal hailstones

Abstract Model calculations are carried out for spheroidal hailstone growth, by taking into account the results obtained by Garcia-Garcia and List from wind tunnel experiments. For the maximum hailstone diameter D varying from 2 to 4 cm, the liquid water contents at the onset of the wet growth and of shedding regimes ( W f,I and W f,E , respectively) are represented as functions of both the air temperature T a and D . The values of these parameters approach the experimental ones for D =2 cm, but show that, for D =4 cm, these transitions occur at 25% smaller values of W f . Using the obtained parameterized equation of W f,E , the dependence on T a of the number of drops N ( d ) shed per km of fall by a particle of D =2 and 4 cm is studied for different values of W f . For D =2 cm, it is shown that shedding is negligible at all temperatures for W f −3 and, even for W f =4 g m −3 a value N ( d ) km −1 =1000 is only reached near the melting level. According to these results, shedding from particles growing in the spongy regime would be much less pronounced than has been previously calculated. The effect of the variation of D on the shedding effect is also discussed.

Crystal structure of droplets frozen on an ice substrate after low speed collision

Abstract In the present work, the freezing behaviour, for droplets colliding at terminal free fall speed with an ice substrate, is studied. The study of the crystal structure of singular frozen droplets was carried out using spectra, having 135 and 75 μm mean volume diameter. The results show that the transition temperature T∗, corresponding to a value P = 0.5 of the probability of nucleation of new orientations in the droplets, decreases with the droplet diameter, varying from -18 to -29°C on a prism (1120) substrate, and from -11 to -13°C on a basal substrate. This behaviour is discussed on the basis of the nucleation theory. Small accretions were also grown in a cold room without ventilation, using droplets having 85 μm mean volume diameter. It is shown that, for air and substrate temperatures Ta = -7 and Ts = -3°C respectively, these accretions mostly followed the substrate orientation, although a few crystals with new orientations were formed. The difference existing between these accretions structures and those obtained in icing wind tunnels, at about the same values of Ta and Ts, is pointed out.

CRYSTAL STRUCTURE OF ICE ACCRETED ON AN ICE SUBSTRATE

Thin ice deposits, grown on a prismatic ice substrate and the first layers of large deposits grown about a cylindrical ice collector, are studied. Experiments were performed at 30 m s-1 wind speed and Ta near -20°C. Progressive structural changes in thin accretions with time of water-injection were observed, when the substrate temperature was Ts > -5 ·C. After 60 s injection, which determined a deposit -1 mm thick, the preferred orientation was established and the mean grain area was -0.01 mm 2 . Such structural changes did not appear along the initial layers of thick deposits. These differed mainly from the subsequent zone because of the larger value of the preferred angle between the c-axis and the growth direction. These effects are discussed by considering the nucleation and orientation selection processes which occurred during droplet freezing and the annealing effects that occurred below the growth front. The different c-axis orientations in the initial and main zones are related to the different Ts values during their formation.