Ice Crystal Interactions Research Articles

Simulation of the Interaction of Ice Crystals with the Surface of a Flying Vehicle

A physical and mathematical model of the movement of nonspherical particles in the two-phase flow over an aerodynamic body, allowing one to estimate the area of wetting of the body surface by these particles depending on their stochastic orientation in the flow, is proposed. On the basis of the heuristic model developed earlier, the coefficients of restoring the velocity components of the ice crystals colliding with the surface of a blunt-nosed body were calculated for values of the governing parameters of the collision process varying within a wide range. It is shown that the initial orientation of these crystals in the aerosol flow over the body influences the trajectories of their movement in the vicinity of the body.

Retrieving Fall Streaks within Cloud Systems Using Doppler Radar

AbstractThe interaction of ice crystals with supercooled liquid droplets in mixed-phase clouds leads to an enhanced growth of ice particles. However, such processes are still not clearly understood although they are important processes for precipitation formation in midlatitudes. To better understand how ice particles grow within such clouds, changes in the microphysical parameters of a particle population falling through the cloud have to be analyzed. The Transportable Atmospheric Radar (TARA) can retrieve the full 3D Doppler velocity vector based on a unique three-beam configuration. Using the derived wind information, a new fall streak retrieval technique is proposed so that microphysical changes along those streaks can be studied. The method is based on Doppler measurements only. The shown examples measured during the Analysis of the Composition of Clouds with Extended Polarization Techniques (ACCEPT) campaign demonstrate that the retrieval is able to capture the fall streaks within different cloud systems. These fall streaks can be used to study changes in a single particle population from its generation (at cloud top) until its disintegration. In this study fall streaks are analyzed using radar moments or Doppler spectra. Synergetic measurements with other instruments during ACCEPT allow the detection of liquid layers within the clouds. The estimated microphysical information is used here to get a better understanding of the influence of supercooled liquid layers on ice crystal growth. This technique offers a new perspective for cloud microphysical studies.

The interaction of ice crystals with hailstones in wet growth and its possible role in thunderstorm electrification

Charge separation during rebounding collisions of ice crystals on hail particles is a probable mechanism of thunderstorm electrification. Modelling of laboratory results successfully predicts the charge structure in thunderclouds under most conditions. An exception is the intense positive ground flashes observed in mid‐continental severe storms which contain large hailstones that are generally in wet growth. It is difficult to see how interacting ice crystals may separate from the wet surface of a hailstone without sticking, although some studies have reported charge separation during this process. Our laboratory experiments show that, during ice crystal interactions with a simulated hailstone in wet growth, a small but significant charge transfer occurs during ice crystal interactions. We present evidence to show that the hailstone surface is not uniformly wet and consists of dry zones, particularly at the edges, where ice crystals are most likely to impact, and hypothesize that a small number of ice crystals rebounding off these zones may give rise to the observed charging current. While this may explain wet growth charging in laboratory experiments, we cannot directly extend the findings to natural thunderstorms where hailstones tend to rotate and gyrate as they fall, but they may experience different growth regimes within the cloud.

Density and surface temperature of graupel and the charge separation during ice crystal interactions

Significant amounts of charge are separated when vapor‐grown ice crystals interact with riming graupel a mechanism widely believed to be responsible for the generation of electric charge in thunderstorms. This study shows that the density of graupel, by itself, is not an effective parameter in determining the sign of the separated charge. The well‐known charge sign reversal temperature is found to be strongly influenced by the surface temperature of the rime at low cloud water content (CWC). At a cloud temperature of −10°C and a CWC of 0.4 g m−3 the charging current to the graupel reversed from positive to negative as the surface temperature of the graupel exceeded −6°C. The maximum CWC at which the sign could thus be reversed increased as the cloud temperature decreased, ranging from less than 0.7 g m−3 at −10°C to less than 0.9 g m−3 at −20°C. At higher cloud water contents the rime surface temperature had no effect on the charging sign. The observations are shown to be broadly explicable in terms of the hypothesis where, during an interaction between two ice particles, the particle that is growing faster from the vapor acquires the positive charge.

The Heat Balance of a Riming Graupel Pellet and the Charge Separation during Ice–Ice Collisions

It has been suggested that the sign of charge acquired by a riming graupel pellet during ice crystal interactions depends on its surface state being negative when it is evaporating and positive when growing by vapor diffusion. Experiments were conducted to determine the surface states of riming cylinders in two previous laboratory studies of thunderstorm electrification. Calculations of the heat balance of the simulated graupel pellets are considered in terms of experimental measurements of the charge transferred under similar conditions. The results show that the contribution from in-cloud supercooled droplets is not always adequate to enable the graupel surface to grow by vapor diffusion in all laboratory cloud conditions where it acquires a net positive charge. It is suggested that factors such as the vapor fields around droplets freezing on its surface and the shape and surface roughness of the graupel may play important roles in determining the surface state and hence the overall sign of the charge transfer. Furthermore, the graupel charges negatively under specific conditions where it is clearly growing from the vapor, confirming that it is imperative to consider the relative growth rate of the two interacting surfaces rather than that of the graupel alone. It is concluded that, during the interaction of two ice particles, the positive charge is generally acquired by the surface that is growing faster or evaporating slower at the point of impact. A suitable physical mechanism may be provided by the liquidlike layer hypothesis of Baker and Dash, and it is suggested how it may be extended to include the cases when one or both of the surfaces are evaporating.

Further laboratory studies of the charging of graupel during ice crystal interactions

Laboratory simulations of thunderstorm electrification by ice crystal collisions with graupel pellets have been extended by the availability of larger crystals than used hitherto. A new cold-room facility permits crystals up to 800 μm to be grown, and these have been interacted with ice targets in the presence of supercooled water droplets leading to substantial charge transfer. The results show that the previously reported strong dependence of charge transfer on crystal size for small crystals is not applicable to large crystals. A velocity-dependence of the charge transfer has been determined, and a function, Q = Ad a V b , describing the charge transfer Q, is given where d is the crystal size, V the speed and A takes certain values for particular ranges of ice crystal sizes. This equation can be used with the appropriate values of A, a and b determined from these experiments, to calculate charging rates in thunderstorms. A new theory of charge transfer due to charged dislocations on the interacting ice surfaces is outlined.

An experimental study of effects of soluble salt impurities on ice crystal processes during growth

Experimental investigations of phenomena associated with the growth and interaction of ice crystals during free-fall in supercooled liquid water clouds are presented. This report focuses on the effects that included inorganic salts have on the microphysical processes. Dilute aqueous solutions of inorganic salts of typical concentration and chemical composition of atmospheric cloud and precipitation water were used to form supercooled liquid water clouds in a cloud chamber. Subsequent nucleation of ice produced a cloud of rapidly growing ice crystals which simulated atmospheric ice crystal initial growth and interaction. The investigations found that the salt composition affected: (1) ice crystal morphology; (2) orientation of aggregate junctions; and (3) probability of secondary ice formation occurring during an experiment. The results were reproducible and were not as strongly affected by other experimental variabilities as by the chemical composition of the aqueous solution. A conceptual model, based upon the observed freezing potential (the Workman and Reynolds effect) which results from freezing of dilute aqueous solutions, is advanced which explains the observed phenomena.

Charge transfer during single crystal interaction with a rimed target

Laboratory experiments were carried out to investigate the charge transfer resulting from a limited number of growing ice crystal interactions with a previously rimed target. Crystal sizes up to 200 μm were used for the study. It was observed that the sign of the charge transfer was controlled by the relative growth rates of the interacting particles with the faster growing particle charging positively. The charge transfer magnitude was of the same order as was obtained in multiple ice crystal collision experiments, thus giving confidence to those measurements.

Charge transfer during multiple large ice crystal interactions with a riming target

Laboratory studies of charge transfer between ice crystals and riming soft hailstones (graupel) have been extended using ice crystals of several hundred micrometers in diameter. The sign of the charge transfer is affected by cloud water content and temperature, being positive to soft hailstones at temperatures higher than about −18°C with typical values of cloud liquid water content. The d4 dependence of charge transfer on crystal size reported previously for smaller ice crystals decreases with larger crystals. The new data can be included in numerical models of thunderstorm electrification.

Ice crystal interactions with a riming target: Charge transfer and collection efficiencies

Recent laboratory studies have shown that the sign and magnitude of electric charge transferred to a riming pellet of soft hail when ice crystals collide then separate are sensitive functions of the cloud liquid water content and the temperature. Certain differences between these results, obtained with crystal/rimer relative velocities of ∼3 m s−1, and those of other workers at ∼10 m s−1 have been reconciled in the present work in terms of a dependence of the sign of the charge transfer on velocity. By measuring the charge transferred to two ice targets of different size, an estimate has been made of the product of collision and separation probabilities of crystals interacting with a 5‐mm‐diameter target.

Laboratory studies of the charging of soft-hail during ice crystal interactions

Laboratory studies of the charging of soft-hail during ice crystal interactions

Laboratory studies of the charging of soft‐hail during ice crystal interactions

AbstractA laboratory investigation of electric charge transfer during the impact of vapour‐grown ice crystals and supercooled water droplets upon a simulated soft‐hailstone target has shown that the magnitude of the charge transferred to the riming surface when crystals separate from it is a function of temperature, crystal dimension, relative velocity, liquid water content, and impurity content of the water droplets and hence the impurity content of the riming target. The sign of the charge transfer depends on temperature, liquid water content and droplet and rime impurity content.In the absence of crystals, no charge transfer was detected during riming. In the absence of supercooled water droplets, crystals impacting at 10ms1 on an evaporating rime target produced a small negative charge on the rime of less than − 0.25fC per separating crystal. When the target surface grew by vapour diffusion it gained a small positive charge during such interactions. Much larger charges and completely different charge transfer behaviour was noted during riming. The target became positively charged at high liquid water contents and temperatures above a critical value, but negatively charged at lower temperatures or with lower liquid water contents. The critical sign reversal temperature at a liquid water content of 1 gm−3 was about − 20°C. At − 10°C with a liquid water content of 2gm−3, a 125 μm crystal impacting at 3ms−1 charged the target by +101C upon separation. The charge transfer increased sharply with impact speed and crystal size. Warming the positively charging rime to cause it to evaporate failed to reverse the sign of the charge transfer. Experiments with impurities showed that the sign reversal temperature increased if the droplets contained contaminants at concentrations found in cloud water.It is suggested that there are two distinct charge transfer processes during crystal interactions with an ice target, the dominant one requiring the presence of supercooled water droplets. Careful control and knowledge of the microphysical properties of the clouds used in these experimental simulations has permitted an examination of charge transfer under many of the conditions used in previous studies. The results provide an understanding of the differences and a reconciliation between some of the previously disparate findings in terms of the two distinct charge transfer regimes.

Aerodynamic shattering of ice crystals in hypersonic flight

2 ~* } but not until recently has significant effort been devoted to the interaction of ice crystals with the shock layer. Wu12 has studied the melting and vaporization of an ice crystal in a hypersonic shock layer and demonstrated a significant reduction in particle size and impact velocity for ice crystals initially of the order of 10-jLt diameter. Larger crystals were relatively unaffected by melting and vaporization. In the same report, Teare12 demonstrated that ice crystals may shatter at the vehicle bow shock. This introduces the possibility that the fractured ice crystal or may deform as it passes through the vehicle shock layer. This deformation will be lateral to the relative velocity and will tend to increase the drag area of the fragment cloud. Hence, the velocity of the fragment cloud will decrease more rapidly than that of a particle, and the velocity with which the fragment cloud impacts the vehicle may be significantly lower than that with which a rigid ice crystal would impact the vehicle. To study this phenomenon, we examine the interaction of the ice crystal with the shock layer over three distinct time scales: fracture, breakup, and deformation. The first time scale is the time required for a stress wave to propagate across the ice crystal and generate multiple fractures. Analysis of this event requires the development of a dynamic fracture criterion. Such a criterion is developed in Sec. II. We consider a monochromatic stress wave and determine an appropriate monochromatic yield criterion. We then assert that a dynamic yield criterion may be obtained by Fourier expanding the applied load and applying the monochromatic yield criterion to each wave. Using this basic hypothesis, we determine the maximum size (Q of the ice crystal fragments allowed by the highest mode that can just fracture the ice crystal. The behavior of a cloud or gas of such fragments is needed to describe the motion of the fractured ice crystal through the vehicle shock layer.