A quantitative study on the growth law of ice crystals

On the basis of analysis and study of the growth of ice crystals, the inadequacy and thus a necessity for improvement of the Kohayashi-Pruppacher model are discussed. Be experimenting with a new wedge-shaped chamber of ice thermal diffusion which provides a stable environment and a wide range of control, a great amount of quantitative data from three-dimensional size measurements have been obtained. A quantitative research method is established, yielding, for the first time, satisfactory quantitative results in the region of low ice supersaturation. These results show that the Kobayashi-Pruppacher inference is inade-quate, and reveal new characteristics of ice-crystal growth in the Wulff growth region, the region of abrupt change of crystal forms and that of low ice supersaturation.     

典型非球形冰晶粒子的凝华增长数值模拟试验

在凝华增长过程中,冰晶的形状随着温度和湿度的改变而改变,准确模拟冰晶粒子的演变对于提高云模式的模拟能力起着非常重要的作用。在现有的云模式中,冰晶形状通常假设为球形,而在实际大气中,冰晶形状十分复杂。本研究中,我们根据冰晶凝华增长理论模型建立了一个单个冰晶粒子增长模型,模拟了温度分别为-1℃~-30℃时,单个典型非球形冰晶粒子的凝华增长过程。与风洞观测数据相对比,该模型能够抓住单个冰晶粒子的轴长,质量以及纵横比随温度和湿度的变化过程。我们进一步将该理论增长模型应用到群粒子的凝华增长过程的模拟。我们釆用欧拉二维正定平流输送法(MPDATA)模拟了典型非球形冰晶群粒子的凝华增长,并对比分析了在不同纵横比分辨率下的模拟效果以及温度变化对冰晶形状的影响,结果表明运用该数值方法可以合理地模拟出群粒子在凝华增长过程中纵横比的演变。与目前采用的拉格朗日-欧拉混合平流算法比较,该算法能够耦合到欧拉动力框架下的分档云模式中去,这对我们研究冰晶粒子形状对云微物理过程和动力过程的影响,以及它们对冰粒子凝华增长的反馈作用具有非常重要的科学意义。     

A comparison of model calculations of ice crystal growth with observations following silver iodide seeding

Abstract

Past theoretical investigations have reported that observed peaks in the diffusional growth of ice crystals in the vicinity of ‐5 and ‐15°C could be qualitatively modelled assuming certain specific crystal properties. This study re‐examines the ice crystal diffusional growth model using more recently available observations of pertinent crystal parameters. The results, indicative of the growth of average crystals, imply a much weaker dependence of the diffusional mass growth rate on temperature in the range of ‐5 to ‐15°C. Extension of the model to include growth by accretion, also suggests that the mass of heavily rimed crystals is not very sensitive to the original crystal habit. The results of the calculations are compared with observed crystal sizes in cumulus clouds following seeding on three different days. The observations indicate that small crystals were continuously being produced as long as 15 min after seeding. In two cases, crystals much larger than those predicted by the model for any reasonable set of parameters suggest that crystal aggregation may have been occurring at a significant rate.     

Enhanced alignment of plate ice crystals in a non-uniform electric field

An atmospheric electric field exerts a torque on both column and plate ice crystals. Columns tend to align along the direction of the field. One diameter of plate crystals also aligns parallel to a field, while a non-uniform diverging field leads to further orientation and proscribes a secondary orientation along the direction of least divergence. A laboratory investigation treats the electrical orientation of thin plate ice crystals in a non-uniform field as a two-step process. In the model, the average field provides the primary torque and aligns one diameter of the plate crystal while simultaneously a spatially varying component of the field aligns the perpendicular diameter. The first element of the process, the primary torque, has been investigated previously. The complete reorientation, caused by a secondary torque, is investigated herein. Experimentally a stronger field with sufficient non-uniformity is required to demonstrate the secondary torque. The realignment caused by this torque depends also on the strength of the aerodynamic torques on the falling ice crystals. Experiments in a laboratory cold chamber with small thin plate crystals in an electric field provided by a charged rod demonstrate crystal reorientation by changing the reflections of a beam of white light. A model incorporating electrical torques is applied to the laboratory observation and used to investigate the possibility of the effects occurring in the atmosphere. Fields near lightning strokes, which are strong but of short duration, may reorient small crystals. Larger crystal aggregates like planar snowflakes are governed by the same electrical torques. It is suggested that the secondary torque realignment might be observable for aggregates large enough to be detected by radar in cases pertaining to weaker but longer duration fields associated with cloud electrification. A simplification of the model is used to investigate electrically caused crystal motion and linear aggregation, with applications to the atmosphere and to lightning initiation.     

A numerical study of the effects of cloud droplets on the diffusional growth of snow crystals

We studied the effect of supercooled cloud droplets on the diffusional growth rate of isolated ice plates and ice columns by approximating the shape of a plate as an oblate spheroid and a column as a prolate spheroid. This work is an extension of the pioneering work of Marshall and Palmer [Marshall, J.S. and Langleben, M.P., A theory of snow-crystal habit and growth, J. Met., 11, 104–120, 1954] who studied the influence of cloud droplets on the growth rate of spherical ice crystals. The vapour density field for oblate and prolate spheroids was obtained by solving Helmholtz's equation with the assumption that the vapour at the ice surface had the equilibrium value. As result, it was possible to calculate separately the temporal evolution of the axis length and the mass of the snow crystals.The results indicate that the presence of cloud droplets around the snow crystal can increase by around 10% of the ice growth rate relative to an environment free of cloud droplets. The magnitude of the growth enhancement is proportional to the liquid water content. The results also corroborate the importance of the shape and aspect ratio on the crystal growth. A general mass growth rate equation for diffusional growth was found, which incorporates effects of cloud droplets and crystal shape through the size and aspect ratio of the snow crystal.     

Two-moment bulk stratiform cloud microphysics in the grid-point atmospheric model of IAP LASG (GAMIL)

A two-moment bulk stratiform microphysics scheme, including recently developed physically-based droplet activation/ice nucleation parameterizations has been implemented into the Grid-point Atmospheric Model of IAP LASG (GAMIL) as an effort to enhance the model’s capability to simulate aerosol indirect effects. Unlike the previous one-moment cloud microphysics scheme, the new scheme produces a reasonable representation of cloud particle size and number concentration. This scheme captures the observed spatial variations in cloud droplet number concentrations. Simulated ice crystal number concentrations in cirrus clouds qualitatively agree with in situ observations. The longwave and shortwave cloud forcings are in better agreement with observations. Sensitivity tests show that the column cloud droplet number concentrations calculated from two different droplet activation parameterizations are similar. However, ice crystal number concentration in mixed-phased clouds is sensitive to different heterogeneous ice nucleation formulations. The simulation with high ice crystal number concentration in mixed-phase clouds has less liquid water path and weaker cloud forcing. Furthermore, ice crystal number concentration in cirrus clouds is sensitive to different ice nucleation parameterizations. Sensitivity tests also suggest that the impact of pre-existing ice crystals on homogeneous freezing in old clouds should be taken into account.     

六棱柱和三棱锥冰晶的散射相矩阵——一种矢量射线追踪方法

本文发展了作者过去的工作(Cai-Liou),利用改进后的数值模式对六角柱冰晶(匀谱和非匀谱)的散射相矩阵进行了计算,所需计算量比Cai-Liou的约小二个量级,且本文的数值模式的结果误差较小。同时,对三棱锥冰晶的散射相矩阵,本文也进行了计算,并与六角冰晶的结果进行了比较。     

Development of a new sea ice growth and lead parameterization

A thermodynamic sea ice model that has been numerically structured to take time steps on the order of a week has been shown to be sensitive to time step size. This sensitivity was caused by the extrapolation of initial ice growth rates over the long time step. A new parameterization of new sea ice growth on open ocean and in leads that can be used over a large range of time step sizes (at least from 0.3 to 12 days) is described here. In this parameterization new sea ice growth is computed as a power law function of the initial energy deficit in the ocean. This power law takes into account the rapid reduction of the ice growth rate as the sea ice gets thicker, and therefore reduces sensitivity to time step size. Tests of this parameterization show that this method does a good job of simulating the rate of new ice growth when compared to data from Mawson, Antarctica, and is relatively insensitive to the length of the time step.     

A modelling study of the effect of ice particle sizes and relative velocity on ice crystal/graupel collisional charge transfer

The effect of ice particle sizes and relative velocity on the sign of charge transfer during graupel/ice crystal interactions in thunderstorms is considered based on the concept of the Relative Growth Rate Hypothesis that the ice surface growing faster by water vapour diffusion charges positively. Diffusional growth rates of ice crystals and riming graupel particles are calculated for various particle sizes and velocities, and the sign of charging during crystal/graupel collisions is determined. The study reveals that larger graupel charges more negatively, while larger ice crystals, or higher relative velocity, lead to increased positive graupel charging.     

冰晶核化对雷暴云微物理过程和起电影响的数值模拟研究

本文在已有的二维对流云模式中采用了一种与气溶胶有关的冰晶核化方案替代原有的冰晶核化经验公式, 并选取个例, 分别就两种方案进行了模拟对比试验。模拟结果表明:(1)新方案所得冰晶比含水量主要分布在-0.1~-7.6℃温区之间, 高于原方案所得的-50.1~-24.2℃温区;在整个雷暴云的发展过程中新方案冰晶的分布高度、温度区间以及最大浓度值均大于原方案。(2)在新方案中, 温度相对较高的过冷区产生大量冰晶, 其争食云中水汽抑制了云滴、雨滴的增长。此外, 与原方案相比, 霰增长受雨滴大幅减小的影响进一步得到限制, 导致生成的霰小于原方案, 且空间分布具有较大区别。(3)两方案在雷暴云初期形成的电荷结构不同;在发展旺盛与消散阶段新方案中电荷空间分布区域和电荷量均大于原方案, 此外, 在不同时刻主正电荷区和主负电荷区的中心高度存在差异。本文对云微物理过程及起电的分析为后继探讨气溶胶与雷暴云起放电过程、电荷结构之间的相互关系提供了有利条件。     

光吸收对冰晶粒子散射相函数的影响

本文利用射线跟踪法对冰晶粒子散射、吸收进行了研究。从理论上对一些常见的近似处理方法进行了讨论，计算了长为300μm，半径为30μm的六棱柱状冰晶粒子在不同波段的光散射相函数以及在波长2.2μm时，五种不同大小冰晶粒子的单次散射反照率。结果表明，即使在有吸收情况下，Snell公式和Lambert公式仍是近似成立的，而吸收效应对散射相函数有明显影响。     

双参数微物理方案的冰相过程模拟及冰核数浓度的影响试验

利用耦合Morrison 2-mon(MOR)双参数微物理方案的中尺度天气研究与预报模式(WRF)中的单气柱模式,对热带暖池国际云试验(TWP-ICE)期间的个例进行数值模拟。通过与观测资料和云分辨率模式的模拟结果进行对比,检验MOR方案对热带对流云系的微物理特征的模拟能力。模拟结果显示:MOR方案能够较好地模拟出热带云系中液相和冰相水凝物的垂直分布以及随时间的演变特征。地表向下长波辐射和大气顶向外长波辐射的量级和时间演变趋势同观测也非常接近。对与冰晶和雪有关的云微物理特征分析之后发现:季风活跃期,冰晶主要的源汇项有凝华增长过程、沉降过程、冰晶向雪的自动转化以及冰晶被雪碰并的过程。由于冰晶主体位于温度低于―20℃的高空,因而它对雨水的形成主要是间接贡献。同时期雪的主要源汇项中,凝华增长和沉降过程占据着主导地位。雪的凝华过程消耗了大量的水汽,可能抑制了冰晶的增长。另外雪的融化过程非常强盛,是产生降水的重要因子。季风抑制期,冰相的微物理过程变得相对简单且整体削弱,以凝华升华和沉降过程为主。凝华凝冻核的数浓度(N_dep)的气溶胶敏感性试验表明:季风抑制期,高空的冰晶云的宏观和微观性质对凝华凝冻核数浓度的响应情况呈现显著的线性特征。冰晶的含量随着N_dep的增加而增加,反之降低。该时期微物理过程主要同冰晶有关,水分的分配较为简单,N_dep增加时,高空冰云中小冰晶粒子数目增多且云顶升高,使得大气顶部向外长波辐射(OLR)值减小,反之冰云主体中冰晶有效半径增加,高空的冰云更加透明,云顶更低,对 OLR值增加起促进作用。而季风活跃期,微物理过程复杂,冰晶云的宏微观特征对N_dep的响应表现出一定的不规律特征。     

冰雪晶碰并勾连增长的实验与观测分析

冰雪晶碰并勾连成雪花或雪团的过程是降水的重要机制之一。冰雪晶形态不同导致各类晶动力特性的差异, 因此, 无论从理论、数值试验还是室内模拟研究这一过程都有很大困难。该文就室内、外场试验和自然云的观测, 讨论分析其发生机理和条件。结果表明:冰晶的增长过程有一个与云滴碰并增长相似的加速过程, 冰晶碰并过冷滴形成霰 (雹胚), 凇附长成冰雹; 冰雪晶相互碰并勾连、攀附增长为雪花或雪团, 都是降水质点加速增长的重要过程。此过程仅在水面饱和、过饱和条件下发生, 而水面欠饱和、无液滴 (无云) 时, 冰晶很薄、晶型简单, 无碰并勾连、攀附现象。冰晶在液滴存在的云雾中伴随气流对流、乱流运动中接触碰并勾连成雪花或雪团, 其碰并勾连效率既受晶体形状影响, 亦受晶体表面附着力的影响, 其机制有勾连亦有粘连, 晶型多样, 以相同晶型为主, 温度范围广 (-3~-17 ℃); 其中-13~-17 ℃碰并勾连效率最高, 该层的枝、星状晶是勾连、攀附的主要区域, 亦为冰晶繁生的主要区域、生长率最快, 是人工增雨播撒人工冰核催化效率较高的温度段。     

双偏振雷达对一次水凝物相态演变过程的分析

联合利用3 GHz双偏振雷达RHI探测数据和温度廓线数据, 建立了云粒子相态反演的模糊逻辑算法, 算法采用Beta型成员函数, 成员变量包括:水平反射率因子、线性退偏比、差分反射率及温度0℃,-40℃对应高度, 反演出的相态包括毛毛雨、雨、低密度干冰晶、高密度干冰晶、湿冰晶、干霰、湿霰、小冰雹、大冰雹、雨夹雪和液态云滴等11种, 并利用雷达的连续探测数据对一次层状云降水过程中水凝物相态的演变情况进行了分析, 得到如下结果:初始阶段层状云相态呈现分层结构, 从上至下依次为高密度干冰晶、湿冰晶和液态云滴; 初始阶段云体中的回波大值区核心区域为大的冰相粒子, 其余部分为液态粒子; 在初始到成熟阶段演变中, 回波大值区上部液态粒子逐步向冰相转化; 消散阶段云中零度层亮带逐步消失, 零度层以上云粒子结构呈现高密度干冰晶包裹湿冰晶的情况。关键词:双偏振雷达; 模糊逻辑; 水凝物相态反演; 层状云降水过程; 水凝物相态演变     

An experimental study of aerosol scavenging by hexagonal plate ice crystals

The scavenging efficiency, E, of small hexagonal plate ice crystals for aerosol particles has been measured in a series of laboratory cloud experiments. The ice crystal diameters, D μm ranged from 35 to 150 μm with aerosol particles in the range 4 to 6 μm. An ice crystal replication technique made possible the individual examination of more than 43m500 individual crystals from which the relation: log₁₀ E = 1.554 − 1.047 log₁₀D was established with values of E in the range 0.2–0.9 corresponding to the range of crystal sizes investigated. For some crystal collectors, values of E extended above unity and this was attributed to wake capture, oscillations and the extended sweep out path of the ice crystals associated with their spiral fall pattern.     

Aggregation of small ice crystals in an electric field

Abstract

Small ice crystals with average diameter of about 30 μm are produced in a large cold room and allowed to fall in a settling chamber in the presence of a quasi‐uniform electric field. Aggregates (flakes) of ice crystals are collected by permanent replicas. Results show that an electric field above a threshold value of about 4 × 10⁴ V m^‐1 rapidly increases the growth of flakes by the capture of small ice crystals. The influence of the electric field upon the growth of ice aggregates is maximum at a field strength of about 1.5 × 10⁵ V m^‐1. Comparison of the results with Jiusto's mathematical model of the growth rate gives values of the collection efficiency at different field strengths. It is very likely that the electric field increases the adhesion (aggregation) efficiency rather than the collision (cross‐section) efficiency.     

A modelling study of the effect of cloud saturation and particle growth rates on charge transfer in thunderstorm electrification

Numerical studies have been made of the importance of cloud saturation to the sign of charge transfer during graupel/ice crystal interactions in thunderstorms. Previous laboratory studies led to the idea that the diffusional growth rates of the interacting ice surfaces may influence the sign of the charge transferred during brief collisional contact. The ice crystals grow by vapour diffusion in a supersaturated environment while the graupel surface grows by diffusion under low accretion rate conditions, but will sublimate when heated sufficiently by riming. The graupel surface is also influenced, even under net sublimation conditions, by the vapour released to it from droplets freezing on its surface. In a cloud, the diffusional growth rates are also affected by ventilation when the supercooled droplets and their local environment flow past the riming surface.The diffusional growth rates of ice crystals and riming graupel particles are calculated for various cloud saturation ratios, then the sign of electric charge transferred during crystal/graupel collisions is determined according to the concept of the relative vapour diffusional growth rates, according to Baker et al., 1987 [Baker, B., Baker, M.B., Jayaratne, E.J., Latham, J., Saunders, C.P.R., 1987. The influence of diffusional growth rates on the charge transfer accompanying rebounding collisions between ice crystals and soft hailstones. Quart. J. Roy. Met. Soc. 113, 1193–1215]. It is found necessary, in order to account for the observation of positive charging of riming graupel at high accretion rates, to modify the assumptions of Baker et al. in order to increase the flux of vapour to the graupel surface. The variable growth parameters available may be adjusted to represent the environmental saturation conditions in various laboratory experiments, including the mixing of clouds from regions having different growth conditions, and are used to determine charge sign sensitivity to cloud saturation ratio, temperature and accretion as measured by the cloud effective liquid water content.     

SCATTERING PHASE MATRICES OF ICE CRYSTALS WITH HEXAGONAL PRISM AND TRIANGULAR PYRAMID FORM—A VECTOR RAY TRACING METHOD

The work presented previously by the authors(Cai and Liou,1982)has been extended in this paper.By making use of our improved model the calculations on scattering phase matrices of hexagonal prismice crystals(HPIC)have been conducted for monodisperse and polydisperse systems.Compared with themodel of Cai and Liou,the required computational quantity is decreased by about two orders of magni-tude and the errors of results are less for the new model.Meanwhile,the scattering phase matrices of triangularpyramid ice crystals(TPIC)are also computed in the paper,and the comparison between the scatterings of thetwo forms of ice crystals is performed.     

冰晶性质对卷云辐射特征影响的模拟研究

介绍了SBDART辐射模式和libRadtran程序包，并利用这两个模式模拟分析了当卷云内冰晶性质如有效半径、冰晶含量以及形状变化时，卷云反照率的变化。模拟分析表明，在相同条件下，不同形状冰晶构成的卷云反照率有所不同，同种冰晶构成的卷云在不同波长的反照率也不同。总的来说，不论是由何种形状冰晶构成的卷云，随着卷云内冰晶含量增大，有效半径减小，卷云反照率增大。这些工作将有助于增进对lib Radtran程序包和卷云性质的了解。     

Comparative Study of Nucleation by Different Alcoholic Solutions of Benzoin, and Benzoin Dust

Nucleation properties of different alcoholic solutions of Benzoin and Benzoin dust have been studied by varying the seeding temperature in a laboratory Cold Chamber. In the present study, three different alcoholic solutions are being used and these are due to the standard Ethyl alcohol, one higher alcohol i.e. Isopropyle alcohol and one lower alcohol i.e. Methyl alcohol. In addition, Benzil dust and droplets of Acetone and Chloroform have been used to examine if these agents have any ice nucleation ability. It has been observed that all alcoholic solutions of Benzoin have sufficient ice nucleation ability at the negative side of 0^°C, with the maximum concentration occurring at −14.4^°C. In case of Benzoin dust, the nucleation ability is better on the positive side of 0^°C than the negative side. On the other hand, Benzil dust has insignificant nucleation ability and the droplets of two other agents have no nucleation ability at all.Considering the chemical as well as crystalline structure, one can conclude that the presence of OH group in the seeding molecule plays the central role in ice nucleation/hydrate crystal formation. In fact, a number of supplementary experiments like crystal growth in an incubator, XRD analysis and NMR study are performed to identify the nature of crystals formed by Benzoin dust above 0^°C. One can conclude that these are hydrate crystals of Benzoin with water caging around Benzoin crystals.