Measurements of natural ice nuclei with a continuous flow diffusion chamber

Measurements of natural ice nuclei were made in winter continental airmasses with a continuous flow thermal gradient diffusion chamber (described in a separate paper). Over the range of temperatures −7°C to −20°C, the concentration of ice nuclei was closely related to ice supersaturation (SS_i) for humidities both below and above water saturation. Measurements below water saturation were interpreted as deposition nuclei with average concentrations (per liter) approximately 0.32 SS_i(%)^0.81. Measurements were made up to 5% above water saturation and activated both deposition and condensation-freezing nuclei. The average concentration of condensation-freezing nuclei was 0.25 e^{−0.15 T(°C)}. Sample residence time in the chamber was probably too small to detect contact nuclei, unless the nucleating aerosols are extremely small. There was large variability in nucleus concentrations, as much as two orders of magnitude at −15°C. Comparisons are made between these ice nuclei measurements and aircraft observations of ice crystal concentrations in winter orographic clouds.     

Water droplet growth study in a continuous flow diffusion cloud chamber

A continuous flow diffusion cloud chamber has been used to study the growth of water droplets on NaCl aerosols of known size. Settling distances of droplets in the chamber have been recorded and compared to the theoretical ones generated on a computer. Using the droplet growth theory of Fukuta and Walter and a thermal accommodation coefficient of 1.0, the condensation coefficient was found to be 0.01.     

An analysis of the filter diffusion chamber and drop freezing methods of determining ice nucleus concentrations

The membrane filters used for sampling at a height of 60 m and at ground level were processed with a drop freezing technique and a static vapor-diffusion chamber. Comparisons of IN concentrations obtained by the two techniques are made. The blank filter counts and ice nucleus measurement reproducibility are analysed. The factors affecting IN concentration measurements are discussed and some favorable operation conditions are suggested. It has been found that: (a) the IN concentrations typically range from 0.2 L-1 to 2.0 L-1 at -20oC, (b) significant time variations in IN concentrations occurs, (c) the IN concentra-tions obtained by the drop freezing technique typically exceed the value given by the diffusion chamber, (d) their variation trends are generally in phase, (e) high IN concentrations are often associated with pre-cipitation events.     

Studies of ice nucleation by Arctic aerosol on AGASP-II

Aerosol particles were collected on filters for studies of their ability to nucleate ice during the second Arctic Gas and Aerosol Sampling Program (AGASP-II) in April, 1986. The ice nuclei (IN) samples were collected from an aircraft at altitudes ranging from the surface to the vicinity of the tropopause in Arctic locations over Alaska, northern Canada and Greenland. Samples of other components of the aerosol were collected and measurements were made of other properties of the aerosol coincident in time with the IN samples. The IN filters were exposed to water saturation in a dynamic developing chamber at –15° C and –25° C. Ice crystals grew on the IN and were counted on the filters at discrete time intervals during the exposure period to determine the rate of ice nucleation and the final concentration of (IN). Results show that Arctic haze aerosol, identified by pollutant signatures, had lower IN concentrations, a lower IN to total aerosol fraction and slower ice nucleation rates than aerosol which had a chemical signature more indicative of the remote unpolluted troposphere. These observations suggest that the Arctic haze aerosol does not efficiently form ice in the arctic troposphere. This may be a factor contributing to the long-range transport of Arctic haze.     

A role of aerosol particles in atmospheric ice nucleation

Considered is the process of ice nucleation in the atmosphere as a result of heterogeneous condensation of water vapor in surface inhomogeneities of aerosol particles and of subsequent heterogeneous crystallization of supercooled water accumulation. It is revealed that the size, structure, and composition of aerosol particles determine the thermal regime of crystallization.     

On Mechanisms of Nucleation of Ice Crystals by Aerodynamic Cooling

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A numerical diffusion model for continuous releases

The purpose of this study was to develop a diffusion model for a continuous point source which takes into account the increase of wind speed with height, and to compare this model with short-range diffusion experiments. The main problem was to find a good expression for the vertical diffusion coefficient. It turned out that good agreement between theory and experiment could only be obtained by introducing a settling speed W for the tracer combined with a conventional expression for the vertical diffusion (K(Z) = K ₀Z^1−p). An empirical relation was found between K ₀ and τ _vU and between W and bar σ _vU².     

Radiometer studies of low-salinity sea ice

Theoretical and experimental studies of ultra-high-frequency (UHF) and microwave radiometry to sense remotely the thickness and other characteristics of low-salinity (less than one per mille) sea ice are described. The experimental studies used a 600-MHz and a multichannel, multibeam, 4.7-GHz radiometer carried by a helicopter to measure the brightness temperature of different types of sea ice in the Gulf of Bothnia in 1975 and 1976.The developed theory and experiments show that the brightness temperature of low-salinity sea ice oscillates as a function of ice thickness at UHF frequencies. Due to higher attenuation, the oscillations quickly die away at 4.7 GHz and the brightness temperature reaches an almost constant value when the thickness exceeds a few decimetres. By using a three-channel UHF radiometer with suitably selected center frequencies, the oscillations can be reduced and the effective brightness temperature will grow more linearly with the ice thickness.Experiments show that ice ridges appear as thick ice at 600 MHz and as thinner ice at 4.7 GHz, thus allowing them to be detected by radio techniques.     

Impact of primary ice nucleation parameterizations on the formation and maintenance of cirrus

Different ice nucleation algorithms are implemented in a cloud microphysical scheme and numerical simulations of clouds are performed using a three-dimensional mesoscale model. The predicted ice crystal fields are found to be sensitive to the different modes of calculation of the number of deposition/ condensation freezing nuclei and contact freezing nuclei. Also a time and supercooling dependence of this sensitivity is established.The general features of the cirrus clouds observed by the research aircraft Falcon during the 1989 (International Cirrus Experiment) mission ICE212 are compared to those of the cirrus clouds generated by the model. The cloud top height, the cloud ice content and the ice number concentrations seem to be reproduced well.     

降雨弹成核率鉴定的误差分析

“三七”高炮降雨弹广泛应用于我国人工影响天气的作业。1978年降雨弹检测小组在重庆对降雨弹的成核率进行了鉴定,发现1克Agl降雨弹的成核率不低于现用降雨弹。本文对重庆实验进行了误差分析,认为此实验具有一定的可信度。如果将现用“三七”高炮降雨弹的配方改成1克Agl,每年将为国家节约几十万元。     

冰晶核化对雷暴云闪电行为影响的数值模拟研究

为了探讨冰晶核化对雷暴云闪电行为的影响,通过已有的三维对流云起、放电模式探讨对比了3种冰晶核化方案,分别为原模式中的经验公式YS方案及与气溶胶相关的DE方案和LP方案。研究表明冰晶核化方案对雷暴云内冰晶微物理发展特征、起电及放电过程均有一定影响。模拟结果显示:(1)考虑了气溶胶的两种新参数化方案中冰晶粒子在高温区(高于-13.8℃)出现,在雷暴云发展过程中DE方案和LP方案冰晶的垂直分布均大于YS方案。(2)DE方案和LP方案中高温区出现的冰晶所带电荷极性有明显的反转现象,导致雷暴云电荷结构产生差异;雷暴云发展旺盛时刻DE方案和LP方案出现三级性电荷结构,而YS方案在整个雷暴云过程都是偶极性,并且DE方案和LP方案中电荷空间分布区域更加广泛。(3)不同核化方案下雷暴云放电特征存在差异,YS方案在偶极性电荷结构背景下没有负地闪产生,而DE方案和LP方案中次正电荷区的存在促进了负地闪的发生,并且负先导出现在较低的高度范围内;DE方案和LP方案中电荷量级较大,因此云闪发生频次以及正、负先导传播次数增加明显。     

A new model of thermal diffusion in a translucent medium

The thermal diffusion equation is solved analytically for an ideally translucent and homogeneous medium with an internal, distributed heat source given by the penetration and absorption of solar radiation. The mean annual temperature profile has the coldest values at the surface; the mean annual flux of sensible heat out of the medium balances the internally absorbed solar radiation for the case of constant mean annual temperatures. A new formula is developed for the evaluation of the true thermal diffusivity of a non-homogeneous conductor from the Fourier components of the waves of insolation and temperature; a constant extinction coefficient is assumed. The new model is applied with moderate success to data from the coastal plateau ice at Mawson, Antarctica.National Research Council Postdoctoral Resident Research Associate. Present Affiliation: Stanford Research Institute, Menlo Park, California 94025.     

Sensitivity of a Greenland ice sheet model to ice flow and ablation parameters: consequences for the evolution through the last climatic cycle

Sensitivity experiments are conducted to test the influence of poorly known model parameters on the simulation of the Greenland ice sheet by means of a three dimensional numerical model including the mechanical and thermal processes within the ice. Two types of experiments are performed: steady-state climatic conditions and simulations over the last climatic cycle with a climatic forcing derived from the GRIP record. The experiments show that the maximum altitude of the ice sheet depends on the ice flow parameters (deformation and sliding law coefficients, geothermal flux) and that it is low when the ice flow is fast. On the other hand, the maximum altitude is not sensitive to the ablation strength and consequently during the climatic cycle it is driven by changes in accumulation rate. The ice sheet extension shows the opposite sensitivity: it is barely affected by ice flow velocity and the ice covered area is smaller for large ablation coefficients. For colder climates, when there is no ablation, the ice sheet extension depends on the sea level. An interesting result is that the variations with time of the altitude at the ice divide (Summit) do not depend on the parameters we tested. The present modelled ice sheets resulting from the climatic cycle experiments are compared with the present measured ice sheet in order to find the set of parameters that gives the best fit between modelled and measured geometry. It seems that, compared to the parameter set most commonly used, higher ablation rate coefficents must be used. Received: 19 September 1995 / Accepted: 30 May 1996     

Depositional ice nucleation on NX illite and mixtures of NX illite with organic acids

Mineral dust particles are known to be efficient ice nuclei in the atmosphere. Previous work has probed heterogeneous ice nucleation on various laboratory dust samples including Arizona Test Dust, kaolinite, montmorillonite, and illite as atmospheric dust surrogates. However, it has recently been suggested that NX illite may be a better representation of atmospheric dust. Hiranuma et al. (2015) performed a laboratory comparison for immersion ice nucleation on NX illite, but here we focus on depositional ice nucleation because of its importance in low temperature cirrus cloud formation. A Raman microscope setup was used to examine the ice-nucleating efficiency of NX illite. Organic coatings on the NX illite particles were also investigated using a mixture of 5 dicarboxylic acids (M5). The ratio of NX illite to M5 was varied from 1:10 to 100:1. It was found that NX illite efficiently nucleates ice with S_ice = 1.07 ± 0.01 at ?47 °C, with S_ice slightly increasing at lower temperatures. In contrast, pure M5 is a poorer ice nucleus with S_ice = 1.30 ± 0.02 at ?40 °C, relatively independent of temperature. Further, it was found that M5 coatings on the order of several monolayers thick hindered the ice nucleating ability of NX illite. Optical images suggest that at colder temperatures (< ?50 °C) 1:1 NX illite:M5 particles and pure M5 particles nucleate ice depositionally, while at warmer temperatures (> ?50 °C) subsaturated immersion ice nucleation dominates. These experiments suggest that mineral dust particles may become less active towards ice nucleation as they age in the atmosphere.     

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.     

On gradient transport turbulence models for stably stratified shear flow

The gradient transport model for stably stratified horizontal shear flow in which eddy diffusivity and viscosity are assumed to depend on the gradient Richardson number, Ri, is augmented with terms representing a finite adjustment time of the exchange coefficients. Barenblatt et al. (J. Fluid Mech., 253: 341–358, 1993) showed that using such a model, initial value problems for the formation of a stepwise structure of the buoyancy distribution are well posed. The model proposed is analysed taking into account the interaction between buoyancy and velocity fields. A condition for the formation of steps is derived from a linear stability analysis. Numerical computations show that a realistic stepwise finestructure develops, provided linear instability is allowed on a finite interval of Ri only.     

An approximate analytical solution to the diffusion equation for short-range dispersion from a continuous ground-level source

An easily-evaluated expression for the dimensionless concentration profile (z/z ₀,/z ₀, z ₀/L) = = cu _*/kQ (or z ₀cu_*/kQ) downwind of a continuous ground-level area (or line) source in the stable surface layer is obtained by integrating the diffusion equation using the Shwetz approximation method (c = concentration, Q = source strength, k = von Kárman's constant). The analytical solution compares closely with concentration profiles obtained using a trajectory-simulation model over a useful range of heights, the important discrepancies occurring at the upper edge of the plume. The analytical solution is used to generate predictions of ground-level concentration for the Project Prairie Grass experiments; good agreement with the observations is obtained at all downwind distances (50 to 800 m).