Observations of condensation growth determined by entity type mixing

It has been speculated for many years that the development of the droplet spectra in cloud is probably influenced by mixing processes. Various theoretical attempts to broaden the droplet spectra by mixing parcels with different velocity histories has shown that that particular effect is small. Similarly, very simpleuniform entrainment procedures did not lead to cloud drop size spectra which were broad enough, although by producing cloud drop size distributions with a double mode these models did substantially improve the drop size spectra of earlier adiabatic models which only exhibited a single mode.Recently a model based on entraining entities representing moving parcels of cloud air within the cloud was detailed byTelford andChai (1980). This study showed that the mixing in of dry air at cumulus turrets could lead to vertical cycling of diluted parcels, and that this cycling, with continual entrainment across the parcel boundaries, will produce much larger drops, as well as smaller drops of all sizes, in the droplet spectra. The entity entrainment concept studied there appears to apply to the observations of stratus cloud discussed in this paper.This paper presents data taken in marine stratus off the California coast which give a particularly clear example of how such droplet spectra modification occurs in practice. Both large drops, and the spread of the spectra to smaller sizes, occur in relation to other variables in such a way as to be consistent with an entity entrainment explanation, with no other obvious possibility.In a marine stratus cloud just over 200 m thick and many tens of miles in extent we find clear evidence that dry air is mixing in at cloud tops. Strong vertical motion is to be found in the cloud, large sized drops are found in cloud parcels where the mixing gives lower droplet concentrations, and there is evidence that newly formed cloud parcels are warmer and contain many more smaller droplets.The observations show that immediately following entrainment of dry air drop diameters are not reduced appreciably, but, in the same parcels, drop concentrations have been reduced by a factor of ten or more. Further down in the cloud big drops, able to start growth by coalescence, are found associated with low total droplet concentrations.Overall, it seems likely from the consideration of these observations that the formation of the large drops which lead to precipitation processes in clouds depends critically on the mixing in of dry air at cloud tops, and very little on the size of the small drops resulting from the condensation nucleus counts. As a conclusion it appears reasonable to state that if entrainment occurs at cloud tops, then big drops will be formed!     

A study of the attachment of radon daughter atoms to aerosol particles

Previous studies of the attachment of radon daughter atoms to aerosol particles in the atmospheric air have given varying results, a few of which did not conform to the diffusion theory developed byLassen andRau (1960). Our studies suggest that the radiolytic aerosols formed by gas-phase reactions in the presence of ionizing radiations interfere in the studies carried out with known aerosols. Radiolytic aerosols are formed spontaneously in the system, depending on the concentrations of aerosol-forming gases in the atmospheric air. It is shown that, under experimental conditions which suppressed the formation of radiolytic aerosols, the attachment rates of Ra-A atoms to aerosol particles in the range of 0.2 to 1.0 m in diameter agree with the diffusion theory developed byLassen andRau (1960). Such investigations of the interactions between atoms, molecules, ions, and aerosol particles are highly useful in atmospheric tracer studies.     

中国东部层积云发展过程中云微物理特征的演变

基于2007—2010年的CloudSat卫星观测数据,以云层液态水路径为指标将层积云的发展过程划分为五个阶段,对比研究了中国东部降水与非降水层积云发展过程中云微物理特征和云微物理机制的演变,并分析了其海陆差异.研究表明:非降水层积云中,云滴增长主要通过凝结过程完成,但云滴的凝结增长有限,难以形成降水,在非降水层积云发展的旺盛阶段,云层中上部云滴发生较弱的碰并过程.降水层积云中云滴碰并增长活跃,当云层液态水路径小于500 g·m~(-2)时,云滴在从云顶下落至云底的过程中持续碰并,并在云底附近出现云水向雨水的转化;当降水层积云液态水路径超过500 g·m~(-2)时,云滴碰并增长主要发生在云层上部,在云层中部,云液态水含量、液态粒子数浓度和液态粒子有效半径达到最大,云水向雨水的转化最为活跃.层积云微物理特征的海陆差异主要是由海陆上空气溶胶浓度和云中上升气流强度不同导致的.在非降水层积云中下部,陆地丰富的气溶胶为云滴凝结增长提供了充足的云凝结核,因而云微物理量的量值在陆地上空更大,而在云层中上部,云滴凝结增长达到极限,海洋充足的水汽输送使云微物理量的量值在海洋上空更大.当降水层积云液态水路径大于500 g·m~(-2)时,陆地层积云中更强的上升气流使大量云滴在云层中上部累积滞留,云滴碰并增长活跃,云层中上部云微物理量的量值在陆地上空更大.     

The 1982 eruptions of El Chichon volcano,Mexico (3): Physical properties of pyroclastic surges

Two major pyroclastic surges generated during the 4 April 1982 eruption of El Chichon devastated an area of 153 km² with a quasi-radial distribution around the volcano. The hot surge clouds carbonized wood throughout their extent and were too hot to allow accretionary lapilli formation by vapor condensation. Field evidence indicates voidage fraction of 0.99 in the surge cloud with extensive entrainment of air. Thermal calculations indicate that heat content of pyroclasts can heat entrained air and maintain high temperatures in the surge cloud. The dominant bed form of the surge deposits are sand waves shaped in dune forms with vertical form index of 10–20, characterized by stoss-side erosion and lee-side deposition of 1–10 cm reversely graded laminae. A systematic decrease in maximum lithic diameter with distance from source is accompanied by decrease in wavelength and amplitude. Modal analysis indicates fractionation of glass and pumice from the surge cloud relative to crystals, resulting in loss of at least 10%–25% of the cloud mass due to winnowing out of fines during surge emplacement. Greatest fractionation from the –1.0–0.0– grain sizes reflects relatively lower pumice particle density in this range and segregation in the formative stages of the surge cloud. Extensive pumice rounding indicates abrasion during bed-load transport. Flow of pyroclastic debris in the turbulent surge cloud was by combination of bed-load and suspended-load transport. The surges are viewed as expanding pyroclastic gravity flows, which entrain and mix with air during transport. The balance between sedimentation at the base of the surge cloud and expansion due to entrainment of air contributed to low cloud density and internal turbulence, which persisted to the distal edge of the surge zone.     

Sur les aérosols intéressants la Météorologie

Summary The lecture is a critical survey of the studies regarding the particles in suspension in atmospheric air and which interest in the phenomenon of water vapor condensation and generally in cloud formation and in the ensuing precipitation.A part of the lecture is devoted to the manifold technics of study introduced, above all, in the last decade, in addition to the method ofAitken counting gear. It dwells particularly on the use of Millipore filter used byLodge and his collaborators for the gathering of the condensation nuclei and of the successive chemical analysis with specific reagents to make evident the nature of the nuclei.The review ends underlining the necessity that the measures of the atmospheric aerosol may be systematically carried on by the synoptical meteorological observatories and the necessity that the experiments in this field may be made with a more and more increasing care and critical mind.     

Über die Bindung der Aerosolteilchen auf der Oberfläche der Wolkenelemente

Summary The importance ofStefan's flow by collection of insoluble aerosol particles on the surface of the evaporating droplets by condensation has recently been emphasized mainly in connection with the possible wash-out mechanism in the atmosphere. A simple theory is deduced for the collection efficiency of insoluble particles on droplets under theStefan's force and a comparision is made with the binding of aerosol particles byBrown's motion and microturbulence of air flow. In general we must take in account the influence ofStefan's flow by calculating the wash-out efficiency.     

The Global Atmospheric Electrical Circuit and Climate

Evidence is emerging for physical links among clouds, global temperatures, the global atmospheric electrical circuit and cosmic ray ionisation. The global circuit extends throughout the atmosphere from the planetary surface to the lower layers of the ionosphere. Cosmic rays are the principal source of atmospheric ions away from the continental boundary layer: the ions formed permit a vertical conduction current to flow in the fair weather part of the global circuit. Through the (inverse) solar modulation of cosmic rays, the resulting columnar ionisation changes may allow the global circuit to convey a solar influence to meteorological phenomena of the lower atmosphere. Electrical effects on non-thunderstorm clouds have been proposed to occur via the ion-assisted formation of ultra-fine aerosol, which can grow to sizes able to act as cloud condensation nuclei, or through the increased ice nucleation capability of charged aerosols. Even small atmospheric electrical modulations on the aerosol size distribution can affect cloud properties and modify the radiative balance of the atmosphere, through changes communicated globally by the atmospheric electrical circuit. Despite a long history of work in related areas of geophysics, the direct and inverse relationships between the global circuit and global climate remain largely quantitatively unexplored. From reviewing atmospheric electrical measurements made over two centuries and possible paleoclimate proxies, global atmospheric electrical circuit variability should be expected on many timescales.     

A comparison of concentrations of fission products,radon 222, and cloud condensation nuclei over the North Atlantic

The concentrations of airborne fission products were observed to be air mass dependent during a cruise of the USNSHayes from Norfolk, Va to Athens, Greece in May–June 1977. Minimum concentrations of fission products, radon, and CCN (cloud condensation nuclei) were measured in maritime air which had previously transited northern North America. Higher fission products, radon, and CCN concentrations were measured in recent, continental air traceable to mid North America or central Europe. These data are consistent with either entrainment by strong winds of previously precipitated fission products (the continental effect) or greater transfer of fission products from the stratosphere to low levels by tropospheric folding.     

A Study of Chloride Aerosol, Total Aerosol and Ice Nuclei in the Indian Regions

--Atmospheric aerosols were sampled from aircraft at various levels over Rihand in northern India during the monsoon season of 1974 and at surface levels at Calcutta, Delhi, Varanasi and Jodhpur during different seasons of 1970 and 1971. Millipore filter assembly was used for this purpose. Chloride nuclei, total aerosol and ice nuclei concentrations were evaluated from millipore filters. At Rihand/Varanasi, during the monsoon, the chloride size spectrum was broader and the distribution bimodal at cloud base level while the spectra were narrower and the distributions unimodal at other levels. At Rihand, chloride mass and larger giant chloride number were considerably higher, total aerosol higher while ice nuclei concentrations were lower on the days with moderate rain as compared to those with light rain. Calcutta (maritime) showed maximum while Varanasi (well-inland) showed minimum chloride number concentrations. As compared with Calcutta, Delhi (continental) had lower chloride and total aerosol, and markedly higher ice nuclei concentrations. Varanasi (high rainfall region) showed all the values, except for small-sized chlorides, notably higher with respect to Jodhpur (low rainfall region). Ice nuclei concentration was lowest in winter and highest in summer at Delhi. Variations of the aerosols during different parts of the day/night are also discussed.     

On the various methods of deducing aerosol size distribution from diffusional decay measurements

Summary The value of three different methods for deducing aerosol size distribution from diffusional decay measurements — the exhaustion method proposed byPollak andMetnieks, the method byFuchs et al. and the method byNolan andScott — is investigated with numerical examples of known distributions and also by applying them to laboratory experiments. It was found that the exhaustion method and the method proposed byFuchs et al. are satisfactory for deducing the mean particle size and that none of the three methods is quite satisfactory for deducing dispersion of the distributions with higher accuracy.     

Characteristic features of atmospheric aerosols over India and their role in warm rain process

Atmospheric aerosols are a crucial link in the physical processes, involved in the formation and growth of precipitating clouds. Extensive aerosol measurements in surface air and in the lower troposphere were made at inland and coastal stations of different regions in India. At inland stations, the hygroscopic fraction of the total aerosol content is found to be a useful characteristic for distinguishing between the monsoon and summer airflow, as well as an indicator for a good or a badly developed monsoon. At coastal stations, however, this feature is not observed.Measurements as a function of height brought out that the aerosol varied widely in air over different seasons. During monsoon, the hygroscopic fraction was found highest at the cloud base level and was closely linked to the development of rain. Details of these investigations are presented.     

On some measurements of the condensation of different vapors upon Aitken condensation nuclei

No final theory on the activity of Aitken nuclei has been established yet. In particular the supersaturation is not known for the Aitken nuclei to grow into droplets, f.e. according toF. Volz only nuclei of radius >0.1 can cause condensation under atmospheric conditions. On the other hand according toH. G. Müller condensation over the continents has to occur at the Aitken nuclei since precipitation washes out the other nuclei.This uncertainty becomes obvious in the interpretation of the experimental results.Chr. Junge found that the necessary activation supersaturation of a continental Aitken aerosol and of artificial aerosols of the same size of nuclei lies between 2 and 20%, 10% being sufficient for the main fraction of nuclei to grow.In contrastW. Wieland was able to activate in a mixing cloud chamber a big portion, if not all, of the nuclei of a continental aerosol at supersaturations below 1.5%. Some of our own results, obtained with the same technic, agree with this. At supersaturations below 0.8% at least half of the Aitken nuclei present are activated. To obtain the same result with benzene and acetone we found, that benzene required a slightly smaller, and acetone a somewhat bigger supersaturation.However later experiments revealed a considerable effect of the geometry of the mixing cloud chamber upon the results. Since the physics of the chamber has not been fully explored the method was abandoned. Instead the principle of cooling by adiabatic expansion was used. An expansion apparatus based on the principle of a fotoelectric nucleus counter was developed allowing us to measure with two cathode ray oscillographs the pressure and simultaneously the change of intensity of a lightbeam due to the scattering on the forming cloud as a function of time. The length of the lightbeam could be chosen between 60 and 200 cm. The overpressure before the expansion was always 180 mm of mercury. The ratio of expansion rates was 12.53060 at the beginning of the expansion. The slowest rate was about 6 seconds, corresponding to a rate of ascent of 210 m/s at the beginning of the expansion. Smaller rates could not be obtained because of heat transfer at the chamber walls.The present work has been performed for the Eidgenössische Kommission zum Studium der Hagelbildung und der Hagelabwehr (Switzerland) at the research station Osservatorio Ticinese Locarno-Monti della Centrale Meteorologica Svizzera     

Toward Improved Solar Irradiance Forecasts: a Simulation of Deep Planetary Boundary Layer with Scattered Clouds Using the Weather Research and Forecasting Model

Accurate forecasts of solar irradiance are required for electric utilities to economically integrate substantial amounts of solar power into their power generation portfolios. A common failing of numerical weather models is the prediction of scattered clouds at the top of deep PBL which are generally difficult to be resolved due to complicated processes in the planetary boundary layer. We improved turbulence parameterization for better predicting solar irradiance during the scattered clouds’ events using the Weather Research and Forecasting model. Sensitivity tests show that increasing the exchange coefficient leads to enhanced vertical mixing and a deeper mixed layer. At the top of mixed layer, an adiabatically ascending air parcel achieved the water vapor saturation and finally scattered cloud is generated.     

Removal of aerosol particles from the atmosphere by growing cloud droplets

Summary When a vapor pressure gradient exists in air containing suspended particulate matter, the particles are propelled toward the region of lower vapor pressure by molecular bombardment of vapor molecules. It has been proposed that this effect is important as a process for removing the particles of natural aerosols from the air surrounding growing cloud droplets.This effect has been investigated experimentally by observing the removal of artificial aerosol particles form an experimental chamber in which a known vapor pressure gradient was established. The velocities imparted to particles under the action of a vapor pressure gradient were determined. The results of the measurements were used to calculate the rate of removal of particles in the vicinity of growing cloud droplets. It was concluded that less than 1% of the particles in the region would be removed during the growth of a typical cloud.This work has been submitted in partial satisfaction of the requirements for the degree of Master of Science.     

An ionometric counter for condensation nuclei

Summary Condensation nuclei detectable in the range of supersaturation used in the expansion method do not play the role expected earlier, for the production of water droplets in natural clouds.However, these nuclei exist in the atmosphere, and they can be of importance in other processes in the air; for example, they can be partners for annihilation of small air ions there. Therefore, a direct investigation of the interaction between the condensation nuclei and the small air ions may be of interest. For this purpose an, ionometric counter for condensation nuclei is designed, in which the air under examination is mixed in a mixing chamber with small air ions produced by a tritium ion-generator, and the presence of condensation nuclei is measured by the decrease of the concentration of air ions recorded with an aspiration ion counter as compared to the concentration recorded when filtered air was originally sucked through the mixing chamber.Comparatively rapid variations of the concentration of nuclei can be continuously recorded. The electrical properties of the aerosol particles destroying air ions can be investigated by inserting an electrical filter. By the ionometric counter condensation nuclei can be investigated without adding water particles to them.     

Impact–entrainment relationship in a saltating cloud

The problem of impact–entrainment relationship is one of the central issues in understanding saltation, a primary aeolian transport mode. By using particle dynamic analyser measurement technology the movement of saltating particles at the very near‐surface level (1 mm above the bed) was detected. The impacting and entrained particles in the same impact–entrainment process were identified and the speeds, angle with respect to the horizontal, and energy of the impacting and entrained sand cloud were analysed. It was revealed that both the speed and angle of impacting and entrained particles vary widely. The probability distribution of the speed of impacting and entrained particles in the saltating cloud is best described by a Weibull distribution function. The mean impact speed is generally greater than the mean lift‐off speed except for the 0·1–0·2 mm sand whose entrainment is significantly influenced by air drag. Both the impact and lift‐off angles range from 0° to 180°. The mean lift‐off angles range from 39° to 94° while the mean impact angles range from 40° to 78°, much greater than those previously reported. The greater mean lift‐off and especially the mean impact angles are attributed to mid‐air collisions at the very low height, which are difficult to detect by conventional high‐speed photography and are generally ignored in the existing theoretical simulation models. The proportion of backward‐impacting particles also evidences the mid‐air collisions. The impact energy is generally greater than the entrainment energy except for the 0·1–0·2 mm sand. There exists a reasonably good correlation of the mean speed, angle and energy between the impacting and entrained cloud in the impact–entrainment process. The results presented in this paper deserve to be considered in modelling saltation. Copyright © 2002 John Wiley & Sons, Ltd.     

Detailed PSC formation in a two-dimensional chemical transport model of the stratosphere

A new two-dimensional zonal model of the stratosphere, based on a formulation in an isentropic framework, with complete chemistry has been coupled with a high resolution detailed microphysical model for polar stratospheric clouds (PSCs). The 2D model chemistry includes all presently known heterogeneous processes on sulfate aerosols and PSCs. The coupling of these two models, with inherently different time scales, is discussed. It is demonstrated that in order to obtain a realistic interrelationship between NO_y and N₂O an accurate simulation of the sedimentation by PSC particles is necessary. A good agreement of model PSC presence and observations is found for the Antarctic polar winter without the need to impose additional artificial temperature variations in the model. The calculated occurrence of polar stratospheric clouds and resulting heterogeneous chemistry during the Antarctic winter are discussed. Sensitivity of the polar stratospheric chemical composition and cloud formation for different perturbations is investigated by studying the effects of transport across the polar vortex boundary and heterogeneous processing by an enhanced sulfate aerosol load. The importance of including sedimentation for all cases is also discussed.     

Towards the production of a reproducible aerosol by radiolytic methods

Summary A convenient constant reproducible aerosol has been a long-felt need for satisfactory research and development purposes in the field of Aitken nucleus studies. An account is given of a recent attempt to produce such an aerosol by radiolytic methods. It consists of the introduction of -particle sources of different activities into nucleus-free air in aerosol containers. Condensation nuclei in the Aitken size-range are produced by radiolytic reactions in the air. The concentration, size and size-distribution of the particles have been measured for sources of various activities and the results are sufficiently consistent to allow us to state that radiolytic production seems a promising approach to the problem of providing aerosols of particles lying in the lower Aitken nucleus size range.     

A model of a homogeneous isotropic turbulent flow and its application for the simulation of cloud drop tracks

Abstract

A model of a homogeneous isotropic turbulent flow is presented. The model provides different realizations of the random velocity field component with given correlation latitudinal and lateral functions and a spatial structure which obeys the Kolmogorov theory of homogeneous and isotropic turbulence. For the generation of the turbulent flow the structural function of the flow in the form suggested by Batchelor (Monin and Yaglom, 1975) was used. This function describes the spectrum of turbulence both in the viscous and inertial ranges. The isotropy and homogeneity of the velocity field of the model are demonstrated.

The model is aimed at simulating the ‘‘fine'’ features of drop's (aerosol particles') motion, such as the deviations of drops’ velocity from the velocity of the flow, detailed structures of drops’ tracks, related to drops’ (particles') inertia. The model is intended also for the purpose of studying cloud drops’ and aerosol particles’ motion and their diffusional spreading utilizing the Monte Carlo methods.

Some examples of drop tracks for drops of different size are presented. Drops’ tracks are very sophisticated, so that the relative position of drops falling initially from the same point can vary drastically. In some cases drops’ tracks diverge very quickly, in other cases all drops move within a turbulent eddy along nearly the same closed tracks, but with different speed. The concentration of drop tracks along isolated paths is found in spite of the existence of a large number of velocity harmonics. It is shown that drops (aerosol particles) tend to leave some areas of the turbulent flow apparently due to their inertia. These effects can possibly contribute to inhomogeneity of drops’ concentration in clouds at different spatial scales.     

Aerosol properties over an urban site in central East China derived from ground sun-photometer measurements

Sun-photometer measurements at Hefei, an urban site located in central East China, were examined to investigate the variations of aerosol loading and optical properties. It is found that aerosol optical thickness (AOT) keeps higher in winter/spring and gets relatively lower in summer/autumn. The large AOT in winter is caused by anthropogenic sulfate/nitrate aerosols, while in spring dust particles elevate the background aerosol loading and the excessive fine-mode particles eventually lead to severe pollution. There is a dramatic decline of AOT during summer, with monthly averaged AOT reaching the maximum in June and soon the minimum in August. Meanwhile, aerosol size decreases consistently and single scattering albedo (SSA) reaches its minimum in July. During summertime large-sized particles play a key role to change the air from clean to mild-pollution situation, while the presence of massive small-sized particles makes the air being even more polluted. These complicated summer patterns are possibly related to the three key processes that are active in the high temperature/humidity environment concentrating on sulfate/nitrate aerosols, i.e., gas-to-particle transformation, hygroscopic growth, and wet scavenging. Regardless of season, the increase of SSA with increasing AOT occurs across the visible and near-infrared bands, suggesting the dominant negative/cooling effect with the elevated aerosol loading. The SSA spectra under varying AOT monotonically decrease with wavelength. The relatively large slope arises in summer, reinforcing the dominance of sulfate/nitrate aerosols that induce severe pollution in summer season around this city.