PHYSICAL PROBLEMS OF WEATHER

Hydrological Sciences-Bulletin~des Sciences Hydrologiques, XXI, 4 12/1976

PHYSICAL PROBLEMS OF WEATHER MODIFICATION* K.E. HAMAN Institute of Geophysics, University of Warsaw, Poland Abstract. There are many contrasting views on the success of experiments into weather modification. This paper reviews a large number of these studies commencing with a summary of some of the pertinent problems in cloud physics and those leading to cloud modification. It deals with the techniques of modification such as by the use of dry ice and silver iodide and gives details of the results of experiments with clouds of different types. The conclusion reached is that the result of methods of modification used at present are not sufficiently predictable to warrant their use commercially and that the results of most of the interesting studies have not been evaluated by sufficiently reliable methods. Des problèmes physiques de la modification artificielle du temps Résumé. Il y a bien d'opinions opposées sur le succès des expériences sur la modification artificielle du temps. Ce mémoire passe en revue un bon nombre de ces études et on commence avec un résumé de quelques-uns des problèmes pertinents des physiques des nuages et quelques-uns de ceux qui mènent à la modification des nuages. Il traite des techniques de modification comme par exemple l'utilisation de la glace carbonique et de l'iodure d'argent et raconte en détail les résultats des expériences avec des nuages de types divers. On conclut que les résultats des méthodes de modification utilisées jusqu'à présent ne sont pas assez prévisibles pour justifier leur utilisation commerciale et qu'on n'a pas utilisé des méthodes assez sûres pour évaluer les résultats de la plupart des études intéressantes.

INTRODUCTION Human life has always been highly dependent on weather and climate; the technological developments of recent decades to a certain extent modified the forms of this dependence, but not its importance. The energy and food crises superimposed on certain climatic anomalies in recent years have served as a dramatic illustration of this fact. It is thus understandable that people have always tried to modify the weather by all available means—from conjurations in primitive societies to science and specialized enterprises in our civilization. Scientifically based studies and efforts in weather modification started soon after World War II and underwent a characteristic evolution-from enthusiastic action towards improvement of nature to anxiety about what would happen if nature's delicate balance were disturbed. Weather modification by direct change of the energy of atmospheric phenomena is practically impossible except on the smallest scale, since sources of energy available now and in the foreseeable future are entirely inadequate by comparison to the amounts of energy released and transformed in atmospheric processes even on a moderate scale. Our weather modification efforts must thus make use of certain 'switches' which can affect the direction of huge natural energy transformations at the relatively low energy expense required to operate the 'switch'. One of the most effective 'switches' of this sort are cloud and precipitation processes.

* This lecture was addressed to the Weather Modification Symposium organized by IAMAP, sponsored by IAHS, and held at Grenoble on 5 September during the XVIth General Assembly of IUGG.

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Changes in the cloud and precipitation regime can substantially affect the whole pattern of atmospheric circulation, mainly due to the high albedo of clouds and snow cover, as well as to the high rank of latent heat of condensation among the atmospheric heat sources and sinks. But the most important present applications of modification of clouds and precipitation are more direct and concentrate mainly on fog dissipation, hail suppression and modification of rain and snowfall aimed towards improvement of the water economy of certain territories, which can be achieved by enhancement; suppression* or simply redistribution (with less emphasis on the amount) of precipitation. Due to the semi-hydrological character of the present symposium, the lecture will be restricted mostly to the last group of problems. The formation of precipitation requires fulfillment of at least three conditions: ( 1) Existence of sufficient amount of water vapour in the air which can be transformed to the liquid or solid phase in the process of cloud formation. (2) Development of dynamic and thermodynamic processes leading to the formation of clouds, with sufficient liquid and/or solid water to permit formation of precipitation. (3) Transformation of small droplets and crystals to precipitation particles, large enough to fall out from the cloud and reach the ground without evaporating in sub-cloud layers. In principle, human activity can affect any of these conditions. For instance, air humidity which is highly dependent upon evaporation from reservoirs and the évapotranspiration of plants, can be affected by agriculture and hydrotechnical activities over extended areas. Such effects have already been observed, but they were not always desirable. Concepts of modifying the climate through such operations as reversing the flows of rivers or closing oceanic straits were fairly popular some twenty years ago. The last serious proposal of that sort known to the author was given by Bergeron in 1968 (Bergeron, 1968). Recently such ideas were abandoned not only because of technical difficulties but mainly because people became more aware of the dangers connected with large-scale modifications of nature without complete knowledge of the possible side effects. The second condition—formation of cloudscan be affected only in the presence of certain unstable dynamic conditions since otherwise the amounts of necessary energy would be prohibitive. Some experiments in this area have been undertaken and will be presented later. At present, the most important approach to cloud modification is by affecting the third condition—the process of formation of precipitation from the cloud water. The description and analysis of this approach require some introductory considerations of cloud physics. SOME BASIC PROBLEMS OF CLOUD PHYSICS Cloud is a colloidal suspension of microscopic water droplets or ice crystals, or both, in the air; the falling speed of these particles is so small that generally they cannot reach the surface as precipitation. Cloud physics can be divided into the following main branches: (1) Cloud microphysics-which deals with the physics of separate cloud particles and the interactions between them; (2) Cloud macrophysics (mainly dynamics and thermodynamics), which treats the cloud as a continuous medium, described in terms of continuous fields of physical parameters (temperature, pressure, velocity, etc.), including fields which refer to liquid and solid phases of water; (3) Physics of the cloud system—which deals with interactions mainly of a mechanical * Theoretically; no serious experiments with precipitation suppression are known to the author.

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and a thermodynamic character between neighbouring clouds or clouds and larger scale meteorological phenomena*. In early phases of cloud physics studies, processes belonging to those branches were analysed separately for the sake of simplicity; in fact, they are strongly interrelated and can be separated only to a limited extent. Investigations of interactions between these branches of processes is one of the most important, though yet poorly developed, tasks of contemporary cloud physics. Particularly important are interactions between the dynamics and microphysics of clouds. A useful, continuous field-type, notion, which refers to the microphysical property, is the size spectrum of cloud particles. The size spectrum gives the number of cloud droplets or crystals in a certain class of sizes measured by radius, diameter or mass. It is usually related to unit volume or mass of air and to a certain point in space and time. After smoothing and normalization, this function can be interpreted as the probability of finding a particle from a given class in a given time and space point of the cloud. Formation of precipitation can be described in terms of the evolution of the size spectrum. Let us now consider some basic properties of the various types of precipitating clouds. Departing slightly from the traditional Atlas of Clouds we shall divide the precipitating clouds into three main types—convective, stratiform, and orographic. All these types of clouds develop due to the condensation of water vapour in updrafts, but the mechanisms of these updrafts are essentially different. In convective clouds they are due to the instability of the hydrostatic equilibrium of the atmosphere or the instability of a large-scale wind field with vertical shear. Release of latent heat of condensation and freezing is essential for liberating those instabilities; but on the other hand, precipitation can trigger downdrafts thus becoming an essential factor in the life cycle and dynamics of a convective cloud. This means that there is strong coupling between the microphysical process of condensation and precipitation and the field of motion. Convective clouds may develop as 'air mass' clouds forming fields with a more or less random spatial distribution of single clouds, or within certain mesometeorological systems like fronts, squalls lines or clusters, generated and maintained either by terrain features or large-scale dynamics or by some internal feedbacks, or by some combination of all those elements. Convective clouds are very sensitive to thermal and topographical inhomogeneities of the surface, which serve as triggers for their development. Vertical currents in convective clouds are relatively strong and may even reach some tens of metres per second. Stratiform clouds form in slow (order of cm/s) large-scale updrafts connected mainly with depressions and frontal systems. The release of latent heat and precipitation, though not completely unimportant, is only a secondary factor in the updraft dynamics, so that the coupling between dynamics and microphysics is here much more unidirectional than in convective clouds. Developing mainly in hydrostatically stable conditions these clouds are governed mostly by large-scale dynamics, though local surface properties can modify them to a certain extent, particularly in hilly or mountain areas. Orographic clouds form in updrafts which are due to the upslope motions on mountain ridges or wave motions generated by them. The updraft dynamics is thus determined mostly by the large-scale wind field and topography. The latent heat release is nearly unimportant for the basic dynamics, except in cases when convection is superimposed on the upslope motion. The characteristic feature of these clouds is their 'linking' to the topography—the

* The terminology of this subdivision is not well established and may sometimes be confusing. The present one has the advantage of being consistent with the concept of the 'microscopic' and 'macroscopic' approach to physical phenomena as used in general physics. However, many authors refer to it as 'microphysics', 'dynamics', and 'mesometeorology of clouds', which seems to be inadequate; particularly the term 'dynamics' seems to be too narrow in that context. In any case this subdivision should not be confused with concepts of 'microscale, mesoscale, and macroscale' as used in dynamic meteorology.

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air is flowing through the cloud which is evidently a thermodynamical process rather than a material entity (this fact, though true also in other groups of clouds is much less visible there and sometimes can be neglected). The coupling between dynamics and microphysics is even more unidirectional than in the case of stratiform clouds. The above description refers of course to typical representatives of each group—in nature some mixed or atypical situations will occur. The differences in degree and direction of coupling between micro and macrophysics in various groups of clouds are essential for the technology of cloud modification. Condensation in clouds starts on so-called condensation nuclei. Condensation nuclei are particles of atmospheric aerosol, mostly of submicron sizes and containing generally a certain amount of water-soluble substances, which reduce the value of the saturation water vapour pressure necessary for condensation. Without them condensation would require multiple supersaturation (measured with respect to a flat surface of pure water), equivalent to more than 700 per cent relative humidity. Usually, there is an abundance of nuclei which become active at a relative humidity slightly exceeding 100 per cent; some become active at even lower humidities—down to 70 per cent. Actual humidity in clouds results from the quasi-balance between the rate of cooling (and supersaturating) of the air and removal of the excess of water vapour by condensation. In natural clouds this balance is reached at humidities of about 100 per cent. The size of a cloud droplet is determined by the quasi-balance between the actual pressure of water vapour and the saturation value for this particular droplet; the latter is a function of size and concentration of dissolved substances. Natural clouds have a range of drop-size spectra from 5-10 jum diameter to 30-40 ;um diameter and only occasionally more. Details of the spectrum depend on the dynamics of the cloud and properties of available nuclei. There is for instance a considerable difference between the spectra of maritime and continental clouds, which results mainly from differences in maritime and continental condensation nuclei. Further transformation of the spectrum towards larger sizes of particles is possible by coalescence. According to present knowledge, the most effective coalescence mechanism is the so-called gravitational coalescence—the effect of collisions between particles of various sizes which fall down with various speeds, determined by a balance between gravitational and air drag forces. But this mechanism is very ineffective if the collecting particles are smaller than 40 pm (diameter) and to become effective enough to produce precipitation, it requires even larger collectors (about 100 /um or more). Thus, there is a gap in the spectrum between typical particles which form by condensation and particles which can effectively coagulate. The existence of a mechanism which permits the cloud to overstep effectively this gap is the most important difference between precipitating and nonprecipitating clouds. There are two well-known mechanisms of this sort in natural clouds. The first of them, the so-called three-phase (or Bergeron-Findeisen) mechanism, is typical for supercooled clouds and works due to the fact that the saturation water vapour pressure for ice is lower than that for supercooled water at the same temperature. Thus a small number of ice crystals, embedded in a cloud saturated with respect to water, can grow very fast by condensation and can reach a size which permits effective coalescence to a precipitation particle, or even direct falling out as fine snow. The crystal can grow by coalescence mainly by collisions with supercooled droplets (riming), and depending on the speed of this process and the exchange of heat liberated during freezing of collected water, the precipitation takes the form of snow, graupel or hail, which can eventually melt and reach the ground as rain. The scavenging ability of ice crystals depends strongly on their habit, which is a complicated function of temperature and supersaturation during their formation. The smallest cloud droplets can remain supercooled down to temperatures as low as -42° C unless they contain a so-called freezing nucleus from which the crystal lattice of ice can start to grow. The best freezing nuclei are ice crystals themselves but there are a great number of various organic and inorganic substances (many of them present in natural atmos590

pheric aerosols) which can serve as freezing nuclei becoming active at various temperatures.* In natural clouds, ice crystals in numbers large enough to effectively start the formation of precipitation by the three-phase process appear at temperatures of about -15 to -20°C. It is believed that freezing droplets and growing ice crystals can fracture and multiply, so that once started, the process of freezing the cloud has the character of a chain reaction. The second mechanism of reaching the stage of effective coalescence--the so-called 'warm cloud mechanism' is connected with the presence of a certain number of 'giant' condensation nuclei which permit the formation of large enough droplets by direct condensation. A few such nuclei per litre may be enough to induce a fairly intensive shower in certain types of cloudiness; such concentrations of giant nuclei in the form of large NaCl crystals are often present in maritime air. The efficiency of this process can be improved through the break-up of droplets due to collisions or loss of capillary stability. In such cases the process may take the form of a chain reaction as noted by Langmuir (1948). The three-phase process is believed to be responsible for the majority of extratropical precipitation, while the warm cloud process is responsible for a considerable fraction of tropical showers. There is, however, evidence that rainfall is sometimes observed from clouds in which both mechanisms are highly improbable. A search of other mechanisms for shifting the condensational spectrum toward larger particles is thus going on, with emphasis put on effects of turbulence, electrical fields as well as on the stochastic character of the coalescence process. Alas, no definite solutions are yet reached. PRINCIPLES AND SOME PROBLEMS OF CLOUD MODIFICATION The three-phase mechanism as well as the warm cloud mechanism provide the potential for cloud modification by the introduction of artificial condensation and freezing nuclei into the cloud. One can expect that this will alter the natural number of precipitation embryoparticles (i. e. particles which are going to grow up to precipitation size) and the natural time of their development. This may result in enhancement, redistribution or suppression of precipitation and by dissipation or conversely, stimulation of the cloud, depending on the details of microphysical-macrophysical interactions in each case. The problem arises which of these possibilities and to what extent they will be realized in a particular case. Modification experiments based on seeding the clouds with artificial freezing nuclei were started soon after World War II, followed (predominantly in the USA) with commercial activities. Early experiments were based on the assumption that the lack of suitable precipitation embryos is the main cause of insufficient rainfall and thus seeding the cloud with freezing nuclei and stimulating the Findeisen-Bergeron process should enhance rain or snowfall. Though some impressive results were obtained in early trials, failures were (and are) observed too, demonstrating that the problem is much more complicated. Namely, we have to remember that the availability of a suitable number of precipitating embryo-particles is a necessary but by far from sufficient condition to produce precipitation able to reach the ground. There must also be a sufficient amount of water available for growth of these embryos and enough time to complete this process (it is not enough to have a 'releaser' cloudthere is also need for suitable 'spender', according to Bergeron's terminology). Thus certain clouds may be improper objects for seeding; others may develop mechanisms compensating for the effects of an increased number of precipitation embryos. In many cases positive effects of seeding could be expected only provided that the place, time and dosage of the seeding agent are very precisely determined. Let us notice that both underseeding and over* In general, freezing of pure water droplets is a random process with the probability of freezing in unit time the greater, the greater is the mass of water and lower temperature. The threshold temperature at which a droplet of given size freezes nearly instantly can be increased by the presence of a suitable freezing nucleus.

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seeding may produce unsatisfactory results— the latter may reduce the size of precipitating particles by increased competition for available water and thus increase evaporation in the undercloud layer. Determination of these elements may be a very difficult task. In order to illustrate these difficulties let us consider the simplest model of monodispersive rainfall, consisting of identical droplets with mass m falling in number TV per second during time T. The total amount/ of precipitated water is then given by the equation I = mNT

(1)

Assuming that the growth of the droplet mass can be described by the equation of continuous coagulation: ^ = ESVW (2) dr (which can be accepted for droplets larger than about 200 jum in diameter which coagulate with typical cloud droplets), and neglecting the falling speed of collected droplets as well as the initial mass of the collector, we can show (see appendix) that the total amount of precipitated water can be given by the formula

j _

47t

S 2

J 3

E-V

3d 24 1L(pc

[ x6 NT

)*J J The meaning of symbols used in formulae (2) and fl(3) is:

(3)

= time, = collection coefficient, = cross-sectional area of the collector, = density of water, = liquid water content of collected droplets per unit volume, = aerodynamic drag coefficient, = air density, T = growth time of raindrop, = averaging over time, (") w = relative velocity of colliding droplets, = gravity. g Let us notice that the dependence of rainfall on the sixth power of E, T and V denotes a great sensitivity to these parameters. For instance a 25 per cent variation of V (which may be even hard to detect by standard measurements) may result in a four-fold variation of rainfall, the difference between flood and drought (Binh, 1974). The collection coefficients, which as a first approximation can be considered constant for raindrops exceeding drizzle size, may become very variable with particle shape when equation (3) is adopted to snow. This denotes sensitivity to the crystallographic forms of snow particles, which may be strongly affected by the introduction of artificial freezing nuclei (change of temperature and supersaturation range in which the crystals are forming). The parameters in equation (3) are generally not quite independent and in certain types of clouds variations of some of them might be compensated by the opposite effect of variations of others. For instance an increase of N can reduce F (by increased scavenging) and thus reduce the final mass m of a raindrop. An increase of V will cause faster growth and thus reduction of r. This may be true particularly in stratiform clouds with their weak dependence on dynamics or microphysics. It also seems very probable that the crystallographic forms of snow crystals forming in such clouds adjust to variations in the number of precipi-

t E S d V CD p

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tation embryos, so that the total rainfall (or snowfall) is only slightly affected. But in convective clouds with their tremendous diversity of internal feedbacks it is not difficult (at least theoretically) to give examples of situations in which seeding with condensation or freezing nuclei will change any of the parameters T, N, V, E, or T in one of several directions either directly or through microphysics-dynamics feedbacks. Let us notice, that though the quantitative considerations based on equations (l)-(3) refer to a very simple and unrealistic model of rainfall, the qualitative conclusions seem to remain valid for more realistic models too. The above remarks yield the conclusion that predicting the response of a given cloud to nuclei seeding operations may be extremely difficult and will require a fairly detailed knowledge of the physics of the seeded clouds as well as the way the seeded material spreads in the cloud. Unfortunately our knowledge in both those respects is very inadequate. Many of the seeding operations were in fact made with only a crude idea of the chain of physical processes that would be started by this operation. This can easily explain why there is so much controversy about the effectiveness of cloud modification. A dependable method of cloud modification must include: (a) a method of at least approximately determining the 'modification potential' of a given cloud or cloud system (i.e. the maximal desired effect which for a given situation might be achieved by the method applied) and (b) a technology which would be able to liberate this modification potential to a reasonable degree. Realization of these points could be much facilitated by a clear physical concept of what is to be accomplished and particularly by good mathematical models of a cloud, able to predict exactly the behaviour of both modified and unmodified clouds. Unfortunately, despite enormous progress in cloud physics and computing technology we do not have such models, and it may be doubtful whether sufficiently precise and universal models will be available in the foreseeable future. On the other hand a purely empirical approach to the problem may also appear hopeless due to the enormous variability of clouds and the number of physical parameters which should be taken into account, not to speak of technical difficulties of measuring many of them, Thus the only realistic way is to improve the existing models of clouds (using empirical data) and use them for the reduction of the number of independent parameters which are to be investigated on an empirical statistical basis. This requires intensive efforts both in experimental cloud physics and mathematical modelling techniques. Mathematical models in cloud physics and weather modification form a separate problem, which would require a separate lecture even for a short introduction. A considerable number of such models exist differing in their physical as well as their mathematical approach and in the range and in degree of complexity. For weather modification, quantitative modelling of micro-macrophysics interactions is of basic importance. Models designed for that purpose generally require high-speed and large-memory computers and even then they can hardly model simultaneously both macro and microphysics processes in satisfactory detail. Progress in computer technology will certainly improve much in this area, however, we must remember that when dealing with very complex mathematical models we may have trouble with separating the purely mathematical properties of the model from the physical properties of the modelled phenomena. Mathematical models have certainly done very much for a better understanding of the mechanisms of cloud modification, but only a few of them and only the simplest have been applied as operational tools in weather modification activities (Hirsch, 1971 ;Orville era/., 1974;Lavoie, 1972; Simpson and Wiggert, 1968). Even the best models require some empirical data (at least for initial and boundary conditions); so observation and climatological analysis of clouds and cloud systems with respect to their modification potential still remain very important tasks (particularly in view of the essential deficiencies of existing models). Especially important parameters which should be investigated are the natural precipitation efficiency of clouds (i.e. ratio of precipitation to

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the integrated flux of condensed water) and the natural content of precipitable water available in a given cloud system. Clouds with a small content of precipitable water can hardly be a good subject for any modification. On the other hand clouds with a high precipitation efficiency will be insensitive to stimulation while clouds with a low efficiency are not suitable for precipitation suppression. Parameters such as top temperature, liquid water content, updraft structure are essential for some seeding techniques and their estimation may be important in modification experiments. Estimation of these properties of clouds is generally made partly with the use of simple models and partly based on empirically determined principles. For certain regions and cloud types these properties may appear well correlated with some easily measurable parameters, like altitude or temperature of the visible or radar top of the cloud, some morphological features or even synoptic situation. Though a number of partial successes have been achieved in recent years, a dependable, universal method of cloud modification, based on a sound, consistent scientific background seems still far ahead. EVALUATION OF CLOUD MODIFICATION ACTIVITIES A very essential thing in weather modification activities (both from the physical and the economical point of view) is the evaluation of results. Unfortunately, the majority of such activities (particularly those undertaken for commercial purposes) were not followed by a correct analysis of the effects. There were two competitive schools of thought on the approach to the evaluation problems: physical evaluation and statistical evaluation. The first school concentrates on the observation of the physical properties of separate cases of modified clouds, searching for changes expected as a result of modification operations. The second school tries to compare the statistics of certain 'measures of success' in sample series of modified and unmodified cases. Recently we become more aware that these two approaches are complementary rather than competitive. Physical evaluation is certainly necessary as a tool of research aimed at improvement of modification techniques and basic cloud physics knowledge as well, but using it solely as an evaluation tool would require a fairly detailed predictability of cloud behaviour if it would remain unmodified, and this is generally extremely difficult and practically impossible. Also our observational possibilities are often inadequate for this purpose. So a certain statistical approach is always inevitable, even in principally 'physical' analysis. On the other hand—as stressed by Warner (1974)—relying only on statistical conclusions, without a reasonable idea about causal relations between the modification operation and the results obtained, is also unacceptable, since such statistical success or failure may be in fact accidental or reflect some incorrectness in designing the experiment. Finally, the nature of some experiments conducted in uncommon meteorological conditions may make relying only on statistical data impossible (due to a prohibitive duration of the experiment necessary for obtaining statistically meaningful results). So in practice certain combinations of those two approaches are generally best. According to Warner (1974) a perfectly planned modification experiment should fulfill certain conditions, which could be summarized as requirements of: (a) a reasonable physical idea of the modification mechanism, (b) correct statistical design, (c) sufficiently long duration in statistically uniform meteorological conditions. Correct statistical design must include: (a) correct choice of the 'measure of success', (b) correct choice of the modified and control samples and (c) correct statistical processing of the results. The two first points cannot be realized without a certain knowledge of cloud and precipitation physics, as well as the climatology of these phenomena. The 'measure of success' should represent a measurable (directly or indirectly) quantity, the changes of which are clearly reflecting the physical effects of modification activities, and 594

which in turn can be clearly related to the ultimate (social or economic) goals of weather modification. * The most natural 'measures of success' in precipitation modification are directly related to the amount of precipitation over the area of interest. Since these parameters are sometimes not available, some indirect measures like crop yield, river flow or insurance data are then used; however, there is a considerable risk that the true modification effectiveness might then be obscured by factors entirely unrelated to the modification operations. Sometimes (more with physical than economic approaches) such parameters as cloud top height, radar reflectivity, first echo time or time of first rain on the ground, number of ice crystals, etc. are used. Then one must remember that variations of these parameters may well reflect some physical response of the cloud to modification, but this response may not be connected in a unique way with the desired improvement in the water economy. Particularly important is the choice of area from which the observations are collected with respect to the area over which the seeding is going on; with a wrong choice there is a risk of taking a redistribution of precipitation for enhancement or reduction. A correct, sound and precise definition for the measure of success is one of the most critical points of any weather modification experiment and can be very essential for reduction of the duration of the experiment necessary for obtaining statistically significant results, as well as for clarifying their physical and economic meaning. Correct selection of the modified and control samples must secure that the control sample is not affected by the modification activities and that both samples represent the same population of cases with respect to the modification potential. It is also very important to exclude from both samples, as far as possible, cases which are obviously unmodifiable, since otherwise the effectiveness of the modification experiment will be apparently reduced. In practice the most common approaches to this problem are: (a) comparison of climatological data before and after the modification operation started, (b) comparison of simultaneous meteorological data from two climatologically similar areas—one under modification, (regression target-control); (c) random choice of periods with modification and without it over investigated area. If a suitable control area exists, the so-called target-control crossover randomization (alternation of modification periods over the two areas) can be applied. In a more physical approach, particularly when working with convective clouds, a random choice between various clouds in the same area is sometimes made. The first approach, though the simplest, is the least satisfactory one, since the natural fluctuations are very large in most areas and general climatological trends in precipitation may completely obscure the effects of modification. Elimination even of evidently unmodifiable cases is generally difficult with this method. The second approach can seldom be applied, since a control area with sufficiently similar climatological properties and distant enough to secure lack of interference is usually hard to find. Randomization is now commonly considered to be the best solution, particularly its crossover version. There are, however, many difficulties in designing correct randomization and securing lack of interference between the modification and control periods. We still know very little about persistence and spreading of various seeding materials, so there is a fear that many of them may appear active much longer and over a much greater area than it is generally presumed. It is also difficult to secure the proper duration for the experiment because of the great natural variability of precipitation; at the present level of knowledge of cloud and precipitation physics, several years or more seem to be necessary in most cases. All those difficulties mean that only few modification experiments made in the past can be considered relatively correct with respect to their evaluation procedure and probably not too many will be undertaken in future. However, as pointed out by Warner (1974) even imperfect experiments might be valuable for the development of modification technology, provided we are conscious of the limitations and careful when claiming success.

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SOME GENERAL REMARKS ON MODIFICATION OPERATIONS At present one of the most important practical problems in weather modification is a reduction of the duration of experiments necessary for obtaining statistically significant results; it is very difficult to find adequate financial support for experiments which continue for years without a guarantee of certain social or economic benefits. The latter problem shows once more the importance of basic investigations in cloud physics and modelling the precipitation phenomena. Good models can considerably reduce the number of factors which, due to inadequate knowledge, we have to consider random and mutually independent and thus permit results to be obtained of equal statistical significance with less observational data than otherwise required. They can also help very much in constructing good 'measures of success' and good, practical criteria for determining the modification potential, the role of which in reducing the experiments duration has already been pointed out. This suggests also that modification operations should be undertaken only in areas in which modifiable situations are frequent enough to justify the undertaken efforts. This stresses the importance of climatological studies with respect to modification potential in areas which are considered as eventual objects of weather modification activities. On the other hand, in undertaking such activities, particularly if they are aimed at certain economic benefits we must take care of possible side effects which sometimes may not be of any benefit. A typical example of such an effect is when successfully performed precipitation enhancement is in fact only a redistribution, with compensatory reduction of precipitation outside the area of interest. Another danger may be connected with underestimating the possible precipitation enhancement and 'replacing drought with flood'. Even such nonmeteorological side effects as the impact on public opinion must be taken into account, since weather modification operations may easily be suspected of causing any undesirable weather phenomena occurring during them. The problem may be particularly difficult in frontier areas, where weather modification activities can easily become an interstate or international political affair. Problems of this type are known from North America, where a fairly developed legislature had to be introduced in order to regulate the scientific and commercial weather modification activities. THE TECHNOLOGY OF CLOUD MODIFICATION As stated before, the most commonly, though not exclusively, exploited principle of cloud and precipitation modification, is the artificial change of the spectrum of cloud particles at certain regions of the clouds—namely those which can play the role of 'releasers' of precipitation. Except for intensive overseeding with ice nuclei, which may result in rapid glaciation of the whole supercooled part of the cloud into small ice particles, that tend not to aggregate with each other and are too small to move fast enough into the lower parts of the cloud (containing liquid water), the artificially modified spectrum of cloud particles is aimed at increasing the number of embryo particles, which are capable of efficient coalescence. This procedure may speed up the formation and increase the number of precipitation particles, which—depending on the conditions—may result in an increase, decrease or redistribution of the precipitation (eventually even changing its form). The problem we are going to discuss now, is the technique of doing this and some physical problems associated with it. The most direct way of obtaining the desired result may be spraying the clouds with water droplets of drizzle size. However, simple estimation shows that to increase precipitation by the order of 1 mm would require a spray of the order 10-100 kg per km 2 . Theoretically one would hope that the same result could be obtained by introducing a few tens of grams of hydroscopic material per km 2 , in order to induce formation of a suitable amount

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of drizzle size droplets from the cloud water. And if formation of precipitation is to be induced by glaciation, even fractions of one gram of ice nucleating agent per km2 may be sufficient, provided it is dispersed into suitably small particles. All these estimates are of course very crude but even so they show, that use of ice nuclei permits manipulation with relatively small amounts of seeding substances. Since the amount of matter which is to be introduced into the cloud is one of the most critical parameters from the technical and logistic point of view, one can easily understand that modification of supercooled clouds by artificially inducing the Findeisen-Bergeron process would be the preferred way. Nevertheless, both the water spray and the hygroscopic substances (mostly NaCl) have also been tried—in most cases with not too convincing results. The substance used for artificial ice nucleation in the earliest experiments was dry icesolid C0 2 . Dry ice fragments produce in their vicinity temperatures low enough to cause spontaneous freezing even of the smallest water droplets, which can later serve as ice nuclei or embryos of precipitation, and eventually multiply by fractioning. They create also a local supersaturation high enough to cause the homogeneous condensation of water vapour into ice crystals. The advantage of the dry ice is its low price and ability to cause ice to nucleate even at temperatures close to 0°C—however, it is troublesome to handle and it can be dispersed only from aircraft, and this limits the situations in which it can be applied. Nevertheless, it is widely used. Another commonly used substance is silver iodide (Agi). In contrast to dry ice, it acts directly as heterogeneous ice nuclei. The exact mechanism of its operation is still subject to intensive research. It becomes effectively active as a nucleant at about -7°C, and its main advantage is that it can be very finely dispersed (up to 1016 particles per gram as smoke, by burning in special burners or in pyrotechnic or explosive mixtures. This makes it a very convenient agent to be dispersed from the ground or from aircraft, by the use of either rockets or shells. Its disadvantage-partly overcome by a suitable technology of burning—is its tendency to become deactivated under ultraviolet radiation; another disadvantage is its relatively high price, which may be a significant problem, even considering the savings on the amount of reagent and tools necessary for its dispersal. Hence, many research centres are looking for other effective ice nuclei. There are hundreds of substances which have been recognized as good ice nuclei but for none of them is the present technology of seeding so well developed as for Agi. The most important competitors of Agi are Pbl 2 (having slightly worse characteristics this is considerably cheaper), CuS and some organic substance like metaldehyde. The latter has the advantage of being nontoxic—a property which under heavy seeding might be worth of consideration. Also use of liquid propane as a cooling agent in place of C0 2 has become popular in recent years. The most common technical tools for dispersing Agi are surface based or airborne flame generators or pyrotechnic flares, surface-to-air or air-to-air rockets or artillery shells. Surface devices can be used, when general meteorological conditions permit one to expect that the Agi smoke will be transported into the precipitation generating part of the clouds by natural air currents. This does not permit the obtaining of very high concentration of ice nuclei in chosen parts of the cloud; seeding from aircraft is better in that respect, though some parts of vigorous convective clouds might be inaccessible to the aircraft. In such cases radar guided shells or the use of rockets seem to be the most effective ways. Obtaining a high concentration of ice nuclei in certain regions of the cloud at a precisely determined moment might be important if the problem of the precise redistribution of rainfall or hail protection is considered. Hygroscopic substances are used when the Findeisen-Bergeron process is not likely to occur or is likely to be supported by direct coalescence. They are generally applied from aircraft or rockets, though surface devices have been used too. The most commonly used hygroscopic material is fine-ground NaCl, though good results with other substances like urea, have been reported. Use of detergents to produce tiny foam bubbles with very high 597

scavenging ability has been under investigation in the Soviet Union. Another unconventional method of affecting the droplet spectrum consists of deactivation of condensation nuclei with monomolecular films or surface active substances introduced into the cloud in the form of .vapour (Bakhanova and Silayev, 1974). However, none of these methods has yet essentially threatened the position of Agi and C0 2 as weather modification tools. A new concept developed in recent years is the dynamic modification of clouds. This includes dissipating young convective clouds by means of downdrafts initiated with impulses from vertically climbing jet aircrafts or from heavy powder loads thrown from above (Khorguani and Kalov, 1974) and modification of cloud dynamics by conventional seeding with ice nuclei. In the latter case the seeding is aimed not so much at changing the cloud particle spectrum, as liberation of latent heat of fusion and invigoration of the cloud. This may increase the liquid water content or time of growth of precipitation particles—both factors to which the precipitation appears very sensitive (as shown before)—and sometimes even the total flux of water vapour into the cloud. Experiments aimed at such effects are being conducted in Florida, USA, with respect to convective clouds (Sax et al, 1973). Also very encouraging results obtained in the USSR by seeding frontal stratiform systems seem to be due to such dynamic effects (Leskov, 1974).

A REVIEW OF CLOUD MODIFICATION EXPERIMENTS During the past 30 years a great number of various weather modification activities have been undertaken, including fog dissipation, cloud dissipation, precipitation modification, hail and lightning suppression and hurricane modification. Space permits only a comprehensive summary of results achieved in operations important from the hydrological point of view. The most important conclusions yielded by these experiments can be presented as follows: (1) The overwhelming majority of hydrologically interesting experiments (with few exceptions) have been aimed towards an increase of precipitation. Most of them have not been evaluated by fully satisfactory methods; positive and negative results have been claimed in various experiments, though predominantly they were found to be inconclusive. Despite this discouraging statement, these experiments gave a tremendous impetus to cloud physics development and our understanding of precipitation mechanisms, creating a better outlook for future activities. (2) The basic technology applied was seeding with Agi (less frequently with C0 2 , NaCl and other agents), aimed at increasing the number of precipitation embryo particles; however, some successful cases should be attributed to the dynamic effects of seeding. (3) The most promising results seem to result from seeding certain orographic cloud systems (up to 30 per cent statistically significant increase). This fact seems understandable, since orographic systems consist of clouds with relatively strong updrafts, high liquid water content and long, stable persistence, in which (due to fast horizontal flow of air through the cloud) there may not be enough time for the formation of natural ice crystals in sufficient amounts and early enough to secure the formation of precipitation, able to precipitate before leaving the cloud on the downwind side. In such cases artificial ice nuclei, active at sufficiently high temperatures, may improve the precipitation efficiency of the cloud. On the other hand, weak dependence of dynamics on microphysics in this type of cloud facilitates the determination of the modification potential and prevents adverse dynamic effects, which could eventually compensate the positive effect of the increased number of precipitation particles. It is worth noting that at low temperatures the natural production of ice crystals may be sufficient for securing a high natural precipitation efficiency and seeding in such cases may reduce precipitation due to the 'overseeding' effect (Chappell, 1972). (4) Inconclusive results have been obtained in the majority of experiments with con-

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vective clouds. The great natural variety of these clouds makes experimental stratification very difficult, while complicated feedbacks between micro and macrophysics may be making them very sensitive to small variations in modification procedures. Nevertheless, this type of cloud is considered to be very promising in the future and is being intensively investigated in many research centres. It must be stressed that in the few cases where at least some local rules concerning modification potential have been established, (Furman, 1974, Gagin and Neumann, 1972), the effects of modification efforts become much clearer. (5) There are only few documented experiments with stratiform frontal clouds. American experiments conducted in 1953 and 1954 were discouraging. Mason (1971) found the modification potential of such clouds rather unpromising, mostly due to their high natural precipitation efficiency. On the other hand, Soviet experiments conducted during the sixties in the Ukraine gave up to 100 per cent increase in precipitation (probably mostly due to the dynamic side effects of seeding). This discrepancy is not easy to explain, since the dynamics of frontal systems needs further investigation. Among the more recent experiments I would point out the following few, particularly interesting due to their unconventional character: (1) Seeding of convective clouds with Agi has been going on in Florida, USA, since 1968. The peculiarity of this experiment is the intentional exploitation of the concept of 'dynamic seeding', aimed at causing rapid glaciation and invigoration of the cloud dynamics due to the liberated latent heat of fusion. There is hope that clouds so invigorated will tend to merge into larger and more effectively precipitating mesometeorological systems. The experiment is guided by a numerical one-dimensional cloud model as a tool for determination of the modification potential and evaluated with a sophisticated single target-area randomization procedure, with both area and particular clouds as experimental units. Until now, the experiment has been found to be inconclusive with respect to precipitation enhancement over a particular area but it seems to give positive results with particular clouds (Sax era/., 1973). (2) Seeding of continental type winter cumuli in Israel with Agi in 1961-1967 and later (Gagin and Neumann, 1972), aimed at an increase of water resources of Lake Tiberias. The experiment was conducted with an outstanding system of statistical evaluation, based on a cross-over randomization scheme, with an additional buffer area for excluding possible cross-contamination by the seeding agent. Parallel physical investigations were going on in order to clear up the details of the precipitation mechanism in these clouds and determine their modification potential. The experiments proved successful in increasing precipitation by 15-20 per cent at a high level cf statistical significance. However, remarkable differences in response between various clouds suggest carefulness when turning to 'industrial scale' modification operations. (3) The already mentioned Soviet experiment (Leskov, 1974), seeding stratiform frontal clouds with C0 2 , is proving the considerable modification potential of such clouds, despite earlier discouraging results in USA. (4) Great Lakes winter snowstorm seeding, conducted in 1968-1971 in the region of Lake Erie in the USA—one of the few aimed not at increasing the precipitation but at redistributing it through overseeding by ice nucleating agents (Agi and C0 2 ) and thus diminishing the size of snowflakes which can be then moved by the wind to greater distances. The clouds were shallow convective systems, thermally induced over a relatively warm lake surface; an interesting three-dimensional mesoscale model of their development (Lavoie, 1972) was used as an aid in the experiment. Though no precise statistical evaluation has been made (insufficient statistics and difficulties in obtaining dependable data on snowfall) a careful investigation of changes in ice crystal habit and number, throw much light on the modification potential of similar shallow convective systems which can be probably found in other areas. Exper-

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iments point out the importance of the cloud top temperature as a criterion for modification potential (Weickmann, 1973). (5) Soviet experiments on extinguishing forest fires in Siberia by the precisely localized seeding of convective clouds and experiments on suppressing young convective clouds in an unstable atmosphere by using mechanical impulses generated by vertically flying jet aircraft, or a heavy powdered load thrown into the cloud (Artsybashev, 1974; Khorguani and Kalov, 1974). Together with successes in the precise introduction of seeding material into developed Cb clouds (achievements connected mostly with hail suppression activities), these experiments create some hopes for possible manipulation of precipitation over a small area, which may help with water management in basins of local importance.

CONCLUSIONS As can be seen the present state of the art of cloud and precipitation modification is insufficiently advanced for application to water management efforts on an industrial scale with satisfactory predictable results. However, pilot experiments are recommended, and if correctly designed they may soon lead to efficient modification techniques, at least in certain geographic areas. In such experiments particular attention should be paid towards determination of modification criteria for various cloud types and systems. Progress in cloud physics and cloud modelling will make this job considerably easier, but some local effects may remain difficult to include in general rules or models. Thus any modification experiments should be accompanied (if not preceded) by local climatological and physical investigations of the clouds. Reliable evaluation of modification effects must be conducted; precise statistical evaluation techniques should be employed whenever possible—particularly in view of the fact that the amount of precipitation increase, which generally might be expected can often be covered by natural fluctuations. Attention should be paid to the fact, that most of the modification techniques can affect the clouds and precipitation in various ways, thus unexpected results can sometimes be obtained, as well as some undesired side effects. Finally a few words should be devoted to inadvertent weather modification, as this is a problem of increasing importance. Inadvertent weather modification can result either as a side effect of purposeful weather modification operations (for instance disturbances in rainfall accompanying hail suppression activities), or it can be a consequence of other human activities. From the point of view of the cloud physicist, the most dangerous might be changes in condensation and freezing nuclei spectra caused by industrial and urban pollution, since this might affect even large-scale weather and climate. There are a number of papers signalling effects of this sort but no estimate of their meteorological and climatological importance can yet be given, but it seems very probable that in forthcoming decades this may become one of the most important problems for meteorologists and hydrologists as well. A wider review of modern problems in weather modification can be found in the book edited by Hess (1974).

REFERENCES Artsybashev, E.S. (1974) Results of experimental and operational work on combating forest fires. In Proceedings of the WMO/IAMAP Scientific Conference on Weather Modification (held at Tashkent, USSR, in October 1973), p. 265 : WMO Publ., Geneva. Bakhanova, P. A. and Silayev, A.V. (1974) On a method of fog modification by passivation of condensation nuclei. In Proceedings of the WMO/IAMAP Scientific Conference on Weather Modification (held at Tashkent, USSR, in October 1973), p.13: WMO Publ., Geneva.

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Bergeron, T. (1968) Cloud physics research and the future fresh water supply of the world. In Proceedings of the International Conference on Cloud Physics (held at Toronto, Canada, August 1968), p. 744. Binh, Pham van (1974) The causes of dry weather over the Red River delta in early summer. Acta Geophys. Polonica, XXII, p. 101. Chappell, C.F. (1972) Airborne seeding of wintertime Wasatch Mountain clouds during Project Snowman. In III Conference on Weather Modification (held at Rapid City, South, South Dakota, USA), p. 129: Amer. Met. Soc. Furman, A.I. (1974) Results of seeding cumulonimbus clouds aimed at modification of precipitation in the steppe part of Ukraine. In Proceedings of the WMO/IAMAP Scientific Conference on Weather Modification (held at Tashkent, USSR, in October 1973), p. 51: WMO Publ., Geneva. Gagin, A. and Neumann, J. (1972) Rain stimulation and cloud physics in Israel. The Hebrew University of Jerusalem, Israel, Hess, W.N. (1974) Weather and Climate Modification: Wiley, New York-London-Sydney-Toronto. Hirsch, J.H. (1971) Computer modelling of cumulus cloud during Project Cloud Catcher. Report 17-7, Inst. Atmos. Sci., South Dakota School of Mines and Technology, Rapid City, South Dakota, USA. Khorguani, V.G. and Kalov, Kh. M. (1974) On the possibility of generating downdrafts by introducing coarse aerosol particles in the atmosphere. In Proceedings of the WMOI I AMAP Scientific Conference on Weather Modification (held at Tashkent, USSR, in October 1973), p. 301: WMO Publ., Geneva. Langmuir, I. (1948) The production of rain by chain reaction at temperatures above freezing. /. Met. 5, P. 175. Lavoie, R.L. (1972) A mesoscale model of lake effect storms,/. Atmos. Sci. 29, p. 1025. Leskov, B.N. (1974) The experimental seeding of frontal clouds in winter in order to increase precipitation. Va Proceedings of the WMO/IAMAP Scientific Conference on Weather Modification (held at Tashkent, USSR, in October 1973), p. 143: WMO Publ., Geneva. Mason, B.J. (1971) The Physics of Clouds, p. 395: Clarendon Press, Oxford. Orville, H.D., et al. (1974) Numerical simulation of cloud seeding experiments. In Proceedings of the WMO/IAMAP Scientific Conference on Weather Modification (held at Tashkent, USSR, October 1973) p. 81 : WMO Publ., Geneva. Sax, R.I., et al. (1973) The EML 1973 Florida area cumulus experiment. In Proceedings of the WMO/ IAMAP Scientific Conference on Weather Modification (held at Tashkent, USSR, October 1973), p. 309: WMO Publ., Geneva. Simpson, J. and Wiggert, V. (1968) 1968 Florida cumulus seeding experiment-numerical model results. Mon. Weath. Rev. 96, p. 87. Warner, J. (1974) Rain enhancement (review). In Proceedings of the WMO/IAMAP Scientific Conference on Weather Modification (held at Tashkent, USSR, October 1973), p. 41: WMO Publ., Geneva. Weickmann, H. (1973) The modification of Great Lakes winter storms. NOAA Technical Report, ERL 265 - APCL 26, Boulder, USA.

Appendix DERIVATION OF EQUATION (3) Let us assume that the droplets are spherical, so that m = f nr3d

(A-l)

where r is the radius of droplets. Then equation (2) can be rewritten as

*: = ëïz

(A-2)

dt Ad If the collecting droplet is of drizzle size or greater (say r > 100 p i ) its falling speed is at least an order of magnitude greater than that of the collected droplets, which typically have a radius of less than 20 fim and a falling speed of a few cm/s. So as a first approximation w can be identified with the falling speed of the collector. This speed is given by the balance between air drag and weight which can be written in the form £-5

= %nr3dg

(A-3) 601

so that w =

(A-4)

3pCD

and (A-2) can be rewritten as dr = EV 6pdCa àt and solved in the following form:

,1/2

(A-5) 1/2

ril2-rV2 = I EV

v 24dpC D .

at

(A-6)

Neglecting the initial radius 77 with respect to the final one and denoting the time of growth from the drizzle size to raindrop size by r = t- tj we get r»> = f J L Y / 2 EV 1 / 2 (A-7) \24dJ .(PCD) and

JpOl

(A-8) 3d2 (24)3 which after substitution in equation (1) gives equation (3). Adoption of equation (3) to solid precipitation can be made by a suitable choice of E and d and by replacing the coefficient Co following from the geometry of the sphere by another coefficient suitable for the given crystallographic form. However, let us notice that both this coefficient as well as d may now be variable with time. m =

602