1 College of Applied Meteorology,Introduction to Weather Modification Dr. Jun Yang College of Applied Meteorology, NUIST, Nanjing, P.R. China October 23, 2007

2 Part II: Principle of Weather Modification

3 Principle of Weather Modification2.1 Precipitation Enhancement 2.2 Hail Suppression 2.3 Fog Dispersal 2.4 Other Severe Weather Phenomena 2.5 Inadvertent modification 2.6 Answers to Questions Most Often Asked

4 2.1 Precipitation Enhancement

5 -Food -Water -Shelter -SpaceWhy do we need water? 4 basic needs -Food -Water -Shelter -Space Corbett and Corbett: -” In the hierarchy of physical priorities for human existence, the need for water ranks above the needs for food, clothing, and shelter.” -Water is ”second only to air as a necessity for survival…”

6 Problem & Solution Insufficient water supply and sources to support rapid growth, therefore development and use of water supply is unsustainable Increase supply Cloud seeding

7 Problem & Solution 6 billion people consume 54% global fresh water presently. It will reach 70% in just due to population increase.

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9 Introduction Growth of precipitation particles is the result of two kinds of instabilities: 1. Larger drops increase in size at the expense of smaller drops in warm clouds by the collision-coalescence mechanism

10 Introduction Growth of precipitation particles is the result of two kinds of instabilities: 2. Existence of ice particles in an optimum range of concentrations in a mixed cloud grow by deposition at the expense of the droplets (and subsequently by riming and aggregation)

11 Introduction Ideas for modifying clouds and precipitation:Introduce large hygroscopic particles into warm clouds to stimulate growth of raindrops by collision-coalescence Introduce artificial ice nuclei into cold clouds in concentrations of 1 per liter to stimulate production of precipitation by the ice crystal mechanism Introduce high concentrations of artificial ice nuclei into cold clouds and reduce the concentrations of supercooled droplets and thereby inhibit the growth of ice particles [dissipate clouds and suppress growth of precipitation particles]

12 2.1.1 Glaciogenic Seeding Seeding of clouds with appropriate ice nuclei (e.g., silver iodide) or cooling agent (e.g., dry ice, liquid propane) to create or enhance the formation of ice crystals, particularly the conversion of supercooled water to ice.

13 2.1.1 Glaciogenic Seeding Silver iodide could act as an ice nucleus at temperatures as high as –4oC. Dry ice dropped into a cloud of supercooled droplets in a deep-freeze box, resulted in the production of numerous small ice crystals. Ice crystals form in the wake of dry ice by homogeneous nucleation because it has such a low temperature (-78oC)

14 2.1.1 Glaciogenic Seeding The two general approaches are1. Static seeding which focuses on microphysical processes; creation of ice crystals and particles; enhances graupel and snow production by increasing the number of ice particles and triggering precipitation process earlier in the cloud’s lifetime. Examples: Climax I and II; Israel; Project Whitetop.

15 2.1.1 Glaciogenic Seeding 1. Static seeding

16 2.1.1 Glaciogenic Seeding (1) Static Seeding: Convective CloudsStatistically significant rainfall increases were not obtained or, in the case of the Israeli experiments, continue to be debated. However, useful results or guidance was obtained which contributes to the current knowledge base in weather modification.

17 2.1.1 Glaciogenic Seeding (1) Static Seeding: Convective CloudsPhysical measurements in clouds are essential to provide an understanding of the underlying processes; High concentrations of ice crystals occur naturally in some cumulus clouds at temperatures as warm as –10°C thus allowing rapid production of precipitation particles; The window of opportunity for enhancing rainfall from a given cloud (system) is limited; Treatment can both enhance and reduce rainfall; Results based on small clouds might not be transferable to dynamically more vigorous and larger cloud complexes.

18 2.1.1 Glaciogenic Seeding (1) Static Seeding: Winter Orographic CloudsRecognition of the complex interactions between terrain and wind flow in determining regions of cloud liquid water and, later, through microwave radiometer measurements, the existence of a layer of supercooled water; Acknowledgment of the need to target and track the dispersion of seeding material and, again later, the demonstration of complex flow including ridge-parallel flows below the ridge crest exist in pronounced terrain; Evidence of marked increases in ice particle concentrations leading to increased precipitation depending upon the availability of supercooled liquid water;

19 2.1.1 Glaciogenic Seeding (1) Static Seeding: Winter Orographic CloudsRe-emphasis of the need for physical data that can be used together with numerical models to identify the spatial and temporal changes in cloud structure; Development of highly efficient silver chloro-iodide ice nuclei and other fast acting, highly efficient ice nucleating pyrotechnic and generator devices; Development of methods to detect traces of seeding agents in snowpack and rain water.

20 2.1.1 Glaciogenic Seeding (1) Static Seeding Winter Orographic CloudsAssociation, W. M., 1996: Weather Modification: Some Facts about Seeding Clouds. Third Edition ed., 18 pp.

21 2.1.1 Glaciogenic Seeding The two general approaches are (Cont.)2. Dynamic seeding The intent was to seed supercooled clouds with large enough quantities of ice nuclei (100–1000 cm-3) or coolant to cause rapid glaciation. Increased buoyancy was expected to cause the cloud to grow larger, ingest more water vapor, and yield more precipitation. It was postulated that increased precipitation would enhance downdrafts and outflows which, in turn, would initiate new convection and extend the effects of treatment. Examples: FACE I and II; Texas.

22 2.1.1 Glaciogenic Seeding 2. Dynamic seeding

23 2.1.1 Glaciogenic Seeding 2. The chain of Dynamic seedingStage 1 (~ 20 minutes or more): Initial vertical tower growth: rapid glaciation of the updraft regions of supercooled water by seeding agent; invigoration of the updraft through buoyancy increase produced by the release of latent heat; pressure falls beneath the actively growing tower. Stage 2 (~ 40 minutes): Horizontal cloud expansion, secondary growth: Enhanced downdraft, convergence at the interface between the downdraft and the ambient low-level flow, growth of secondary towers; horizontal enlargement. Stage 3: Interaction with neighboring clouds: Seeding of secondary towers, additional growth and merger of clouds on the mesoscale. Stage 4: Increased area rainfall: more rainfall is obtained from the available moisture than would have obtained naturally; there is an enhancement of moisture supply to the area.

24 2.1.1 Glaciogenic Seeding 2. Dynamic seedingThe complexities of ice formation in clouds where ice and supercooled water have been found at temperatures as high as –10°C and as low as –38°C, respectively; The dependence of ice formation upon CCN concentrations and sizes (e.g., freezing of large drops) and the role of primary and secondary ice formation in graupel production which have emerged from these experiments are areas of uncertainty; The importance of coalescence (and hence aerosols) on cloud structure, evolution and rain production; The importance and relationship between cloud dynamics and microphysics and the induced changes resulting from seeding; The power and limitations of existing radar systems as integral experimental tools and as possible means of verification of seeding results.

25 2.1.1 Glaciogenic Seeding Note:Static and Dynamic Seeding are not mutually exclusive because they both result in increased ice crystal concentrations and affect cloud dynamics. The same seeding material is used in both seeding concepts and only the quantity of seeding material is varied. While the dynamic seeding concept is primarily applicable to convective clouds, the static seeding concept has been widely utilized in orographic and layer-type clouds as well as in convective clouds. In convective clouds, both “static” and “dynamic” responses can occur in a mutually interactive fashion.

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27 Wing-tip Generators

28 Wing-tip IN Generators

29 Acetone-silver iodide generator attached to a Piper Aztec

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32 Long rectangular holes cut through a stratus cloud using crushed dry ice at rate of about 1 kg/km. Such holes develop in less than an hour and become at least 2 km wide. Project Cirrus, 1974. Association, W. M., 1996: Weather Modification: Some Facts about Seeding Clouds. Third Edition ed., 18 pp.

33 Modification of cold cloudsOverseeding of a supercooled cloud – dry ice

34 Airborne Flare Racks

35 Airborne Flare Racks

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37 Dry Ice Hoppers

38 An equipment for seeding liquid N2

39 2.1.2 Hygroscopic Seeding As opposed to glaciogenic seeding, is directed at promoting the coalescence of water droplets in the cloud. The intention is to promote particle growth through coalescence and thereby improve the efficiency of the rainfall formation process.

40 2.1.2 Hygroscopic Seeding 1. Large hygroscopic particle seeding, which seeds clouds with large salt particles (e.g., >10 μm dry diameter) to short-circuit the condensation growth process and provide immediate raindrop embryos to start the coalescence process. Examples: Project Cloud Catcher, India, Thailand.

41 2.1.2 Hygroscopic Seeding 2. Hygroscopic flare seeding, which focuses on broadening the initial drop spectrum during the nucleation process by seeding with larger than natural CCN (0.5μm to 3μm dry diameter) to enhance the coalescence process in warm and mixed-phase clouds. Examples: South Africa, Mexico experiments.

42 2.1.2 Hygroscopic Seeding Both the South African and Mexican experiments produced remarkably similar statistical results in terms of the differences in radar estimated rainfall for seeded versus non-seeded groups; In the South African and Mexican experiments, reevaluation of the results showed an increase in rain mass 30–60 minutes after seeding, significant at the 96 percent level (α=0.04) or higher;

43 2.1.2 Hygroscopic Seeding Marked differences in concentrations of ice particles were found in maritime clouds (high) versus continental clouds (low) signifying the active role of collision and coalescence in maritime clouds compared to continental clouds; Freezing temperatures increased with increasing drop size because larger droplets contain or have a higher probability of colliding with ice nuclei;

44 2.1.2 Hygroscopic Seeding Relatively large droplets (>24μm) played a role in ice multiplication processes, including mechanical fracturing during melting and evaporation and ice splinter formation during riming; A delayed response in radar-derived storm properties was a possible function of seeding-induced dynamic processes beyond the classical cloud physics results that links cloud condensation nuclei and droplet spectra to rain production; Hygroscopic seeding might overcome inhibiting effects on rainfall of air pollution.

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46 Wing-tip hygroscopic Nuclei Generators

47 Ground Based Flare Trees

48 Ground Based Generators

49 Effective Location Example: AgIprecipitation: 20 minutes;Above cloud base V = 10 m/s = 600 m/min; below cloud base V = 5 m/s Raindrop from cloud base to ground: 8 minutes Then: Effective Location is 12 km apart.

50 Cloud Seeding DifficultiesCloud vertical extent - If the cloud is of small vertical extent, the entire cloud may be converted to ice crystals before any are large enough to act as precipitation particles. The cloud may actually dissipate as the crystals fall slowly into dry air below cloud base. In towering clouds, wind shear may blow cloud tops away from their bases so that ice-crystals fall into clear air instead of growing in their descent through the cloud. Overseeding - when too many crystals are formed, destabilization does not occur because the crystals compete with each other to grow. Cloud lifetime - clouds must exist for ~30 min

51 2.2 Hail Suppression

52 Hail damage Much of the damage from hail is on crops. Hail is named the white plague by farmers. Damage to vehicles, buildings, particularly roofs and landscaping are also damaged during hail storms. Susceptibility to crop damage depends on the crop type, its stage of development, the size of the hail, and the magnitude of any wind accompanying the hail.

53 Hail damage

54 Hail damage

55 Hail damage

56 Hail damage

57 Positive points Ice storms and hail can be beneficial. They can knock down dead branches, releases seeds, and provides nesting and sleeping shelters for birds and animals.

58 Hail damage mitigationMany thunderstorms produce hail because they are slow to make ice, or make relatively few ice particles. These hail embryos grow rapidly in supercooled cloud, resulting in large hailstones that cannot melt completely after they fall from the parent storm before reaching the ground.

59 Hail damage mitigationEssential elements of Theory of hail growth for suppression: (1) Hail embryo formation process, including the microphysics of particle growth and the region or regions in the storm where such growth occurs; (2) Transport of embryos to regions of abundant supercooled liquid water where the further growth to hail is possible; (3) Growth trajectory of the hailstone itself as it passes through the strong updraft of a storm; (4) The time evolution of the storm’s updraft and cellular development.

60 Hail damage mitigationTreatment of the developing cloud towers with glaciogenic materials accelerates the precipitation development process, while producing many, more ice particles, which them compete for the available cloud water. More rain, and less and small hail results.

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62 Hail damage mitigationCritical aspects Correctly identifying hail cloud Seeding time Proper target point Quantity of seeding agent

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64 Mobile rocket launcher

65 rocket launching

66 Chinese Antiaircraft Gun

67 2.3 Fog Dispersal

68 2.3 Fog Dispersal Airport visibility problemNumber of fog events identified from surface observations between , for stations located in Rhode Island, Connecticut, southern New York, New Jersey, eastern Pennsylvania and northern Delaware. The number of events divided by the total number of valid observations (in %) is also shown for each station in table form.

69 2.3 Fog Dispersal Different techniques are being used to disperse warm (i.e. at temperatures greater than 0°C) and cold fogs. The relative occurrence of warm and cold fogs is geographically and seasonally dependent.

70 2.3 Fog Dispersal

71 2.3 Fog Dispersal How can cloud seeding help visibility?Visibility in a fog is inversely proportional to the number concentration of droplets and to their total surface area Improve visibility by decreasing either the concentration or the size of the droplets Clearing by this method has not been widely used due to its expense and lack of dependability

72 2.3 Fog Dispersal Most effective methods for dissipating warm fogsEvaporate fog droplets by gound-based heating (e.g. burning hydrocarbon fuels) This method of fog dissipation was used quite successfully at 15 airfields in England during World War II

73 2.3 Fog Dispersal Most supercooled fogs form when nocturnal radiational cooling or moisture-laden air moves over a much colder surface. Treatment of the supercooled fogs with glaciogenic agents accelerates the ice-formation process, producing many more ice particles to compete for the available fog (cloud) water, quickly depleting it.

74 2.3 Fog Dispersal The resulting ice crystals settle slowly to the ground, collecting any remaining cloud droplets as they fall. Visibility is markedly improved, often to the point of sunlight reaching the ground.

75 2.3 Fog Dispersal The thermal technique, which employs intense heat sources (such as jet engines, petroleum burning) to warm the air directly and evaporate the fog has been shown to be effective for short periods for dispersal of some types of warm fogs.

76 2.3 Fog Dispersal These systems are expensive to install and to use. Another technique that has been used is to promote entrainment of dry air into the fog by the use of hovering helicopters or ground based engines. These techniques are also expensive for routine use.

77 2.3 Fog Dispersal To clear warm fogs, seeding with hygroscopic materials has also been attempted. An increase in visibility is sometimes observed in such experiments, but the manner and location of the seeding and the size distribution of seeding material are critical and difficult to specify. In practice the technique is seldom as effective as models suggest.

78 2.3 Fog Dispersal Result of cloud seeding to disperse fog at airport in Alaska Hector Vasquez, NWS Phoenix

79 2.4 Other Severe Weather Phenomena

80 Lightning suppressionCloud-to-ground (CG) lightning has been a major cause of fires in man-made structures and in forests, and it has been the cause of many human deaths. The concept usually proposed involves reducing the electric fields within thunderstorms so that they do not become strong enough for lightning discharges to occur.

81 Lightning suppressionSeeding by silver iodide; extra ice crystals will increase the leakage current between the two main charge centers in the thunderstorm Experiment results inconclusive Type of lightning in seeded storms had a larger percentage of short-duration strokes- less likely to ignite forest fires LSafety40min.ppt

82 Lightning suppressionSeeding using tiny metallic chaff Observed lightning discharges in the seeded clouds were about one third or less of those in the unseeded clouds Model results showed that the chaff produced large numbers of positive and negative ions, leading to a decrease in the vertical electric field in the cloud.

83 Lightning suppressionLightning is triggered by launching a small rocket trailing a grounded wire. It has been found that lightning flashes can be triggered from clouds to ground roughly 50 percent of the time. The University of Florida’s Lightning Research Center The International Center for Lightning Research and Testing at Camp Blanding, Florida Other research centers

84 Lightning suppressionHowever, these few studies are qualitative in nature and are not statistically significant due to limited evaluation capabilities at the time.

85 Hurricanes Postulated that seeding with artificial ice nuclei just outside of the eye-wall cloud will broaden the “warm core” (through the release of the latent heat of fusion) of the storm and spread out the region of low pressure in the eye  reduction in wind speeds Esther (1961), Beulah (1963), and Debbie (1969) were seeded with silver iodide. All storms showed some wind speed reductions following seeding. However, the reductions were not outside the range of natural variability

86 Hurricanes Results were inconclusive, and there currently is no generally accepted scientific conceptual model suggesting that hurricanes can be modified.

87 Tornadoes Although modification of tornados and other storms producing damaging winds is desirable for safety and cost reasons, there presently is no scientifically acceptable physical hypothesis to accomplish such a goal.

88 Freezing Drizzle and RainSpeculations can be made about the possibilities of reducing aircraft icing episodes or mitigating icing of highways and roads by seeding nearby supercooled cloud regions, but there is no physical, conceptual model on how to mitigate these hazards and no work has been done in this field.

89 Flash Floods and Large-Scale FloodingNo physical conceptual model exists to mitigate these events and no work has been conducted in this field. If the precipitation processes were fully understood, then perhaps procedures could be designed to decrease rains from flood-producing rain clouds. Accurate numerical modeling of such conditions would be necessary for such studies.

90 2.5 Inadvertent modification

91 Inadvertent modificationHuman activity is inducing inadvertent effects in the atmosphere on scales ranging from the local (a given point source of pollution, urban heat island, contrails, etc.) to the global (changes in greenhouse gases and aerosols and associated cloud effects).

92 Inadvertent modification1957, Gunn and Phillips: The detrimental effects of air pollution CCN on clouds and precipitation 1974, Twomey: Increased pollution results in greater CCN concentrations and numbers of cloud droplets, which in turn increase the reflectance of clouds. 1984, Twomey et al.: Enhanced cloud albedo has a magnitude comparable to that of greenhouse warming and acts to cool the atmosphere. Biomass burning and other anthropogenic sources of aerosols affect the radiative properties of clouds and precipitation processes in clouds, leading also to changes in the dynamical processes in clouds (i.e., effects on cloud lifetimes). 1989, Albrecht: Increased CCN lead to higher droplet concentrations and a narrower droplet spectrum (which manifests itself as a higher cloud albedo), which leads to suppressed drizzle formation and longer lasting stratiform clouds (e.g., ship-track studies).

93 Inadvertent modificationOutputs from a single paper mill affects the valley in which it is situated, and adjoining areas out to about 30 km

94 Inadvertent modificationPaper mill in WA state Mean annual precip in a region downwind of the mill was more than 30% greater, relative to the surrounding regions, during its active period than during an earlier period when production was low Mill emits about 1017 s-1 of large CCN Clouds situated in the mill plume contain higher concentrations of cloud drops with diameters > 30 mm than clouds unaffected by the plume Increases the efficiency of the collision-coalescence mechanism for the production of precipitation Heat and water vapor emissions from the mill also play a role

95 Inadvertent modificationThis smoke stack of a mining complex in Manitoba, Canada, causes the pollution track originating at the white asterisk. Satellite remote sensing image of yellow pollution tracks in the clouds, due to reduced droplets size. SOURCE: Photo provided by W. L. Woodley, Woodley Weather Consultants. Image adapted from Rosenfeld (2000).

96 Inadvertent modificationSatellite-retrieved effective droplet (reff) radius near cloud top for polluted cases (solid lines) and corresponding pristine locations (broken lines). This suggests substantial alteration of cloud properties by anthropogenic influences in ways that might inhibit precipitation. SOURCE: Ramanathan et al. (2001).

97 Inadvertent modificationSlash and burn agriculture practices Burning agricultural waste introduces large concentrations of small CCN into the atmosphere; these tend to produce a continental-type droplet distribution in clouds and decreases rainfall

98 Inadvertent modificationOther miscellaneous effects Lead particles from combustion can combine with other chemicals to form ice nuclei Large cities; sources of aerosol, gases, heat, and water vapor, modify radiative properties of the earth’s surface, “heat islands”, anomalous precipitation in their vicinity (e.g. St. Louis)

99 2.6 Answers to Questions Most Often AskedAssociation, W. M., 1996: Weather Modification: Some Facts about Seeding Clouds. Third Edition ed., 18 pp.

100 Q1. Does cloud seeding really work?A. Yes! Fifty years of research and operations in more than 40 countries have demonstrated that properly designed programs operated by competent persons can dissipate supercooled fog, beneficially increase seasonal rainfall or snowfall, and decrease seasonal hail damage.

101 Q2. Is there a large amount of silver iodide or other seeding material in the precipitation which falls from seeded clouds? A. No. The amounts are very small. The typical concentration of silver in rainwater or snow from a seeded cloud is less than 0.1 micrograms per litre (one part in 10,000,000,000).

102 Q3. Is such a concentration harmful to people or the environment?A. No. The silver concentration in rainwater from a seeded storm is well below the acceptable concentration of 50 micrograms per litre as set by the U. S. Public Health Service. Many regions have much higher concentrations of silver in the soil than are found in precipitation from seeded clouds. The concentration of iodine in iodized salt used on food is far above the concentration found in rainwater from a seeded storm. No significant environmental effects have been noted around operational projects, including projects of 30 to 40 years duration.

103 Q4. What about downwind effectsQ4. What about downwind effects? Do you sometimes "rob Peter to pay Paul?" A. No. The idea that rainfall increases in one area must be offset by decreases elsewhere is a misconception. Precipitation data from a number of cloud seeding projects have been examined in detail for evidence of "extra-area" effects. In some cases, there have been weak indications of increased precipitation at dis-tances of 150 km (90 miles) or more downwind from the target areas. There are no significant indications of rainfall decreases downwind from any long term cloud seeding projects.

104 Q5. If microscopic particles in the atmosphere have such a strong influence on Nature's ability to produce clouds and precipitation, don't all of the particles from homes, industry, automobiles, and even campfires in wilderness areas influence the weather? A. Yes. Every particle in the atmosphere, regardless of how it originates, may influence the formation and growth of clouds, including their ability to produce precipitation and to modify the radiative properties of the atmosphere.

105 Q6. In that case, does natural weather exist any more?A. If "natural weather" means weather completely unaffected by human beings, there has been no such thing since human beings learned to use fire. However, impacts of human activities on weather and climate were much smaller before the Industrial Revolution than they are today.