cloud seeding 1

8/13/2019 Cloud Seeding 1

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Cloud seeding is a form of weather modification. It can be used to

disperse fog, suppress hail, or control winds, but is most often used to

increase precipitation. In order to understand the process, however, a

basic understanding of clouds and how precipitation is formed isneeded.

As warm air rises from the Earth, it begins to cool and forms tiny

droplets of water that condense into cloud droplets. Cloud droplets are

formed around particles of dust, salt, or soil (called cloud condensationnuclei) that are always present in the atmosphere. These cloud droplets

group together into clouds, which can form precipitation in one of two

ways. In warm temperatures, the droplets in the clouds merge with many

other droplets and become heavy enough to fall to the Earth as rain. (It

takes millions of cloud droplets to form a single raindrop.) In colder

temperatures, the droplets of water form ice crystals. Other droplets

freeze onto these ice crystals, which grow larger and heavier until they

fall to the ground as rain, snow, or hail.



Cloud seeding is actually a very complex process. In the

simplest terms, it introduces other particles into a cloud to

serve as cloud condensation nuclei and aid in the

formation of precipitation. There are three types of cloudseeding: static mode, dynamic mode, and hygroscopic

seeding.

Static mode cloud seeding seeks to increase rainfall by

adding ice crystals (usually in the form of silver iodide ordry ice) to cold clouds. Dynamic mode cloud seeding

increases rainfall by enhancing "vertical air currents in

clouds and thereby vertically process more water through

the clouds." Basically, in this method of seeding, a much

larger number of ice crystals are added to the cloud than

in the static mode. In hygroscopic seeding, salt crystals are

released into a cloud. These particles grow until they are

large enough to cause precipitation



HOW DID CLOUD SEEDING BEGIN?

In the late 1940’s, a discovery was made at

the General Electric labs in Schenectady, NewYork. During an unrelated experiment, it was

noticed that dry ice shavings had the ability to

convert super cooled water droplets (thoseexisting as water at temperatures lower than

freezing) to ice crystals. Later experimentation of

those observations led to a series of laboratorytrials which confirmed the nucleating properties of

various materials in certain cold cloud conditions.

The laboratory results were promising and trials



The main objective of the ``static mode'' of cloud seeding is to increase the efficiency of

precipitation formation by introducing an ``optimum'' concentration of ice crystals in

supercooled clouds by cloud seeding. It was originally thought that clouds were

deficient in ice nuclei and therefore additions of modest concentrations of ice nuclei

should result in a more efficient precipitation-producing cloud system. All that was

needed was to introduce seeding material from the ground or at the base of clouds

which would then enhance ice crystal concentrations and thereby increase rainfall.

Cotton and Pielke (1995) concluded that physical studies and inferences drawn from

statistical seeding experiments over the last 50 years suggests that there exists a much

more limited window of opportunity for precipitation enhancement by the static-mode

of cloud seeding than was originally thought. The window of opportunity for cloudseeding appears to be limited to:

1. clouds which are relatively cold-based and continental;

2. clouds having top temperatures in the range -10 to -25 C;

3. a time scale limited by the availability of significant super cooled

water before depletion by entrainment and natural precipitation

processes.



While the fundamental concept of the ̀static mode' of cloud seeding is

that precipitation can be increased in clouds by enhancing their

precipitation efficiency, alterations in the dynamics or air motion in

clouds due to latent heat release of growing ice particles, redistribution ofcondensed water, and evaporation of precipitation is also inevitable.

Alterations in the dynamics of clouds, however, is not the primary aim of

the strategy. By contrast, the focus of the `dynamic mode' of cloud

seeding is to enhance the vertical air currents in clouds and thereby

vertically process more water through the clouds resulting in increased

precipitation. The main difference in implementation of the strategy is

that larger amounts of seeding material are introduced into clouds. A

goal in the static mode of seeding is to achieve something like 1 to 10 ice

crystals per liter at temperatures warmer than -15C. In the dynamic modeof seeding the target ice crystal concentration is more like 100 to 1000

ice crystals per liter, which corresponds to seeding as much as 200 to

1000 g of silver iodide in flares dropped directly into the high super

cooled liquid water content updrafts of cumuli.



In the 1960's to the 1980's, the hypothesized chain of

physical responses to the insertion of such large quantities

of seeding materials as summarized by Woodley et al.

(1982) included the following: (1) the nucleated ice crystalsglaciate a large volume of the cloud releasing the latent heat

of freezing and vapor deposition, (2) this warms the cloud

yielding additional buoyancy in the seeded updrafts, (3)the

updrafts with enhanced buoyancy accelerate causing thecloud towers to ascend deeper into the troposphere, (4)

pressure falls beneath the seeded cloud towers and

convergence of unstable air in the cloud wil l as a result

develop, (5) downdrafts are enhanced, (6) new towers wi l ltherefore form, (7) the cloud wil l widen, (8) the likel ihood

that the new cloud wil l merge with neighboring clouds wil l

therefore increase, and (9) increased moist air is processed

by the cloud to form rain.



As a result of their experience in Texas, Rosenfeld and

Woodley (1993) proposed an altered conceptual model

of dynamic seeding as follows:1) NONSEEDED STAGES (i) Cumulus growth stage

The freezing of supercooled raindrops plays a major

role in the revised dynamic seeding conceptual model.

Therefore, a suitable cloud is one that has a warm base

and a vigorous updraft that is strong enough to carry

any raindrops that are formed in the updraft above the

0 C isotherm level. Such a cloud has a vast reservoirof latent heat that is available to be tapped by natural

processes or by seeding.



(ii)Supercooled rain stage At this stage a significant amount of supercooled cloud and rainwater exists between

the 0 and the -10 C levels, which is a potential energy source for future cloud growth.A cloud with active warm rain processes but a weak updraft will lose most of the water

from its upper regions in the form of rain before growing into the supercooled region.

Therefore, only a small amount of water remains in the supercooled region for the

conversion to ice. Such a cloud has no dynamic seeding potential.

(iii)The cloud-top rain-out stage If the updraft is not strong enough to sustain the rain in the supercooled region until it

freezes naturally,most of it will fall back toward the warmer parts of the cloud without

freezing. The supercooled water that remains will ultimately glaciate. The falling rain

will load the updraft and eventually suppress it, cutting off the supply of moisture andheat to the upper regions of the cloud, thus terminating its vertical growth. This is a

common occurrence in warm rain showers from cumulus clouds.



(iv) The downdraft stage At this stage, the rain and its associated downdraft reach the surface,

resulting in a short-lived rain shower and gust front.

(iv) The dissipation stage The rain shower, downdraft, and convergence near the gust front weaken

during this stage, lending no support for the continued growth of

secondary clouds, which may have been triggered by the downdraft and

its gust front.



2) SEEDED STAGES

(i) Cumulus growth and supercooled rain

These stages are the same for the seeded sequence as they are for natural

processes.

(ii) The glaciation stage

The freezing of the supercooled rain and cloud water near the cloud top

at this stage may occur either naturally or be induced artificially byglaciogenic seeding. This conceptual model is equally valid for both

cases.

The required artificial glaciation is accomplished at this stage through

intensive, on-top seeding of the updraft region of a vigorous supercooled

cloud tower using a glaciogenic agent (e.g., AgI). The seeding rapidly

converts most of the supercooled water to ice during the cloud's growth

phase. The initial effect is the formation of numerous small ice crystals

and frozen raindrops



(i i i ) The unloading stage The greater precipitation mass in the upper portion of the tower eventually moves

downward along with the evaporatively cooled air that was entrained from the drier

environment during the tower's growth phase. When the precipitation descends throughthe updraft, it suppresses the updraft. If the invigorated pulse of convection has had

increased residence time in regions of light to moderate wind shear, however, the

precipitation-induced downdraft may form adjacent to the updraft, forming an enhanced

updraft-downdraft couplet. This unloading of the updraft may allow the cloud a second

surge of growth to cumulonimbus stature.When the ice mass reaches the melting level, some of the heat released in the updraft

during the glaciation process is reclaimed as cooling in the downdraft. This downrush

of precipitation and cooled air enhances the downdraft and the resulting outflow

beneath the tower.

(iv) The downdraft and merger stage The precipitation beneath the cloud tower is enhanced when the increased water mass

reaches the surface. In addition, the enhancement of the downdraft increases the

convergence at its gust front



.

(v) The mature cumulonimbus stage

The enhanced convergence acts to stimulate more neighboring cloud growth,

some of which will also produce precipitation, leading to an expansion of the cloud

system and its conversion to a fully developed cumulonimbus system.

When this process is applied to one or more suitable towers residing within a

convective cell as viewed by radar, greater cell area, duration, and rainfall are the result.

Increased echo-top height is a likely but not a necessary outcome of the seeding,

depending on how much of the seeding-induced buoyancy is needed to overcome the

increased precipitation loading.

(vi) The convective complex stage

When seeding is applied to towers within several neighboring cells, increased

cell merging and growth will result, producing a small mesoscale convective system

and greater overall rainfall.

This is an idealized sequence of events. Dissipation may follow the glaciation

stage or at any subsequent stage if the required conditions are not present.''



One method of seeding clouds to enhance

precipitation is to introduce hygroscopic particles

(salts) which readily take on water by vapor depositionin a supersaturated cloudy environment. The

conventional approach is to produce ground salt

particles in the size-range of 5-100 , and release these

particles into the base of clouds. These particles grow

by vapor deposition and readily reach sizes of 25 to 30

in diameter or greater. They are then large enough to

serve as ``coalescence'' embryos and initiate orparticipate in rain formation by collision and

coalescence



Seeding of tropical cumulus clouds, and indeed any clouds, requires that they contain

supercooled water--that is, liquid water colder than zero Celsius. Introduction of a

substance, such as silver iodide, that has a crystalline structure similar to that of ice will

induce freezing. In mid-latitude clouds, the usual seeding strategy has been based upon

the vapor pressure being lower over water than over ice. When ice particles form in

supercooled clouds, they grow at the expense of liquid droplets and become heavy

enough to fall as rain from clouds that otherwise would produce none.

Seeding of tropical cumuli sought to exploit the latent heat released by freezing as well.

This strategy of "dynamic seeding" assumed that the additional latent heat would add

buoyancy, strengthen the updrafts, ensure more low-level convergence, and ultimately

cause explosive growth of properly selected cumuli.



The above sketch illustrates an aircraft dispensing

pyrotechnics doped with silver iodide into asupercooled cloud that is invigorated by the latent heat

released as the boundary between liquid and frozen

hydrometeors (blue horizontal line in the cloud on the

left) moves down to the zero Celsius isotherm (green

horizontal line). It was this transformation that the

experimenters hoped to use for construction of an

artificial outer eye wall during Project STORMFURY.



Historically, there are three main ways to introduce seeding agents:

ground-based generators Clearly, ground-based generators must use

seeding agents that will survive normal ground temperatures, so the

generators typically produce a "smoke" that contains the seeding agent.

The ability of ground-based generators to deliver seeding agents to theappropriate level in the cloud is open to question, except perhaps in

situations where the clouds are being seeded to enhance snowfall in

mountainous regions (where the ground temperatures are already below

freezing and the generators are, effectively, already within the clouds).rockets or artillery shells Again, these delivery methods must use

chemical seeding agents. The Soviets used artillery for many years to

introduce seeding agents as a mechanism for hail suppression. I'll have

more to say on this later.

aircraft Delivery methods based on aircraft are relatively expensive, but

have the clear advantage that it should be possible to put a large fraction

of the seeding agent more or less just where it is intended to go. Using

aircraft, some of the cheaper alternatives to chemical agents become

viable, such as dry ice or crushed water ice.

cloud seeding 1

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