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Cloud Seeding: Conditions, Processes, and Challenges

1. Introduction to Cloud Seeding

'The underlying concept is to treat those storms or portions of storms that are naturally inefficient to make them more efficient through cloud seeding'. (ASCE)

2. Cloud Types and Processes

2.1 Convective vs. Stratiform Clouds

Convective Clouds Stratiform Clouds
Formed by rising air currents Form in horizontal layers
Puffy, cauliflower-like appearance Generally produce lighter, widespread precipitation
Range from small fair-weather cumulus to large cumulonimbus Examples: stratus and altostratus clouds
Often associated with intense, localized precipitation

'The effects of convective cloud seeding have been mixed and often inconclusive since the discovery of the effects of dry ice on supercooled stratiform cloud precipitation process efficiency'... but 'results indicated that rainfall was increased downwind of the seeding activity'. (ASCE) Randomized hygroscopic seeding in South Africa and Thailand reached statistical significance and increased rainfall up to 30-60%.

2.2 Orographic Clouds

Clouds that form as a result of air being forced to rise over topographical features like mountains or hills

'Shallow widespread winter orographic cloud systems provided the best potential for precipitation augmentation through cloud seeding operations because their supercooled liquid water is long lasting and distributed over a large area'. (ASCE)

Orographic clouds are targeted for cloud seeding for several reasons.

  • Predictable formation: Orographic clouds form reliably when moist air is forced up over mountains, making them easier to target.
  • Supercooled liquid water: These clouds often contain supercooled liquid water, which is essential for glaciogenic seeding.
  • Natural precipitation enhancement: Mountains already enhance precipitation naturally, so seeding can potentially augment this effect.
  • Water resource management: Many regions rely on mountain snowpack for water supplies, making orographic cloud seeding attractive for water resource management.

Formation Process:

  1. Moist air encounters a topographical barrier (e.g., mountain)
  2. Air is forced to rise up the windward side
  3. As air rises, it cools and expands
  4. If air cools to its saturation point, water vapor condenses into cloud droplets

Characteristics:

  • Typically form on the windward side of mountains
  • Can appear as a "cap" over mountain peaks or a "collar" around them
  • May dissipate on the leeward side due to descending air motion (rain shadow effect)
  • Common cloud types: Altocumulus, Stratocumulus, Cumulus

3. Conditions for Cloud Seeding

3.1 Supercooled Liquid Water (SLW)

SLW refers to water droplets that remain liquid below the freezing point

SLW is a necessary but not sufficient condition for initiating glaciogenic cloud seeding (atmospheric stability and vertical wind profiles need to be examined to determine if seeding is feasible).

SLW:

Characteristic Range
Vertical thickness 100-500 m (typical), up to 1-2 km
Horizontal extent Few hundred meters to over 100 km

3.2 Temperature and Humidity Requirements

For glaciogenic seeding:

Temperature Range Effect
Above -10C Seeding materials less effective at nucleating ice
-10C to -25C Optimal range for seeding
Below -25C Natural ice crystal concentrations may already be high

3.3 Atmospheric Stability

Atmospheric stability plays a crucial role in the effectiveness of cloud seeding operations

Condition Effect on Seeding
Stable May limit vertical mixing of seeding agents
Unstable Can enhance vertical transport but may lead to rapid dispersal

3.4 Background Aerosol Concentrations

Natural vs. Anthropogenic Sources

Natural Sources Anthropogenic Sources
Sea spray (ocean aerosols) Industrial emissions
Dust from arid regions Vehicle exhaust
Volcanic emissions Biomass burning
Biological particles (pollen, spores, etc.) Agricultural activities

Impact on Cloud Microphysics

  • Aerosols serve as cloud condensation nuclei (CCN) and ice nuclei (IN)
  • Higher aerosol concentrations can lead to more numerous but smaller cloud droplets
This can affect:
  • Cloud lifetime
  • Precipitation efficiency
  • Radiative properties

Role in Cloud Seeding Effectiveness

Background aerosol concentrations can significantly impact the effectiveness of cloud seeding

Effects on Seeding:

High Concentrations Low Concentrations
Can reduce seeding effectiveness May allow seeding particles to have a more significant impact
Compete with seeding particles for available water vapor
Potentially overwhelm the impact of added seeding material

The composition of background aerosols (e.g., their hygroscopicity) also affects seeding outcomes

Optimal Seeding Conditions:

  • Generally, moderate background aerosol concentrations are ideal for seeding
  • Too clean environments may lack sufficient CCN for initial cloud formation
  • Too polluted environments may reduce the relative impact of seeding particles

4. Cloud Seeding Processes

4.1 Vapor Deposition

The process by which water vapor directly transforms into ice on a surface, without going through the liquid phase

Process:

  1. Water vapor molecules in supersaturated air diffuse towards ice particles or droplets
  2. Molecules adhere to the surface, growing the particle
  3. Growth rate depends on factors like temperature, pressure, and particle size
  4. As particles grow, they release latent heat, affecting further growth

4.2 Epitaxial Riming

Growth of small ice particles on the surface of larger ice particles along the same crystal axis

  • Results in faceted particles
  • Small particles inherit the same lattice structure as the underlying ice crystal
  • More common at higher temperatures (near 0C)
  • Tends to occur on larger ice particles

4.3 Collision-coalescence

Process by which cloud droplets collide and merge to form larger droplets

Key Steps:

  1. Cloud droplets of different sizes fall at different speeds
  2. Larger droplets fall faster and collide with smaller droplets
  3. Upon collision, droplets merge to form even larger droplets
  4. Process continues until droplets are large enough to fall as rain
In Hygroscopic Seeding:
  • Hygroscopic particles released at cloud base rise in updrafts
  • As they ascend, they grow by vapor deposition, becoming larger cloud droplets
  • Larger droplets have higher collection efficiency and fall speed than natural droplets
  • As they fall relative to smaller droplets, they collide and coalesce, growing larger
  • Process initiates earlier and faster than in unseeded clouds
  • Larger droplets continue growing as they fall, potentially becoming raindrops

5. Optimal Seeding Strategies

5.1 Cloud Selection

What makes a cloud a good candidate for precipitation / cloud seeding?

Clouds that have significant cloud condensate but lack an appropriate mechanism for hydrometeor growth to a precipitation-sized hydrometeor within their lifetime (i.e. are naturally inefficient / colloidally stable).

5.2 Timing and Location

Where and when is it most efficient to seed?

If we have a cloud that is 32F at its base and 25F at the top, we want to seed at the bottom of the cloud, where the temperature is closest to 32F.

Even though seeding at temperatures farther below freezing is associated with a higher ice nucleation rate and growth rates, it's actually worse due to a nucleation delay -- seeding at this level leads to fewer activated ice nuclei. You miss out on opportunities to form precipitation that may have been present had you seeded closer to freezing.

It's generally understood that there is a short window of opportunity for most effective seeding: generally, when the top of the growing tower passes from 0C to -10C to -12C.

Hygroscopic Seeding usually occurs just below cloud base in the area of maximum updraft to ensure that the cloud ingested the seeding agent early in the warm convective cloud's lifetime before the warm rain process is firmly established.

Because the effects of seeding are not instantaneous, seeding agents must be applied to moving cloud systems sufficiently in advance of their entering the target area. The width of this buffer zone clearly is dependent on the speed at which the system is moving.

5.3 Energy and Heat Considerations

The rapid conversion of supercooled water droplets to ice particles adds heat (latent heat of fusion) to the cloud w.r.t. the surrounding air, which increases buoyancy. This heated air is less dense than the surrounding air, and thus rises, leading to even greater cloud development. So seeding individual convective clouds results in more updrafts -> increased condensation -> increased precipitation.

6. Challenges in Cloud Seeding

6.1 Distinguishing Seeded vs. Natural Precipitation

  • One of the primary challenges in cloud seeding research
  • Requires sophisticated measurement techniques and statistical analysis
  • Recent advances include Improved radar technology and Tracer studies

6.2 Variability in Cloud Microphysics

  • Cloud properties can vary significantly even within a single cloud system
  • Makes it difficult to predict and quantify seeding effects
  • Requires careful monitoring and adaptable seeding strategies

6.3 Environmental and Regulatory Concerns

  1. Potential long-term impacts of seeding agents on ecosystems
  2. Regulatory frameworks vary by region and country
  3. Ongoing research into more environmentally friendly seeding materials