The Microphysical Processes of Snow Formation
Meteorologist Anthony PhillipsBall State University
Historic Snowstorm
1993 Storm of the Century
•Formed: March 11, 1993•Dissipated: March 15, 1993•Lowest Pressure: 960 mb•Max Winds: 110mph, Boone, North Carolina•150 mph derecho, Cuba•Fatalities: 300•Damages: $6 – 11 billion
Historic Snowstorm
What is Snow?
•Particles of white or translucent ice formed within a cloud that become heavy enough to fall to the ground
•Hexagonal form and often agglomerate into snowflakes
•Ice pellets and hail are not considered snow
Structure of an Ice Crystal
•As liquid water beings to freeze, hydrogen and oxygen align to form a crystalline lattice with hexagonal symmetry
•Tetrahedral bonding angle• 109.5°• Reason why ice is less dense
than liquid water
•Ice cannot exist above 0°C…or can it?
Multiple Types of Ice?
• We know that ice in our freezer (or anywhere on Earth’s surface), if brought above freezing , will melt• Known as Ice Ih
• There are however, 15 additional “phases” of solid H2O.• All other phases exist at lower
temperatures and/or very high pressures
Multiple Types of Ice?
Ice Crystal Formation
•Four processes control crystalline formation & growth:
1. Nucleation (formation)2. Diffusion (growth)3. Wegener-Bergeron-
Findeisen Process4. Collision-Collection
•Let’s look at these in-depth with regards to solid hydrometeors…
Ice Nuclei
•Ice Nuclei, abbreviated “IN”:• Can only be particles that have a similar molecular
structure as ice• Natural ice nuclei include:• Fine clay such as kaolinite• Bacteria and amino acids• Soot from forest
fires, volcanoes, etc• Other ice crystals• Seeder-feeder
mechanism
Ice Nuclei
•Ice Nuclei:• Manufactured substances:• Silver iodide• Lead iodide• Cupric sulfide
Some IN Critical Temperatures
•Source: Stull (2000)
Ice Crystal Formation - Nucleation
•Nucleation: the onset of a phase transition (i.e., water vapor to liquid by condensation)
•Two types of ice nucleation:1. Homogeneous nucleation2. Heterogeneous nucleation
Nucleation of carbon dioxide bubbles around a finger.
Ice Crystal Formation - Nucleation
•Homogeneous nucleation:• The spontaneous freezing of
liquid water droplets near -40°C• No ice nuclei or impurity is
needed• Most clouds are too warm for
this type of nucleation
Ice Crystal Formation - Nucleation
•Heterogeneous nucleation:• Predominant process in the
atmosphere• Takes place in the presence of
ice nuclei within a saturated environment• Several types:
1. Deposition nucleation2. Immersion freezing3. Condensation freezing4. Contact freezing
Ice Crystal Formation - Nucleation
•Deposition nucleation:• Water vapor deposits directly
on an ice nucleus• Unlikely on particles < 0.1μm• Colder temperatures increase
deposition nucleation, as does greater supersaturation
Ice Crystal Formation - Nucleation
•Immersion freezing:• Occurs with a liquid droplet that contains an
undissolved ice nucleus• As external cooling occurs, the droplet reaches its
critical temperature and freezes.• Larger droplets = more ice nuclei = better chance for
freezing at warm temperatures• To freeze half of a clouds droplets with radius R, the
temperature must fall to T, given statistically by:
)/ln(21 oRRTTT
mRKTKT o 53235 21
Immersion Freezing Problem
•Example:• How cold must a cloud become so that half of the 100
μm radius droplets would freeze due to immersed nuclei?
• Use the formula:
)/ln(21 oRRTTT
)5/100ln()3()235( mmKKT
CKT 29244
Ice Crystal Formation - Nucleation
•Condensation freezing:• A cross between deposition nucleation and immersion
freezing• Supercooled water condense around ice nuclei• Instantly freeze
• Nuclei particles are more attractive as condensation nuclei (compared to deposition nuclei) Why?
Ice Crystal Formation - Nucleation
•Contact freezing:• An uncontaminated supercooled water droplet makes
contact and hits an ice nucleus• Instant freezing occurs if the droplet is colder than the
critical temperature of the nucleus• Similar to when supercooled “freezing” rain makes
contact with cold trees and power lines• Ice crystals within the atmosphere are good contact
nuclei for supercooled water
End Part I
•Part II - Wednesday, December 8th:• Ice crystal habits• Ice crystal growth: • Diffusion• Wegner-Bergeron-Findeisen Process• Collision & Collection
Ice Crystal Growth by Diffusion
•Diffusion deposition:• Water vapor deposits directly on an ice crystal,
freezing instantly. • Due to differences in vapor pressure over water vs.
over ice
•Ice Crystal Habits• The hexagonal lattice
structure of solid water allows ice crystals to grow into a variety of shapes, known as habits.
More Ice Crystal Habits
Ice Crystal Growth by Diffusion
• Growth Rates by Diffusion:• Crystalline growth is best measured by mass rather
than radius length (which is used for liquid droplets)• Dependent on ice crystal’s habit• Column & thick plates (3-D):
• Dendrites (2-D):
• Needles and sheaths (1-D):
23
)( tSDm
2)( tSDm
2
1
)(exp tSDm
Ice Crystal Growth by Diffusion
• Growth Rates by Diffusion:
• m: mass of ice crystal (units kg)• D: diffusivity term:
• Where, c= 2.11x10-5 m2s-1
• P0= 101.3 kPa• T0= 273.15 K
• S: supersaturation fraction• t: time
23
)( tSDm 2)( tSDm
2
1
)(exp tSDm
94.1
0
0
TT
PPcD
Ice Crystal Growth by Diffusion
• Example: Calculate the mass of a dendritic ice crystal after 45 minutes of growth in a 20% supersaturation environment and under the following conditions:
P= 100 kPa T= -10°C
1. Use the formula for finding diffusivity:94.1
0
0
TT
PPcD
94.1125
15.273263
1003.101)1011.2(
KK
kPakPasmxD
1251099.1 smxD
Ice Crystal Growth by Diffusion
2. Now use the equation for finding the mass of a dendrite:
We know,
This is approximately the mass of a snow crystal
1251099.1 smxD
2)( tSDm
20.0%100/%20 S
min45t
2125 min4520.01099.1( smxm
kgxm 81021.3
Ice Crystal Growth by Diffusion
• 3-D crystals grow slowest over time• 2-D crystals, such as the dendrites, grow faster than 3-D ones• 1-D crystals with single linear dimensions grow fastest
The Wegener-Bergeron-Findeisen (WBF) Process
• In 1911, Alfred Wegener, a geologist and originator of the theory of continental drift, originally proposed a theory of ice crystal growth based on the difference in saturated water-vapor pressure between ice crystals and supercooled water droplets.
The Wegener-Bergeron-Findeisen (WBF) Process
• In the 1930's, the Swedish meteorologist Tor Bergeron and the German meteorologist Walter Findeisen contributed further to the theory which became known as the Wegener-Bergeron-Findeisen (WBF) Process, or more simply the Bergeron Process.
The Wegener-Bergeron-Findeisen (WBF) Process
Initial conditions:• Air parcel near surface• Saturated environment• RH = 100%
• Water droplets form (CCN)
The Wegener-Bergeron-Findeisen (WBF) Process
Time 1: • Air parcel rises & cools• Only supercooled liquid water
exists• Supersaturated environment • RH > 100% wrt water
• Water droplets grow
The Wegener-Bergeron-Findeisen (WBF) Process
Time 2: • Air parcel rises & cools further• Ice nuclei become activated • Ice crystals form and grow• Liquid droplets continue to
grow• RH > 100% wrt water and ice
The Wegener-Bergeron-Findeisen (WBF) Process
Time 3: • Ice crystals and liquid droplets
continue to grow• Supersaturated environ-
ment• However, ice crystals grow
slightly faster• Ice crystals are further
from ice saturation line• More supersaturated compared to liquid droplets
• RH is still > 100% wrt water and ice
The Wegener-Bergeron-Findeisen (WBF) Process
Time 4: • Water vapor continues to be
removed from the air• Supersaturation is reduced
further• RH < 100% wrt water• Liquid droplets begin to
evaporate• RH > 100% wrt ice• Ice crystals continue to grow (still
supersaturated)
• Net result: ice crystals grow at the expense of the evaporating liquid droplets (see bold arrows)
The Wegener-Bergeron-Findeisen (WBF) Process
Time 5: • Growth stops when one or
more of the following occur:1. No liquid droplets are present to provide Wv
2. RH drops below 100% wrt ice3. Ice crystals become to heavy and fall from cloud
• If the atmosphere below the cloud is unsaturated (dry), then ice crystals fall and evaporate• Evaporation cools the column, lowering the LCL, and eventually allows ice crystals & snow to reach the ground
The Wegener-Bergeron-Findeisen (WBF) Process
The Wegener-Bergeron-Findeisen (WBF) Process
Things to consider:1. The difference between ice and liquid saturation
vapor pressures is greatest between -8°C and -16°C.
The Wegener-Bergeron-Findeisen (WBF) Process
Things to consider (cont):2. The WBF Process requires cold clouds (<0°C)• Also known as the cold cloud process
3. If a large number of ice nuclei exist in the atmosphere, a large number of ice crystals will form…therefore the ice crystals are too small to precipitate.
4. If only a few ice nuclei exist in the atmosphere, only a few, large ice crystals will rapidly form…leaving behind many small liquid droplets
Collision and Coalescence
• Collision is the only way ice crystals merge• Remember Coalescence is a warm cloud process
• When ice particles collide and stick to one another, this is known as aggregation• When ice particles collide with supercooled liquid droplets, this is known as accretion (or riming)• Hydrometeors that become heavily rimed to the
point that the original crystalline habit is obscured are called graupeln (singular: graupel)• Graupel is less dense than hail (which forms
when liquid water does not instantly freeze to a solid hydrometeor)
Collection
Aggregation
Accretion/Riming
Brief Summary
• Cloud droplets can form on either:1. Cloud Condensation Nuclei, CCN, within warm
clouds (>0° C) OR2. Ice Nuclei, IN, within cold clouds (<0° C)
• Ice crystals can exist in air along with supercooled liquid drops• Heterogeneous nucleation is the predominant process of crystalline formation in our atmosphere
1. Deposition nucleation2. Immersion freezing3. Condensation freezing4. Contact freezing
Brief Summary
• Ice crystals grow by one or more of the following:1. Diffusion deposition• WBF Process
2. Aggregation3. Accretion
• Ice crystals grow at the expense of evaporating liquid droplets (Diffusion deposition)• Ice crystal collisions with other hydrometeors can result in merging to form snow aggregates (crystal-crystal collision), accreted/rimed ice crystals (crystal-liquid collision), or graupeln.
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