The Microphysical Processes of Snow Formation

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.


•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


•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


•Deposition nucleation:• Water vapor deposits directly

on an ice nucleus• Unlikely on particles < 0.1μm• Colder temperatures increase

deposition nucleation, as does greater supersaturation


•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


•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?


•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


• 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


• 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


• 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


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


• 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.


• 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.


Initial conditions:• Air parcel near surface• Saturated environment• RH = 100%

• Water droplets form (CCN)


Time 1: • Air parcel rises & cools• Only supercooled liquid water

exists• Supersaturated environment • RH > 100% wrt water

• Water droplets grow


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


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


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)


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



Things to consider:1. The difference between ice and liquid saturation

vapor pressures is greatest between -8°C and -16°C.


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.

Documents

The Microphysical Processes of Snow Formation