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The Microphysical Processes of Snow Formation

The Microphysical Processes of Snow Formation. Meteorologist Anthony Phillips Ball State University. Historic Snowstorm. Historic Snowstorm. 1993 Storm of the Century Formed: March 11, 1993 Dissipated: March 15, 1993 Lowest Pressure: 960 mb Max Winds: 110mph,

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The Microphysical Processes of Snow Formation

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  1. The Microphysical Processes of Snow Formation Meteorologist Anthony Phillips Ball State University

  2. Historic Snowstorm

  3. 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

  4. 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

  5. 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?

  6. 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

  7. Multiple Types of Ice?

  8. Ice Crystal Formation • Four processes control crystalline formation & growth: • Nucleation (formation) • Diffusion (growth) • Wegener-Bergeron-Findeisen Process • Collision-Collection • Let’s look at these in-depth with regards to solid hydrometeors…

  9. 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

  10. Ice Nuclei • Ice Nuclei: • Manufactured substances: • Silver iodide • Lead iodide • Cupric sulfide

  11. Some IN Critical Temperatures • Source: Stull (2000)

  12. Ice Crystal Formation - Nucleation • Nucleation: the onset of a phase transition (i.e., water vapor to liquid by condensation) • Two types of ice nucleation: • Homogeneous nucleation • Heterogeneous nucleation Nucleation of carbon dioxide bubbles around a finger.

  13. 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

  14. 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: • Deposition nucleation • Immersion freezing • Condensation freezing • Contact freezing

  15. 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

  16. 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:

  17. 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:

  18. 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?

  19. 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

  20. End Part I • Part II - Wednesday, December 8th: • Ice crystal habits • Ice crystal growth: • Diffusion • Wegner-Bergeron-Findeisen Process • Collision & Collection

  21. 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.

  22. More Ice Crystal Habits

  23. 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):

  24. 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

  25. 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 • Use the formula for finding diffusivity:

  26. Ice Crystal Growth by Diffusion • Now use the equation for finding the mass of a dendrite: • We know, • This is approximately the mass of a snow crystal

  27. 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

  28. 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.

  29. 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.

  30. The Wegener-Bergeron-Findeisen (WBF) Process • Initial conditions: • Air parcel near surface • Saturated environment • RH = 100% • Water droplets form (CCN)

  31. 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

  32. 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

  33. 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

  34. 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)

  35. The Wegener-Bergeron-Findeisen (WBF) Process • Time 5: • Growth stops when one or • more of the following occur: • No liquid droplets are • present to provide Wv • RH drops below 100% • wrt ice • 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

  36. The Wegener-Bergeron-Findeisen (WBF) Process

  37. The Wegener-Bergeron-Findeisen (WBF) Process • Things to consider: • The difference between ice and liquid saturation vapor pressures is greatest between -8°C and -16°C.

  38. The Wegener-Bergeron-Findeisen (WBF) Process • Things to consider (cont): • The WBF Process requires cold clouds (<0°C) • Also known as the cold cloud process • 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. • 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

  39. Collision and Coalescence Collection • 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)

  40. Aggregation

  41. Accretion/Riming

  42. Brief Summary • Cloud droplets can form on either: • Cloud Condensation Nuclei, CCN, within warm clouds (>0° C) OR • 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 • Deposition nucleation • Immersion freezing • Condensation freezing • Contact freezing

  43. Brief Summary • Ice crystals grow by one or more of the following: • Diffusion deposition • WBF Process • Aggregation • 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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