1. Lecture 13: Cold Clouds

2. PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
Cold Cloud Processes
Wang and Olsen.
Warm Cloud Processes
PHYS Clouds, spring ‘04, lect. 5, Platnick 2

4. ICE Crystal Habits
For Ice crystals, there are many unknown habits…

6. 6

7. PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
Ice Crystal Habits
main types
Variables
Temperature
primary
Supersaturation
secondary
Electric Field
minor
Wang and Olsen slide.
PHYS Clouds, spring ‘04, lect. 5, Platnick 7

8. PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
c axis
basal face
prism face
a axis
Growth on
T(C)
Growth habits
prism
0 – -4
thin plates
basal
-4 – -10
needles
-10 – -20
plates, dendrites
-20 – -50
hollow columns
Wang and Olsen slide.
PHYS Clouds, spring ‘04, lect. 5, Platnick 8

9. Ice Cloud Microphysics CRYSTAL-FACE, A. Heymsfield
25 July 2002
(VIPS)
25 July 2002
(VIPS)
CPI: 7 July 2002
For ice crystals, you have another unknown - particle habit.
C-F Citation:
VIPS - video particle imager (down to 5-10 µm), well-defined volume, capable of determining size distributions
CPI - down to ~ 20 µm
Andy says in strong updrafts, high concentration of small spherical-like ice particles from homogeneous nucleation
=> Some assumption regarding habit must be made to enable the retrieval method previously described.
PHYS Clouds, spring ‘04, lect. 5, Platnick 9

10. Ice cloud microphysics, cont.
sep slide.
PHYS Clouds, spring ‘04, lect. 5, Platnick 10

11. -dependency on temperature and supersaturation
Ice Crystal Habits
-dependency on temperature and supersaturation
Wang and Olsen slide. Heymsfield et al. ???
Where you see largest ice crystal? 11

12. MODIS ice crystal library habits/shapes
Aggregates include rough surfaces a al Cox and Munk, otherwise smooth
D is maximum dimension (e.g, size dist. x-axis)
Heymsfield replicator images?
PHYS Clouds, spring ‘04, lect. 5, Platnick 12

13. Ice Clouds have particles of many sizes and shapes
3 layer model
*1 km 10 to 50 um spheres
*1 km 300 um columns
*2 km 150 um aggregates
and bullet rosettes. 13 13 13

14. Magano & Lee (1966)
Wang and Olsen slide. 14

15. MODIS Ice Cloud Size Distributions (based on FIRE II IFO, re ~ 7 – 60 µm)
King, M. D., S. Platnick, P. Yang, G. T. Arnold, M. A. Gray, J. C. Riédi, S. A. Ackerman, and K. N. Liou, 2004: Remote sensing of liquid water and ice cloud optical thickness and effective radius in the arctic: Application of airborne multispectral MAS data. J. Atmos. Oceanic Technol., 21, 857–875. 15

16. ICE FORMATION AND CONDENSATIONAL GROWTH

17. PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
Ice Clouds:
Nucleation and Growth
Nucleation
– Homogeneous, heterogeneous, ice nuclei
– Habits (shapes)
Ice crystal growth
– Growth from vapor (diffusion)
– Bergeron process (growth at expense of water droplets)
– Ice multiplication process
– Collision/coalescence (riming, aggregation)
Size distributions
– Microphysical measurements, temperature dependencies
PHYS Clouds, spring ‘04, lect. 5, Platnick 17

19. Many different types of particulates in the atmosphere can act as ice nuclei, both natural and anthropogenic, including those composed of minerals, soot, organic matter and sulfate. However, the exact nucleation potential of each type varies greatly, depending on the exact atmospheric conditions. Very little is known about the spatial distribution of these particles, their overall importance on global climate through ice cloud formation and whether human activity has played a major role in changing these effects. However, snowflakes appear to be often formed around biological ice nucleators,[2] typically bacterial cells.[3]

20. Class Activity What is ice nuclei (IN)?
What is the difference between IN and CCN?
Q: Given a mixed cloud containing water and ice,
which type of particle (ice v.s. water) will grow more quickly and why????

21. Precipitation Growth in Cold Clouds - Warm versus Cold Clouds
Our previous discussion regarding droplet growth by condensation and collisions is valid for warm clouds:
warm clouds - have tops warmer than about 0°C
comprised entirely of water

22. Cold Clouds
cold clouds are defined as those clouds with tops colder than 0°C
can be comprised of:
-water
-super-cooled water - liquid droplets observed at temps less than 0°C
-ice
Notice that super cooled water is found at altitudes where:
-40°C < Temp < 0°C
only ice is found at altitudes above -40°C
Q: So how does frozen precipitation form in cold clouds?

23. Ice crystal formation by Homogeneous Freezing
Pure water drops do NOT freeze at 0°C
it needs to be colder
bigger water drops will freeze at warmer temperatures than smaller drops
smaller water drops require colder temperatures to freeze
hence, you will find more smaller drops than larger drops higher in the cloud

24. Saturation vapor pressure over water and ice
First, note that es(water) > es (ice)
Q: where is this difference
a maximum???

25. Difference in saturation vapor pressure over water and ice
es(water) - es (ice) is a maximum at -15°C
Hence, it is near this temperature in a cold cloud that ice particles will grow more readily than water particles

26. Ice Crystal Growth through Deposition
Vapor can deposit onto ice nuclei (IN) in a cloud
There tends to be more cloud condensation nuclei (CCN) than ice nuclei (IN):
therefore, there tends to be more super cooled water droplets formed by condensation than ice particles formed by deposition at altitudes where -40°C < Temp < 0°C

27. Ice Crystal Growth through Contact Freezing
Occurs when a supercooled drop comes in "contact" with an ice nuclei
causes the supercooled drop to freeze

28. Accretion/Riming
Occurs when super cooled drops freeze onto ice particle
the resultant particle is often referred to as graupel

29. Aggregation
Occurs when two ice particles stick together, forming one larger particle
All of the aforementioned processes are occurring in a cold cloud to form ice particles
-homogeneous freezing
-deposition
-contact freezing
-accretion
-aggregation
Q: Given a mixed cloud containing water and ice,
which type of particle (ice v.s. water) will grow more quickly and why????

30. Growth of ice and water particles
Consider three situations:
In a hypothetical cloud, let's say that es (water) = 5 mb and es (ice) = 2 mb
Now, if:
the vapor pressure in cloud (e) = 6 mb, will the water and/or ice grow in this cloud? ANSWER
the vapor pressure in cloud (e) = 4 mb, will the water and/or ice grow in this cloud? ANSWER
the vapor pressure in cloud (e) = 1 mb, will the water and/or ice grow in this cloud? ANSWER

31. Cloud Seeding - Weather Modification
Often, cold clouds do not precipitate for two reasons:
cloud does not have enough ice nuclei (remember, ice particles will likely grow more readily than water particles)
not enough vapor in the cloud (e is too low)
Hence, the idea of cloud seeding:
consider the cloud to the right -->>
es (water) = 5 mb
es (ice) = 2 mb
e (cloud) = 3 mb
ice could grow, but don't have enough ice nuclei....., what to do????
add more ice nuclei to a cloud - form more ice particles in the cloud that may grow to precip size

32. Cloud Seeding - Addition of Ice Nuclei to a Cold Cloud
To add more ice nuclei to the cloud, simply fly an airplane over it or under the updraft and release ice nuclei:
silver iodide
dry ice
It is possible to release to many ice nuclei into the cloud:
in this situation, you will have many small ice particles competing for vapor to grow
there there is not enough vapor, then you end up with a cloud full of small ice particles
can't grow to precip size - suppressed precipitation!!
It's also possible to naturally seed clouds

33. QUESTION FOR THOUGHT:
Suppose that a thick nimbostratus cloud contains ice crystals and cloud droplets all about the same size. Which precipitation process will be most important in producing rain from this cloud? Why?
2. When cirrus clouds are above a deck of altocumulus clouds, occasionally a clear area, or "hole," will appear in the altocumulus cloud layer. What do you suppose could cause this to happen?
3. During a recent snowstorm, Denver, CO received 7 cm of snow. 60 km east of Denver, a city received no measurable snowfall, while 150 km east of Denver another city received 10 cm of snow. Since Denver is located to the east of the Rockies, and the upper-level winds were westerly during the snowstorm, give an explanation as to what could account for this snowfall pattern.

34. Precipitation Types- Rain Rain - drop diameter > 0.5 mm
drizzle - drop diameter < 0.5 mm (largely produced by stratus)
virga - rain leaving cloud base and evaporates before hitting the ground
often visible as evaporating streaks of precipitation
Rain Events:
Showers - localized, sometime heavy rain events
usually associated with Cb
sometimes called a "cloud burst"
continuous rain - from nimbostratus....

35. Precipitation Types - Snow
Snow - often visible as fall streaks associated with high cirrus
Snow Events:
Flurries - weak, intermittent - produced from developing Cu
Snow squalls - brief, heavy snow fall - produced from Cu
Steady Snow - continuous for hours - produced from Nb
Blizzard - low temperatures, strong winds, blowing snow... good stuff!!!!!

36. PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
Ice Clouds:
Nucleation and Growth
Nucleation
– Homogeneous, heterogeneous, ice nuclei
– Habits (shapes)
Ice crystal growth
– Growth from vapor (diffusion)
– Bergeron process (growth at expense of water droplets)
– Ice multiplication process
– Collision/coalescence (riming, aggregation)
Size distributions
– Microphysical measurements, temperature dependencies
PHYS Clouds, spring ‘04, lect. 5, Platnick 36

37. Ice Clouds – Nucleation
Some nucleation pathways
Homogeneous freezing of solution droplets (w/out assistance of aerosol particles)
requires very cold temperatures (~ -40 C and below)
Heterogeneous freezing (via aerosol particles that may or may not contain/be imbedded in water). Ice nuclei not well understood.
Contact freezing (ice nuclei contact with solution droplet)
Deposition on ice nuclei
reference: P. Demott, p. 102, “Cirrus”, Oxford Univ Press, 2002; Rogers and Yau, “A short course in cloud physics”.
PHYS Clouds, spring ‘04, lect. 5, Platnick 37

38. Ice Clouds – Homogeneous Nucleation
Overview
Liquid water has transient molecular groupings [liquid-like (close packed), ice-like, …]
Ice-like structures rare and only for temperatures near and below freezing (0 C).
Ice nucleation (formation) will occur when supercooled temperatures are cold enough such that ice-like structures can survive (i.e., supercooling vs. supersaturation for nucleation of water droplet out of the vapor phase).
Theoretical studies: modification to water homogeneous nucleation to account for free energy from liquid to ice state and surface free energy for ice-water interface. Many of the quantities are highly uncertain.
Laboratory experimental studies provides some consensus that homogeneous nucleation of pure water is ~ -40 C.
Solution droplets: nucleation rate decreases for a given temperature, i.e., nucleation occurs at lower temperature than for pure water (depression of freezing temperature).
PHYS Clouds, spring ‘04, lect. 5, Platnick 38

39. Homogeneous Freezing - conceptual schematic
Water molecules arrange themselves into a lattice.
Embryo grows by chance aggregation .
Ice nucleus cluster number/concentrations are in constant flux
in equilibrium, molecular clusters in Boltzman distribution
Chance aggregation number/concentrations increases with decreasing temperature.
ice
embryo
Wang and Olsen. 39

40. Ice Molecules Arranged in Lattice
Liquid
water
Freezing
Wang and Olsen.
Ice 40

41. From Wang and Olsen. 41

42. Ice Clouds – Heterogeneous Nucleation
Overview
Vapor deposition directly to aerosol particle (insoluble or perhaps dry soluble particles).
Contact freezing: particle collides with water droplet
Condensation freezing: from mixed aerosol particle (soluble component of particle initiates condensation, insoluble component causes freezing instantly)
Immersion freezing: same as above but insoluble particle causes freezing at a later time, e.g., at a colder temperature (but at temperatures greater than for homogeneous freezing)
Theoretical basis less certain than for homogeneous freezing.
Ice nuclei
minerals (clay), organic material (bacteria), soot, pure substances (AgI)
Deposition requires high supersaturation w.r.t. ice (e.g., 20% for AgI at -60 C, Detwiler & Vonnegut, 1981).
H. Houghton: Mason & van end Heuvel (1959), si~12% at -12 C for AgI.
PHYS Clouds, spring ‘04, lect. 5, Platnick 42

43. Heterogeneous Freezing - conceptual schematic
Freezing is aided by foreign substances, ice nuclei
Ice nuclei provide a surface for liquid water to form ice structure
Ice embryo starts at a larger size
Freezing occurs at warmer temperatures than for homogeneous freezing
ice
nuclei
Wang and Olsen. 43

44. Heterogeneous Freezing - conceptual schematic
Contact
Water droplet freezes instantaneously upon contact with ice nuclei
Condensation followed by instantaneous freezing
Nuclei acts as CCN, then insoluble component freezes droplet
Wang and Olsen. 44

45. Heterogeneous Freezing - conceptual schematic
Immersion
Ice nuclei causes freezing sometime after becoming embedded within droplet
Deposition
Ice forms directly from vapor phase
Wang and Olsen. 45

46. Ice Clouds – Ice Nuclei (IN)
Measured ice particle number concentration: < 1/liter to ~10/liter
Large discrepancies between measured IN and ice number concentration
IN vary with temperature, humidity, supersaturation.
Secondary production (limited understanding):
shattering of existing crystals
splintering of freezing drops
Other
In situ measurement problems
PHYS Clouds, spring ‘04, lect. 5, Platnick 46

47. PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
Cold Cloud Processes
Wang and Olsen.
Warm Cloud Processes
PHYS Clouds, spring ‘04, lect. 5, Platnick 47

48. PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
Ice Nuclei (IN), cont.
Internal nuclei
Water ice lattice held together by hydrogen bonds. Aerosol with hydrogen bonds at surface with similar bond strengths, as well as rotational symmetry which exposes H-bonding groups allowing interaction with water molecules, will be good IN. Example: organics.
Geometrical arrangement of aerosol surface molecules also important. Surface matching ice lattice structure will serve as good IN (e.g., AgI). Best IN will have similar bond length. Bond length differences give rise to stresses which creates an energy barrier to nucleation. Therefore expect easier nucleation at colder temperatures. See Pruppacher & Klett, Fig
Lab experiments indicate that ice nucleation is a local phenomenon proceeding at different active sites on the surface.
Contact nuclei
An electric dipole effect? Nucleates at ~5-10 C warmer than same nuclei inside droplet.
P&K + Krider notes.
PHYS Clouds, spring ‘04, lect. 5, Platnick 48

49. Ice Particle Growth - Condensation
Diffusion growth (C is “capacitance” of particle in units of length, current flow to a conductor analogy for molecular diffusion ):
C is a useful analogy, but difficult to analytically quantify except for simple shapes). Note that C = r for spheres, 2p/r for hexagons, …
With ice supersaturation defined as:
Rogers and Yau.
Solution: same as water droplet with C vs. r, Ls vs. L, es,i vs. es,w
PHYS Clouds, spring ‘04, lect. 5, Platnick 49

50. Wang and Olsen slide. 50

51. Ice Multiplication Process
Fracture of Ice Crystals
Splintering of Freezing Drops
During ice particle riming under very selective conditions:
Temperature in the range of –3 ° to –8 °C.
A substantial concentration of large cloud droplets (D >25 m).
Large droplets coexisting with small cloud droplets.
Wang and Olsen slide. 51

52. Ice Precipitation Particles
At surface:
Hail: alternating layers of clear ice (wet growth) & opaque ice
Graupel: “soft” hail < 1 cm diameter, white opaque pellets, consists of central crystal covered in rimed drops
Sleet: transparent ice, size of rain drops
Snow: coagulation of dendritic crystals
Krider notes.
PHYS Clouds, spring ‘04, lect. 5, Platnick 52

53. extras 53

61. Precipitation Types- Ice Habits
Environmental Temperature(°C) Crystal Habit
0 to thin plates
-4 to needles
-6 to columns
-10 to plates
-12 to dendrites, plates
-16 to plates
-22 to hollow needles

62. Precipitation Types- Ice Habits