MET215: Advanced Physical MeteorologyIce Clouds: Nucleation and Growth
Sources: Steve Platnick
Menglin S. Jin
Water Droplet Growth - Condensation
Evolution of droplet size spectra w/time (w/T∞ dependence for G understood):
large droplets : r(t) ro2 2Gsenv t
T (C) G (cm2/s)* G (µm2/s)
-10 3.5 x 10-9 0.35
0 6.0 x 10-9 0.60
10 9.0 x 10-9 0.90
20 12.3 x 10-9 12.3
With senv in % (note this is the value after nucleation, << smax):
T=10C, s=0.05% => for small r0:
r ~ 18 µm after 1 hour (3600 s)r ~ 62 µm after 12 hours
* From Twomey, p. 103.
Diffusional growth can’t explain production of precipitation sizes!
Platnick
Review:
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Co
ld C
lou
d P
roce
sses
War
m C
lou
d
Pro
cess
es
review
Ice Clouds: Nucleation and Growth
• Nucleation
– Homogeneous, heterogeneous, ice nuclei– Habits (shapes)
Sources: Steve 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
Sources: Steve Platnick
Platnick
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”.
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
Boltzmann distribution • Chance aggregation
number/concentrations increases with decreasing temperature.
ice embryo
Ice Molecules Arranged in Lattice
Freezin
g
Liquid
water
Ice
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).
• 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
Heterogeneous Freezing - conceptual schematic
ice nuclei
• Contact– Water droplet freezes instantaneously upon contact with ice
nuclei
• Condensation followed by instantaneous freezing– Nuclei acts as CCN, then insoluble component freezes droplet
Heterogeneous Freezing - conceptual schematic
• Immersion– Ice nuclei causes freezing sometime after becoming embedded
within droplet
• Deposition– Ice forms directly from vapor phase
Heterogeneous Freezing - conceptual schematic
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
main types
Ice Crystal Habits
• Variables
– Temperature
• primary
– Supersaturation
• secondary
– Electric Field
• minor
Platnick
Growth on T(C) Growth habits
prism 0 – -4 thin plates
basal -4 – -10 needles
prism -10 – -20 plates, dendrites
basal -20 – -50 hollow columns
c axis
basal face
prism face
a axis
Platnick
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. 9-12.
– 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.
Ice Cloud MicrophysicsCRYSTAL-FACE, A. Heymsfield
25 July 2002(VIPS)
25 July 2002(VIPS)
CPI: 7 July 2002
Platnick
Ice cloud microphysics, cont.
Platnick
Ice Crystal Habits-dependency on temperature and supersaturation
Platnick
MODIS ice crystal library habits/shapes
Platnick
Magano & Lee (1966)
dm
dt 4CD () s(r)
C is a useful analogy, but difficult to analytically quantify except for simple shapes). Note that C = r for spheres, 2/r for hexagons, …
With ice supersaturation defined as:
Diffusion growth (C is “capacitance” of particle in units of length, current flow to a conductor analogy for molecular diffusion ):
Ice Particle Growth - Condensation
Platnick
si e
es,i 1
e
es,w
es,wes,i
1
Solution: same as water droplet with C vs. r, Ls vs. L, es,i vs. es,w
dm
dt
4CD ss,if (Ls,es,i(T))
Ice Multiplication Process
• Fracture of Ice Crystals
• Splintering of Freezing Drops
During ice particle riming under very selective conditions:
1. Temperature in the range of –3 ° to –8 °C.
2. A substantial concentration of large cloud droplets (D >25 m).
3. Large droplets coexisting with small cloud droplets.
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
Platnick
extras
PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
Ice Nuclei (IN), cont.• Internal nuclei
– Nuclei 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. 9-12.
• Contact nuclei
– An electric dipole effect. Nucleates at ~5-10 C warmer than same nuclei inside droplet.
a axis length (A)
c axislength
organics
claysAgI.
5 20
10
5
15
