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:

Platnick

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