Transcript Lecture: Ice Cloud Processes

MET215: Advanced Physical Meteorology
Ice Clouds: Nucleation and Growth
Menglin S. Jin
Sources: Steve Platnick
Review:
Water Droplet Growth - Condensation
Evolution of droplet size spectra w/time (w/T∞ dependence for G understood):
large droplets: r(t) 
ro2  2G senv  t
With senv in % (note this is the value after nucleation, << smax):

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
*
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
Platnick
Warm Cloud
Processes
Cold Cloud Processes
review
Ice Clouds: Nucleation and Growth
•
Nucleation
–
–
Homogeneous, heterogeneous, ice nuclei
Habits (shapes)
Sources: Steve Platnick
Ice Clouds: Nucleation and Growth
•
Nucleation
–
–
•
Ice crystal growth
–
–
–
–
•
Homogeneous, heterogeneous, ice nuclei
Habits (shapes)
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
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”.
Platnick
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
Liquid
water
Freezing
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).
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
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
Heterogeneous Freezing - conceptual schematic
• Immersion
– Ice nuclei causes freezing sometime after becoming embedded
within droplet
• Deposition
– Ice forms directly from vapor phase
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
Ice Crystal Habits
•
Variables
– Temperature
• primary
– Supersaturation
• secondary
– Electric Field
• minor
main types
Platnick
c axis
a axis
basal face
prism face
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
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.
Platnick
Ice Cloud Microphysics
CRYSTAL-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)
Ice Particle Growth - Condensation
Diffusion growth (C is “capacitance” of particle in units of length, current flow
to a conductor analogy for molecular diffusion ):
dm
 4CD ()  s (r)
dt
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:
si 
e
e es,w
 1 
 1
es,i
es,w es,i
Solution: same as water droplet with C vs. r, Ls vs. L, es,i vs. es,w

4CD ss,i
dm

dt
f (Ls,es,i (T))
Platnick
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
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.
c axis 15
length 10
5
organics
.
AgI
5
•
clays
20
a axis length (A)
Contact nuclei
–
An electric dipole effect. Nucleates at ~5-10 C warmer than same
nuclei inside droplet.
PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick