Transcript Lecture 13: Cold Clouds

Lecture 13: Cold Clouds
Cold Cloud Processes
Warm Cloud
Processes
PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
ICE Crystal Habits
For Ice crystals, there are many unknown habits…
Ice Crystal Habits
•
Variables
– Temperature
• primary
– Supersaturation
• secondary
– Electric Field
• minor
main types
PHYS 622 - Clouds, spring ‘04, lect. 5, 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
PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
Ice Cloud Microphysics
CRYSTAL-FACE, A. Heymsfield
25 July 2002
(VIPS)
25 July 2002
(VIPS)
CPI: 7 July 2002
PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
Ice cloud
microphysics, cont.
PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
Ice Crystal Habits
-dependency on temperature and supersaturation
Where you see largest ice crystal?
MODIS ice crystal library habits/shapes
PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
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.
Magano & Lee (1966)
MODIS Ice Cloud Size Distributions
(based on FIRE II IFO, re ~ 7 – 60 µm)
ICE FORMATION AND
CONDENSATIONAL GROWTH
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
PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
• 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]
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????
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
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?
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
Saturation vapor pressure over
water and ice
• First, note that
es(water) > es (ice)
Q: where is this difference
a maximum???
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
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
Ice Crystal Growth through
Contact Freezing
• Occurs when a
supercooled drop
comes in "contact"
with an ice nuclei
• causes the
supercooled drop to
freeze
Accretion/Riming
• Occurs when super
cooled drops freeze
onto ice particle
• the resultant particle
is often referred to as
graupel
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????
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
•
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
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
QUESTION FOR THOUGHT:
1. 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.
• Precipitation TypesRain
• 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....
•
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!!!!!
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
PHYS 622 - Clouds, spring ‘04, lect. 5, 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”.
PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
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 622 - Clouds, spring ‘04, lect. 5, 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
Boltzman 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).
PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
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
PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
Cold Cloud Processes
Warm Cloud
Processes
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. 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.
PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
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))
PHYS 622 - Clouds, spring ‘04, lect. 5, 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
PHYS 622 - Clouds, spring ‘04, lect. 5, Platnick
extras
Precipitation Types- Ice Habits
• Environmental Temperature(°C)
0 to -4
-4 to -6
-6 to -10
-10 to -12
-12 to -16
-16 to -22
-22 to -40
Crystal Habit
thin plates
needles
columns
plates
dendrites, plates
plates
hollow needles
Precipitation Types- Ice Habits