Precipitation development; Warm and Cold clouds

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Precipitation developmentWarm and Cold clouds
gt0C
lt0C
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Last lectures from me

• Cloud droplet formation (micro-scales)
• Cloud/fog formation processes (macro-scales)
• This lecture return to the micro-scales

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Cloud droplets and Raindrop sizes
r radius in ?m n numberconcentrationper
litre v terminalfall speedin cm/s
How do droplets grow and become raindrops?
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Q
Why doesnt it always rain when there are clouds?
A Updrafts can keep small cloud droplets
suspended
Radius (?m) Terminal Velocity (cm s-1) Type of Particle
0.1 0.0001 Condensation (Aitken) nuclei
10 1 Typical cloud droplet
100 70 Large cloud droplet
1000 1 mm 650 Typical raindrop
2500 2½ mm 900 Large raindrop
Need stronger updraughts to support larger drops
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Q
What do rain drops look like?
equivalent diameter (mm) of rain drop
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How do cloud droplets (radius 10 ?m) turn into
rain drops (1 mm) ?
Q
Initial growth by condensation, but this is
limited by diffusion They never get a chance to
grow into raindrops by condensation alone this
process would take D A Y S . . .

• There are 2 main processes
• In warm clouds with cloud top T gt -15 C
• In cold clouds with cloud top T lt-15 C

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Raindrop formation by collision and
coalescence in warm clouds
It takes about 106 small cloud droplets (10 mm)
to form one large raindrop (1000 mm)
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Stochastic model of collisions and droplet growth
Start with 100 drops In 1 timestep, 10
grow Next step, repeat
End up with a logarithmic size distribution Actu
ally, more complicated
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Cascade process
Raindrops reaching Earths surface rarely exceed
5 mm (5000 mm). Collisions or glancing blows
between large raindrops break them into smaller
drops. Also surface tension is too weak to hold
the larger drops together
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Distribution of raindrop sizes raindrop spectra
No. of drops in each class size per m3
the Marshall-Palmer distribution
n(D) noe-?D
no 8 x 103 ? 4.1 Rh-0.21 where Rh is the
rainfall rate (mm h-1)
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Depth of cloud influences type of rain
Stratus thin cloud (lt500 m) and has a slow
upward movement (lt 0.1 ms-1). Growth by
coalescence wouldnt produce a droplet more than
about 200 mm.
If RH below the cloud is high, then the droplets
will arrive at the ground as drizzle, defined as
diameter of drop lt 500 mm (0.5mm).
Thicker clouds, formed by convective motion, can
have stronger updrafts and can keep larger cloud
droplets aloft, permitting them to join
(coalesce) with more droplets and grow to greater
sizes.
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1 Low Nimbostratus (Ns)
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3 Cumulonimbus (Cb)
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Supplementary feature virga

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Cold clouds (temperate latitudes and polewards).
Q
Does water always freeze at 0 C ?
A
It depends on its volume and the presence of
ice nuclei.
For ice to form all the water molecules must
align in the proper crystal structure in a
large volume there is a high chance a few of the
molecules will line up in the proper manner
whereas in a small volume of water the chances
are reduced, simply because there are fewer
molecules
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Ice nuclei
Ice or freezing nuclei aid the freezing
process c.f aitken nuclei (lt0.2 mm) for
condensation nuclei.
1 cm3 of pure water in a test tube wouldnt
freeze until T was about -3 to 5 C.
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Ice nuclei
- are less common than Aitken nuclei

• most effective ones have the same crystal shape
• as ice crystals hence ice can form around and
  on them easily.

- kaolonite (clay) minerals are effective ice
nuclei
- are most effective at about -10 C

• because of the relative sparseness of ice
  nuclei, ice crystals
• and supercooled water can coexist at the same
  time.

• this last point is crucial in the formation of
  precipitation
• in cold clouds as it gives rise to the Bergeron
  process.

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Bergeron process arises since svpiceltsvpwater so
ice grows at the expense of supercooled water
droplets
vapour pressure
Super-cooledwater
0 C
ice
temperature
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One of the reasons you have to defrost your
freezer regularly
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Bergeron process
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Lab ice crystal growing from super-cooled water
drops
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Why are snowflakes hexagonal? its complicated!
http//www.uwgb.edu/dutchs/PETROLGY/Ice20Structur
e.HTM
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Shape of H2O molecule and H-bonding gives rise to
hexagonal crystals
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Clouds can be a mixture of water droplets and ice
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Summary

• Cloud particle size limited to a few mm by fall
  velocity
• Droplets (µm) grow to raindrops (mm) by two main
  routes
• Warm clouds condensation, collision, coalescence
  (then break-up)
• Cold clouds super-cooled water freezes on ice
  nuclei producing larger ice particles often
  melt en route to surface
• Precipitation can evaporate en route

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0000 Fri 06 Nov
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1200 Friday
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1200 Saturday
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1200 Sunday
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0000 Monday