Precipitation Formation

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Precipitation Formation
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Warm Cloud Processes

• Condensation of water on nuclei even with
  electric effects
• Collision Coalescence process
• Still too slow to produce much rain except in
  rare cases

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e
c
b
RH



-
1
3
e
rT
r
s
K
H
Water Vapor Scavenging
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Various Distributions Used in Warm Cloud Models
Condensation Nuclei Distribution
k
N
cr

d
Gamma Distribution (Cloud droplets)
r
p
-
-
1
e
x
f
r
(
)

p
(
)
G
Truncated Log-normal Distribution (rain)
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Time
r
p
-
-
1
e
x
f
r
(
)

p
(
)
G
Number/interval
Mist
Rain
Cloud
Radius of Drops
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Wegoner-Bergeron-Findeisen Ice Crystal Mechanism

• Process involved in formation of large drop Rain

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Supercooling of Liquid Drops

• Cloud droplets do not necessarily freeze at 32
  Fo, 0 Co.
• Cloud droplets may remain liquid at -40 Co (or
  Fo)
• Larger drops tend to freeze first

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Processes

• Freezing of Drops
• Growth of Snowflakes

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Growth of Snowflakes

• Diffusional growth similar to droplets
• Faster
• Growth to radius of 2-3 mm in tens of minutes
• Clears out liquid water droplets fast

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Latent Heat from Ice to Water
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Ice phase Scavenging
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Freezing Level
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Big Drop Rain From Clouds

• Diffusional Growth of Droplets
• Collision Coalescence
• Bergeron Process
• Scavenging of Moisture to Crystals
• Collision and Coalescences of Crystals
• Melting of Snow Clumps
• ------------------------------------
• Large Drop Rain

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