Formation of precipitation

Chapter 6

(pages 153 - 158)

How does the atmosphere produce rain?

What to know from today

• Know what precipitation is
• Know what the collision/coalescence process is and when it occurs
• Know how the residence time of water in a cloud varies with size
• Know what the Bergeron process is and when it occurs

• Formal definition: any form of water that falls from the atmosphere and reaches the ground
• Three main processes for formation:
• Condensation/deposition (well understood, beginning of rain droplet formation)
• Collision/coalescence (warm clouds processes; not well understood)
• Bergeron process (cold clouds processes; not well understood)

• High humidity: condensation exceeds evaporation: cloud particle grows
• Growth is fast initially, then slower (Fig 6.1)
• Particle needs to be 0.2 millimeter (200-micrometer) in diameter
• If much smaller, doesn’t fall
• If somewhat smaller, falls but evaporates
• It would take days to make precipitation by this process
• This process only important until cloud particle become cloud droplets (roughly 20 micrometer in size)
• 1 million average size cloud droplets needed to produce an average size rain drop of 2000 micrometers

• Only important in warm clouds (technically the temperature everywhere in the cloud is  > 0  degrees C)
• Large cloud droplets fall but encounter air resistance (Fig. 6.3)
• Depends on size of drop and falling speed
• Speed increases until air resistance equals pull of gravity --> terminal velocity
• Larger drops have smaller surface-area-to-weight ratio --> fall faster --> collide with smaller drops

• Merging of cloud droplets
• Large drops fall faster, overtake smaller drops
• Some small drops moved out of way
• Rest collide
• Some break up (more likely with small drops: higher surface tension)
• Others merge together: coalescence
• Collection efficiency: # coalesced/droplets in path
• Coalescence enhanced in thunderstorms where strongly charged droplets exist in strong electrical field

• Still air:
• Large cloud droplet takes:
• 12 minutes to travel through 500 m thick cloud
• 1 hour to travel through a 2500 m thick cloud
• Strong updraft maximizes amount of time in the cloud --> larger drops
• For example:
• Stratus: 500 m thick, updraft 0.1 m/s, r ≈ 200 mm, drizzle
• Tropical cumulus cloud: for typical updraft of 6.5 m/s, cloud droplet of 100 mm rises, collides, grows to 1000 mm, falls again and grows to 5000 mm
• Largest droplets fall out in beginning of rain shower

• Ice nuclei rare compared to CCN
• Ice nuclei examples:
• Clay minerals
• Bacteria
• Ice crystals formed by freezing of supercooled water

• Figure 6.5
• Water and ice at same temperature: equilibrium vapor pressure less over ice (fewer water vapor molecules in air near ice)
• When water and ice mix: water vapor will attempt to balance --> move from near water to near ice
• more water vapor near ice means supersaturation: so deposition (Fig 6.6)
• Less water vapor near water: not saturated: so evaporation
• Ice grows at expense of water
• Can be 1 million water droplets for every ice particle: ice particle becomes very large

• Important in mid- and high-latitudes: cold clouds (can also occur in tropics as well)
• If T > 0 degrees C: water droplets only
• If -40  degrees C < T < 0  degrees C :
• Supercooled water droplets
• Ice particles
• If T < -40 degrees C : ice particles only

• Ice crystal begins to fall
•  Collides with supercooled droplets
• Freeze on contact and stick together: accretion or riming
• Creates graupel
• Graupel falls and splinters
• Collision with other crystals may freeze hundreds of supercooled particles
• Aggregation: ice crystals bond to create snowflakes at T > -10 degrees C
• Snow may melt before reaching ground: rain
• Much of the rain in middle- and high-latitudes begins as snow

• Ratio of 1:100,000 to 1:1,000,000 ice crystals to water droplets
• If fewer: crystal grows, falls, leaves rest of cloud untouched (very little precipitation)
• Otherwise lots of small ice crystals that don’t fall