Precipitation Processes
- Cloud Droplets
- Why don't all cloud droplets fall to the ground as precipitation?
- Cloud droplets are composed of liquid water and are heavier than air
- Droplets accelerated downward by gravity
- Opposing force is air resistance (frictional drag)
- Downward acceleration due to gravity is constant
- Upward acceleration due to air resistance increases with velocity
- Eventually balance is reached and downward motion continues at constant speed
- Terminal Velocity
- Speed at which downward gravitational force = upward frictional force
- No further acceleration and downward speed remains constant
- Gravitational force = mass x gravitational acceleration
- Force proportional to cube of radius
- Frictional drag force = constant x surface area x velocity squared
- Force proportional to square of radius
- Equating the forces to get terminal velocity:
- So terminal velocity is:
- Terminal velocity is proportional to square root of droplet radius
- Cloud droplets are too small to fall to the ground fast enough
- Cloud droplets: 10 microns - terminal velocity = 1 cm/sec
- Takes 8.3 hr to fall 300m
- Large cloud droplets: 50 microns - terminal velocity = 27 cm/sec
- Takes 18.5 min to fall 300m
- Rain drops: 1000 microns (1 mm) - terminal velocity = 6.5 m/sec
- Takes 45 sec to fall 300m
- Upward velocities in clouds frequently greater than 1 cm/sec, so droplets often remain suspended in cloud
- Air below cloud base is unsaturated, so droplets evaporate after leaving cloud
- Growth of Raindrops
- Need 1,000,000 cloud drops to make 1 raindrop
- Growth by continued condensation theoretically possible, but way too slow
- Need to identify cloud processes that accelerate rate of droplet growth
- Warm Cloud Process
- Collision and coalescence of droplets
- Clouds contain wide range of droplet sizes with different terminal velocities
- Larger, faster droplets overtake smaller droplets and collide with them
- Drops grow by coalescence
- Collision efficiency can be low, as small droplets get swept aside by "bow wave"
- Works best when droplets remain inside cloud for long periods
- Thick clouds with deep, strong updrafts
- Clouds with high liquid water content - especially warm clouds in tropics
- Cold Cloud Process
- Three phase or Bergeron process
- Requires cloud with water in all three phases in same region
- Most clouds in middle and high latitudes are partially or entirely below freezing
- Cool clouds - base above freezing but upper reaches below freezing
- Cold clouds - whole cloud below freezing
- Ice nuclei required for crystals to form by deposition (gas to solid)
- Ice nuclei much rarer than condensation nuclei
- Clouds below freezing often contain supercooled water droplets
- Liquid droplets at temperatures below freezing
- Saturation vapor pressure larger over water surface than over ice surface
- Cloud in equilibrium with water and vapor is supersaturated when ice crystal forms
- Ice crystal grows through increased deposition
- Vapor content reduced below saturation level for water
- Droplet shrinks through increased evaporation
- Water migrates from droplet through vapor phase to ice crystal
- Mechanism allows for relatively rapid growth
- Principle behind cloud seeding
- Inject ice nuclei (usually silver iodide) into clouds with supercooled droplets to start process
- May increase precipitation by around 10% under right conditions
- Difficult to know when right conditions exist and what would have happened with no seeding
