Chapter 8 - Precipitation zwang/atsc2000/Ch8.pdfآ 1 Chapter 8 - Precipitation Rain Drops, Cloud...
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Chapter 8 - Precipitation
Rain Drops, Cloud Droplets, and CCN • Recall the relative sizes of rain drops, cloud drops, and CCN:
– raindrops - 2000 μ m = 2 mm • fall at a speed of 4-5 ms-1
– cloud drops - 20 μ m = 0.02 mm • remain suspended in the air
– CCN - 0.2 μm = 0.0002 mm • remain suspended in the air
• To get a droplet (20 μ m) to grow to raindrop size (2000 μ m) it must increase in size by a factor of 100 (two orders of magnitude): – 2000 μ m/20 μ m = 100 – this occurs in about 30 minutes in a
• How does this happen? – condensation – collision/coalescence – ice-crystal process
Cloud Droplet Growth by Condensation • Consider pure water in equilibrium with air above it
– then the RH = 100% – evaporation = condensation – vapor pressure (e) = saturation vapor pressure (es)
• if evaporation > condensation, water is ________ • if evaporation < condensation, water is __________ • Now, a droplet surface is not flat as shown to the right ->
instead, it has curvature.....
Flat versus Curved Water Surfaces • More energy is required to maintain the "curvature" of the
drop – The water molecules on the surface of the drop have more energy – Evaporation rate off curved surface > evaporation rate off of flat
surface • Since air above both surfaces is saturated
– evaporation rate = condensation rate – condensation rate onto droplet > condensation rate onto flat water
surface – esdrop > esflat
• Therefore: – if RHflat = 100%, then RHdrop
> 100% – the air surrounding the drop
must be supersaturated!! • This is called the
curvature effect – the curvature effect is larger
for smaller drops
Solution Droplets • Note that the previous discussion
is valid for a pure water drop • if a droplet is comprised of a
solution – it can be in equilibrium with the
environment at a much lower RH – this explains the formation of haze
• This process of condensation will grow drops , but not to precipitation sizes ~2 mm
• Q: So, if a droplet grows to some size by condensation, how can it continue to grow to precipitation size?
Droplet fall speed is called its terminal velocity
Collision/Coalescence • This is a dominant process for
precipitation formation in warm clouds (tops warmer than about -15°C)
• Some cloud droplets will grow large enough and will start to fall in the cloud -->>
• Since the bigger drops fall faster than the smaller drops, they will "collect" the smaller drops - the bigger drop grows
• Need droplets of different sizes for this process to really work
• Q: what determines the droplets fall speed relative to the ground?? – A: droplet size and updraft
Collision/Coalescence - cloud droplet growth by collision
Life cycle of a droplet 1. The drop initially forms in
the updraft of the cloud near cloud base
2. It grows in size by condensation and collisions
3. then the drop will begin to fall when Vt > w
4. Continue growth by coalescence
• Factors promoting growth by collision/coalescence – Different drop sizes --> – thicker clouds – stronger updrafts – consider a shallow stratus
• Vg = w + Vt – Vg = ground relative fall speed of the
drop – w = updraft velocity – Vt = drop's terminal velocity
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?
The distribution of ice and water in a cumulonimbus cloud.
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
• Any liquid water droplet, no matter how small, must freeze spontaneously below –40°C
Ice formation through Ice nuclei
Ice Crystal Growth through Deposition
• After ice crystals are initialized through homogeneous freezing or ice nuclei, ice crystal will growth initially through vapor deposition.
• The deposition is normally slow, but in the presence of supercooled water, the growth can be fast.
Saturation vapor pressure over water and ice
• Recall that es(water) > es (ice)
• Where is this difference a maximum?
• es(water) - es (ice) is a maximum at -15°C
• Given a mixed cloud containing water and ice, which type of particle (ice v.s. water) will grow more quickly? – ice particles will grow more
readily than water particles
The ice-crystal (Bergeron) process Ice crystals grow
larger at the expense of the surrounding water droplets.
The fate of falling large ice crystals
Collect super cooled water droplets
Break up Collect small ice crystals
Accretion/Riming Ice collect water
• Occurs when super cooled drops freeze onto ice particle
• The resultant particle is often referred to as graupel
• 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
Natural seeding by cirrus clouds may form bands of precipitation downwind of a mountain chain.
Ice crystals falling from a dense cirriform cloud into a lower nimbostratus cloud
How ice crystals grow and produce precipitation in clouds with a low liquid-water content and a high liquid-water content.
Precipitation Types- Rain • 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
Precipitation Types- Rain
Rain Events: • Showers - localized, sometime heavy rain
events – usually associated with Cb – sometimes called a "cloud burst"
• continuous rain - from nimbostratus....
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,
Average annual snowfall over the United States. (NOAA)
The dangling white streamers of ice crystals beneath these cirrus clouds are known as fallstreaks. The bending of the streaks is due to the changing wind speed with height
Precipitation Types- Ice
Computer color-enhanced image of dendrite snowflakes.
Other types of frozen precipitation
• The vertical variation of temperature near the ground can have a dramatic influence on the type of precipitation that is observed at the ground: – Sleet – Freezing rain
Vertical Temperature Profile for Sleet • If a deep freezing layer exists at low levels, sleet
may form • sleet - tiny ice pellets that are transparent
Vertical Temperature Profile for Freezing Rain
• If a shallow freezing layer exists at low levels, freezing rain, or glaze may form upon striking a cold object.
• This is the type of temperature structure that is necessary for an ice storm
• Note: at low levels on will experience freezing rain, at higher levels, it will be rain
Rime Ice • Rime ice - super cooled drops in fog/cloud
that freeze onto objects on ground • Big problem on Mt. Washington
A heavy coating of freezing rain (glaze) covers Syracuse, New York, during January, 1998, causing tree limbs to break and power lines to sag.