METR215- Cloud Droplet Formation

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METR215- Cloud Droplet Formation

• http//apollo.lsc.vsc.edu/classes/met130/notes/cha
  pter7/ccn_drop_prec.html
• RY book, Chapter 6
• S. Platnick notes

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Water Cloud Formation

• Water clouds form when RH slightly greater than
  100 (e.g., 0.3 supersaturation). This is a
  result of a subset of the atmospheric aerosol
  serving as nucleation sites (to be discussed
  later). Common ways for exceed saturation
• Mixing of air masses (warm moist with cool air)
• Cooling via parcel expansion (adiabatic)
• Radiative cooling (e.g. ground fog, can lead to
  process 2)

PHYS 622 - Clouds, spring 04, lect. 2, Platnick
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Concepts
PHYS 622 - Clouds, spring 04, lect. 2, Platnick
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Saturation Vapor Pressure (Clausius-Clapeyron
equation)
At equilibrium, evaporation and condensation have
the same rate, and the air above the liquid is
saturated with water vapor the partial pressure
of water vapor, or the Saturation Vapor Pressure
(es) is
Where Tstriple point temperature (273.16K), L is
the latent heat of vaporization (2.5?106 J/kg),
es(Ttr) 611Pa (or 6.11 mb). Rv is the specific
gas constant for water vapor (461.5 J-kg?1-K?1).
PHYS 622 - Clouds, spring 04, lect. 2, Platnick
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Saturation Vapor Pressure
An approximation for the saturation vapor
pressure (Rogers Yau)
Over liquid water L latent heat of
vaporization/condensation, A2.53 x 108 kPa, B
5.42 x 103 K.
Over ice L latent heat of sublimation, A3.41
x 109 kPa, B 6.13 x 103 K.
PHYS 622 - Clouds, spring 04, lect. 2, Platnick
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Example Microphysical Measurements in Marine Sc
Clouds (ASTEX field campaign, near Azores, 1992)
Data from U. Washington C-131 aircraft
PHYS 622 - Clouds, spring 04, lect.2, Platnick
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Rain Drops, Cloud Droplets, and CCN
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• relative sizes of rain drops, cloud drops, and
  CCN
• raindrops - 2000 mm 2 mm
• fall at a speed of 4-5 ms-1
• cloud drops - 20 mm 0.02 mm
• remain suspended in the air
• CCN - 0.2 mm 0.0002 mm
• remain suspended in the air
• To get a droplet (20 mm) to grow to raindrop size
  (2000mm) it must increase in size by a factor of
  100 (two orders of magnitude)
• 2000mm/20mm 100
• this occurs in about 30 minutes in a
  thunderstorm!!!
• this is like a 150 lb person growing in size to
  15,000 lbs in half an hour!!!
• Q How does this happen??

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Processes for Cloud Droplet Growth

• How does this happen??
• By
• condensation
• collision/coalescence
• ice-crystal process

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Water Droplet Growth Condensation Collision

• Condensational growth diffusion of vapor to
  droplet
• Collisional growth collision and coalescence
  (accretion, coagulation) between droplets

PHYS 622 - Clouds, spring 04, lect.4, Platnick
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Water Droplet Growth - Condensation
Flux of vapor to droplet (schematic shows net
flux of vapor towards droplet, i.e., droplet
grows)

• Need to consider
• Vapor flux due to gradient between saturation
  vapor pressure at droplet surface and environment
  (at 8).
• Effect of Latent heat effecting droplet
  saturation vapor pressure (equilibrium
  temperature accounting for heat flux away from
  droplet).

PHYS 622 - Clouds, spring 04, lect.4, Platnick
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Cloud Droplet Growth by Condensation

• Consider pure water in equilibrium with air above
  it

C-C equation to calculate es
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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 gt condensation, water is _________
• if evaporation lt condensation, water is ________
• Now, a droplet surface is not flat, instead, it
  has curvature.....
• Q how does curvature affect the
  evaporation/condensation process??

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Equilibrium
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Flat versus Curved Water Surfaces
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Flat versus Curved Water Surfaces curvature
effect

• more energy is required to maintain the
  "curvature" of the drop
• therefore, the water molecules on the surface of
  the drop have more energy
• therefore, they evaporate more readily that from
  the flat water surface (compare the length of the
  red arrows)
• therefore evaporation rate off curved surface gt
  evaporation rate off of flat surface
• since air above both surfaces is saturated, then
• evaporation rate condensation rate
• therefore, condensation rate onto droplet gt
  condensation rate onto flat water surface
• therefore, esdrop gt esflat
• therefore
• if RHflat 100, then RHdrop gt 100
• the air surrounding the drop must be
  supersaturated!!
• This is called the curvature effect

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Curvature Effect

• Curvature effect --gt
• notice that for the droplet to be in equilibrium
• (evaporation off drop condensation onto drop),
• the environment must be supersaturated
• also notice that the curvature effect
• is larger for smaller drops
• this makes sense since smaller drops
• have more curvature that larger drops

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Class activity-Curvature Effect

• Q what will happen to a drop 1.9 mm in size that
  is in a cloud where the RH is 100.05?
• Q what will happen to a drop 1.9 mm in size that
  is in a cloud where the RH is 100.15?   

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QUESTIONS FOR THOUGHT

• 1. At what relative humidity will pure water
  droplets of the following sizes grow by
  condensation
• a. 10 microns
• b. 4 microns
• c. 1 micron
• 2.  Explain why very small cloud droplets of pure
  water evaporate even when the relative humidity
  is 100.

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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 --gt
• this explains the formation of haze
• This process of condensation will grow drops ,
  but not to precipitation sizes š 2 mm
• Q So, if a droplet grows to some size by
  condensation, how can it continue to grow to
  precipitation size???

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QUESTION FOR THOUGHT

• Haze particles can form when the relative
  humidity is less than 100. Are these haze
  particles pure water droplets or solution
  droplets? Why?

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Collision/Coalescence

• Collision/Coalescence - cloud droplet growth by
  collision
• this is a dominant process for precipitation
  formation in warm clouds (tops warmer than about
  -15C)
• some cloud droplets will grow large enough and
  will start to fall in the cloud --gtgt
• since the bigger drops fall faster than the
  smaller drops, they will "collect" the smaller
  drops - the bigger drop grows
• droplet fall speed is called its terminal
  velocity
• need droplets of different sizes for this process
  to really work
• Q what determines the droplets fall speed
  relative to the ground??

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Droplet Fall Speeds and Droplet Growth

• Q what determines the droplets fall speed
  relative to the ground??
• A droplet size and updraft strength --gt
• given a growing cu with an updraft strength of 4
  ms-1
• if the particle terminal velocity is -2 ms-1, the
  particles fall speed is ANSWER
• if the particle terminal velocity is -4 ms-1, the
  particles fall speed is ANSWER
• if the particle terminal velocity is -6 ms-1, the
  particles fall speed is

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Life cycle of a droplet

• Growth by collision
• the drop initially forms in the updraft of the
  cloud near cloud base
• it grows in size by collisions
• since Vg w Vt
• Vg ground relative fall speed of the drop
• w updraft velocity
• Vt drop's terminal velocity
• then the drop will begin to fall when Vt gt w

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Factors promoting growth by collision/coalescence

• Different drop sizes --gt
• thicker clouds
• stronger updrafts
• consider a shallow stratus deck....

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Droplet Growth in a Shallow Stratus Deck

• Often, drops will evaporate from shallow stratus
  before reaching the ground
• or you may get drizzle if they are large enough

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QUESTION FOR THOUGHT

• 1.  Why is a warm, tropical cumulus cloud more
  likely to produce precipitation than a cold,
  stratus cloud?
• 2.  Clouds that form over water are usually more
  efficient in producing precipitation than clouds
  that form over land. Why?

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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 0C
• comprised entirely of water

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Cold Clouds

• old clouds are defined as those clouds with tops
  colder than 0C
• can be comprised of
• water
• super-cooled water - liquid droplets observed at
  temps less than 0C
• ice
• Notice that super cooled water is found at
  altitudes where
• -40C lt Temp lt 0C
• only ice is found at altitudes above -40C
• Q So how does frozen precipitation form in cold
  clouds?

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Precipitation Types- Ice Habits
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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,
  blowing snow... good stuff!!!!!