1. METR215- Cloud Droplet Formation
R&Y book, Chapter 6
S. Platnick notes

2. PHYS 622 - Clouds, spring ‘04, lect. 2, Platnick
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 Clouds, spring ‘04, lect. 2, Platnick 2

3. PHYS 622 - Clouds, spring ‘04, lect. 2, Platnick
Concepts T e
es(T)
(T1,e1)
(T2,e2)
unsaturated
saturated
Mixing
Radiative
Cooling
Adiabatic expansion
PHYS Clouds, spring ‘04, lect. 2, Platnick 3

4. Saturation Vapor Pressure (Clausius-Clapeyron equation)
Water
Air and water vapor T
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 Ts=triple 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 Clouds, spring ‘04, lect. 2, Platnick 4

5. Saturation Vapor Pressure
An approximation for the saturation vapor pressure (Rogers & Yau):
Over liquid water:
L = latent heat of vaporization/condensation,
A=2.53 x 108 kPa, B = 5.42 x 103 K.
Over ice:
L = latent heat of sublimation, A=3.41 x 109 kPa, B = 6.13 x 103 K.
Need a sketch of solid-liquid-vapor phase regions.
PHYS Clouds, spring ‘04, lect. 2, Platnick 5

6. Example Microphysical Measurements in
Marine Sc Clouds (ASTEX field campaign, near Azores, 1992)
Data from U. Washington C-131 aircraft
PHYS Clouds, spring ‘04, lect.2, Platnick 6

7. Rain Drops, Cloud Droplets, and CCN

8. relative sizes of rain drops, cloud drops, and CCN:
raindrops 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 mm = mm
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??

9. Processes for Cloud Droplet Growth
How does this happen??
By:
condensation
collision/coalescence
ice-crystal process

10. Condensation & Collision
Water Droplet Growth
Condensation & Collision
Condensational growth: diffusion of vapor to droplet
Collisional growth: collision and coalescence (accretion, coagulation) between droplets
PHYS Clouds, spring ‘04, lect.4, Platnick

11. 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 ∞).
Effect of Latent heat effecting droplet saturation vapor pressure (equilibrium temperature accounting for heat flux away from droplet).
From Bill Olsen
PHYS Clouds, spring ‘04, lect.4, Platnick

12. Wang and Olsen slide. 12

13. Cloud Droplet Growth by Condensation
Consider pure water in equilibrium with air above it
C-C equation to calculate es

14. 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, instead, it has curvature.....
Q: how does curvature affect the evaporation/condensation process??
Answer 1: Being added to
Answer2: Being removed from

15. Equilibrium

16. Flat versus Curved Water Surfaces

17. 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 > evaporation rate off of flat surface
since air above both surfaces is saturated, then
evaporation rate = condensation rate
therefore, condensation rate onto droplet > condensation rate onto flat water surface
therefore, esdrop > esflat
therefore:
if RHflat = 100%, then RHdrop > 100%
the air surrounding the drop must be supersaturated!!
This is called the curvature effect

18. Curvature Effect Curvature effect -->
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

19. 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 %?
Q: what will happen to a drop 1.9 mm in size that is in a cloud where the RH is %?   

20. 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%.

21. 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 š 2 mm
Q: So, if a droplet grows to some size by condensation, how can it continue to grow to precipitation size???

22. 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?

23. Collision/Coalescence
Collision/Coalescence - cloud droplet growth by collision
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
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??

24. Droplet Fall Speeds and Droplet Growth
Q: what determines the droplets fall speed relative to the ground??
A: droplet size and updraft strength -->
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:
2ms-2
0ms-1
2ms-1 downward

25. 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 > w

26. Factors promoting growth by collision/coalescence
Different drop sizes -->
thicker clouds
stronger updrafts
consider a shallow stratus deck....

27. 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

28. 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?

29. 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

30. Cold Clouds
old 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?

31. Precipitation Types- Ice Habits

32. 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!!!!!