Transcript Rain-cloud-CCN

METR215- Cloud Droplet
Formation
http://apollo.lsc.vsc.edu/classes/met130/notes/chapter7/ccn_drop_prec.html
R&Y book, Chapter 6
S. Platnick notes
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:
1.
Mixing of air masses (warm moist with cool air)
2.
Cooling via parcel expansion (adiabatic)
3.
Radiative cooling (e.g. ground fog, can lead to process 2)
PHYS 622 - Clouds, spring ‘04, lect. 2, Platnick
Concepts
es(T)
(T1,e1)
e
Radiative
Cooling
saturated
Mixing
(T2,e2)
unsaturated
T
PHYS 622 - Clouds, spring ‘04, lect. 2, Platnick
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:

es (T)  es Ttr  e
Air and
water vapor
T
T Water
L 1 1
( 
)
R v T Ttr
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
gas constant for water vapor (461.5 J-kg1-K1).
specific
PHYS 622 - Clouds, spring ‘04, lect. 2, Platnick
Saturation Vapor Pressure
An approximation for the saturation vapor pressure
(Rogers & Yau):

e s (T )  Ae
B
T
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.
PHYS 622 - Clouds, spring ‘04, lect. 2, Platnick
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
Rain Drops, Cloud Droplets, and
CCN
• 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??
Processes for Cloud Droplet
Growth
• How does this happen??
• By:
– condensation
– collision/coalescence
– ice-crystal process
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
Water Droplet Growth - Condensation
Flux of vapor to droplet (schematic shows “net flux” of vapor towards
droplet, i.e., droplet grows)
Need to consider:
1.
Vapor flux due to gradient between saturation vapor pressure at droplet
surface and environment (at ∞).
2.
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
Cloud Droplet Growth by
Condensation
• Consider pure water in equilibrium with air
above it
C-C equation to calculate es
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??
Equilibrium
Flat versus Curved Water
Surfaces
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
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
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%?
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%.
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???
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?
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??
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:
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
Factors promoting growth by
collision/coalescence
• Different drop sizes ->
• thicker clouds
• stronger updrafts
• consider a shallow
stratus deck....
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
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?
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
•
•
•
•
•
•
•
•
•
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?
Precipitation Types- Ice Habits
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!!!!!