Transcript Understanding Weather and Climate Ch 7

Why does it rain on us???
Review of last lecture
 3 cloud properties, 9 ISCCP cloud types
 Why do clouds constitute a wildcard for climate
change? Competition between greenhouse
effect and albedo effect
 Convection: 3 types of stability. Two factors
limiting the height of clouds
Satellite observation of precipitation
•
•
•
•
Infrared-derived or visible-derived (GPI)
Microwave-derived (MSU, SSM/I, TMI)
Radar: Tropical Rainfall Measurement Mission (TRMM)
Merged with surface gauge measurements and model
forecast
Global distribution of precipitation
Precipitation formation - cloud drop growth
• Not all clouds precipitate due to their small
sizes and slow fall rates
• Balance between gravity and frictional
drag  eventually become equal to
achieve terminal velocity VT, which is
proportional to the square root of cloud
drop radius VT=c r0.5 ,where r is drop
radius and c is a constant.
• For a cloud drop to fall, its terminal
velocity must exceed the vertical velocity
of the upward-moving air parcel.
Otherwise it will be carried up.
• Cloud drop growth is required for
precipitation to form
Fgravity
Fdrag
Mechanisms for cloud drops to grow larger
1. Collision Coalescence (warm clouds, T > 0 C, form rain)
2. Bergeron Process (cool/cold clouds, T < 0 C, form snow)
Cold Clouds
Cool Clouds
1. Collision Coalescence: Growth in Warm Clouds
• Process begins with larger collector drops
which have higher terminal velocities
• Collector drops collide with smaller drops and
merge with them (coalesce). Coalescence
efficiency is generally very high, indicating
that most collisions result in the two drops
joining.
• If collector drop is too big: compressed air
beneath falling drop forces small drops aside
• If collector drop is too small (same size as
other drops) it will fall at same speed and no
collision will occur
• So, collection efficiency is greatest when the
size of collector drop is slightly larger than the
size of the other drops
• After the collector drops become large, the
larger one among them can serve as a “supercollector” to collide with other collector drops
Raindrop shape and maximum size
• Determined by competition between surface
tension and frictional drag. Frictional drag is
larger at the bottom than at the top
• Small drop (<0.08in): frictional drag <<
surface tension  Sphere shape
• Medium-size drop (0.08in<size<0.25in):
frictional drag approaches surface tension 
Parachute shape
• Large drop (>0.25in): frictional drag at bottom
> surface tension  Split (The surface tension
at the top allows the raindrop to remain more
spherical while the bottom gets more flattened
out.)
• Maximum drop size of about 0.25in or 5 mm
Video: The Science of Snowflakes
• https://www.youtube.com/watch?v=fUot7XSX8uA
Formation of snow and hails
2. Bergeron Process: Growth in Cool/Cold Clouds
• Clouds are usually composed of:
liquid water, super-cooled water,
and/or ice (supercooled water
exists down to T= -40C !!)
• Supercooled water can exist at
T<0C because ice formation
requires ice nuclei, which,
unlike condensation nuclei, are
rare unless the temp. is very
cold
• Coexistence of ice and supercooled water is critical to the
creation of cool/cold cloud
precipitation - the Bergeron
Process
http://www.uwsp.edu
Bergeron Process (cont.)
• Key: Saturation vapor pressure of
ice < that of super-cooled water
at the same temperature.
• When air is in saturation wrt
super-cooled water, it’s oversaturated wrt ice - deposition of
water vapor over ice.
• When air is in saturation wrt ice,
it’s sub-saturated wrt supercooled water - evaporation of
super-cooled water into water
vapor.
• In this way, ice crystals grow
rapidly at the expense of supercooled drops
http://www.uwsp.edu
Further growth: Riming and Aggregation
• Bergeron Process usually not enough to
produce large enough crystals for preciptation
• Further growth is due to collisions between
falling crystals and drops  riming and
aggregation
• Riming (or Accretion) = liquid water freezing
onto ice crystals
• Aggregation = the joining of ice crystals
through the bonding of surface water builds
ice crystals, producing snowflakes
• Collision combined with riming and
aggregation allow formation of crystals large
enough to precipitate within 1/2 hour of initial
formation
Shape of snowflakes depend on formation
conditions (humidity and temperature)
Dendrite ice
crystals
Plate ice
crystal
Wilson Bentley, a Vermont farmer, took photographs of snowflakes
under a microscope as a hobby. These photographs were published in
the "Monthly Weather Review" in 1902.
Shapes/sizes depend on formation conditions
(humidity and temperature)
Cloud Seeding
• Induce precipitation  injection of dry ice or silver iodide
into clouds
• Convert super-cooled droplets to ice initiate Bergeron
process
• Dry ice (frozen CO2) – lowers temperature to -40C,
• no ice nuclei are required for water droplets to freeze
• Silver iodide – acts as ice nuclei at warmer temp (-5C)
• Effectiveness is debated
Summary
• Forces acting on a cloud/rain droplet. Terminal velocity.
How does it change with cloud drop radius?
• Growth mechanisms for rain and snow (Warm clouds,
cool clouds, cold clouds)
• Formation of rain: coalescence process (the collector is
larger than the cloud droplets but not too large)
• Bergeron process: happens with coexistence of ice and
super-cooled water. Key: Saturation vapor pressure of
ice < that of super-cooled water at the same
temperature.
• Further growth of ice crystals (riming and aggregation)
Works cited
• http://www.edudemic.com/study-finds-most-people-thinkcloud-computing-is-run-on-actual-clouds/
• http://hyperphysics.phyastr.gsu.edu/hbase/electric/diph2o.html
• http://nyffetyff.deviantart.com/art/Raindrop-189805290
• http://www.its.caltech.edu/~atomic/snowcrystals/photos/p
hotos.htm
• http://www.crh.noaa.gov/unr/?n=06-04-99_pg1
• http://www.clker.com/clipart-cartoon-sun.html
• http://pmm.nasa.gov/node/145