Download Understanding Weather and Climate Ch 7

Snow and wintertime blizzards
Review of last lecture
• Formation of clouds: 3 types of stability. Two factors
limiting the height of clouds.
• 3 cloud properties. 9 ISCCP cloud types. Why do clouds
constitute a wildcard for climate change?
• Forces acting on a cloud/rain droplet. Terminal velocity.
How does it change with cloud drop radius?
• Growth mechanisms for rain and snow
• Formation of rain: coalescence process (the collector is
larger than the cloud droplets but not too large)
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)
Bergeron Process
• Coexistence of ice and super-cooled
water (exists down to T= -40C !!) is
critical to the creation of cool/cold
cloud precipitation - the Bergeron
Process
• Key: Saturation vapor pressure of
ice < that of super-cooled water at
the same temperature.
• When air is in saturation wrt supercooled water, it’s over-saturated wrt
ice - deposition of water vapor over
ice.
• When air is in saturation wrt ice, it’s
sub-saturated wrt super-cooled
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
Extensive riming in severe storms
• Graupel – ice crystals that undergo extensive riming
– Lose six sided shape and smooth out
– Either falls to the ground or provides a nucleus for
hail
• Hail – concentric layers of ice build around graupel
– graupel carried aloft in updrafts  high altitudes
freezing temperatures
– water accreting to graupel freezes, forming a layer
– Hail begins to fall, carried aloft again by updrafts,
process repeats
– Hailstones are very heavy – high density
– Capable of tremendous amounts of damage
– Great Plains = highest frequency of hail events
Video: Weather fronts
http://www.youtube.com/watch?v=tkK4
_F0VKhM
Air masses

An airmass is a large (usually thousands of km across) volume of air that
has horizontally uniform properties of temperature and moisture.
Airmasses acquire their properties from spending days to weeks over the
same part of the Earth.
“Polar” airmasses are colder than “tropical” airmasses
“Maritime” airmasses are wetter than "continental" airmass

Other specific airmass types include "arctic", "equatorial", and “monsoon”



Bergeron classification of air masses



3 letters: e.g. mTk, cPw
1st letter for moisture properties: c - continental, m - maritime
2nd letter for thermal characteristics: T - tropical, P -polar, A Artitic/Antarctic, M - monsoon, E - equatorial, S -superior air(dry air
formed by significant downward motion in the atmosphere)

3rd letter for stability: k/w - air colder/warmer than ground
Fronts


A weather front is a boundary separating two air masses
Types: cold front, warm front, stationary front, occluded
front, dry line, squall line
Cold Fronts
• A cold front is a mass of
cold air advancing towards
warm air.
• Typically associated with
heavy precipitation, rain or
snow, combined with rapid
temperature drops.
• Since friction decreases with
height, winds move faster at
higher altitude. Then the
surface of cold front
becomes more steeper
through time, leading to a
narrow belt of precipitation.
• Moving speed up to 30mph
Warm Fronts
• Warm fronts are warm air
moving towards cold air.
• Friction decreases with height,
so winds move faster at higher
altitude. This causes the
surface of the front to become
less steep through time. Then
clouds will be spread to a wider
region.
• Shallow stratus clouds
dominate and bring light
precipitation. Frontal fogs
may occur as rain
evaporates in the colder air
near the surface.
• Moving speed about 12 mph
Winter blizzards: mid-latitude cyclone


The mid-latitude cyclone
is a synoptic scale low
pressure system that has
cyclonic (counterclockwise in northern
hemisphere) flow that is
found in the middle
latitudes (30N-55N, 30S55S).
It has a larger size than a
tropical cyclone
How does a mid-latitude cyclone form?
In mid-latitude there is a boundary
between northern cold air and
southern warm air
In the boundary a initial cyclone
can advect warm air northward
and cold air southward
Mature stage. Cold air begins to
catch up with warm air (occluded).
If the upper level low is to the west
of surface low, the cyclone will
amplify and precipitation will form.
Cold air cools down the cyclone.
Dissipation.
Regions of cyclogenesis and typical tracks
• Gulf of Mexico, east coast
• Alberta Clipper from eastern side of Canadian Rockies
• Colorado Low from eastern slope of American Rockies
 Lee-side lows, lee cyclogenesis
Summary
• Bergeron process: happens with coexistence of ice and supercooled water. Key: Saturation vapor pressure of ice < that of
super-cooled water at the same temperature.
• Further growth of ice crystals (riming and
aggregation)Extensive riming in strong updrafts (graupel, hail)
• Definition of airmasses. Bergeron classification of air masses
(3 letters).
• Fronts: 6 types (cold, warm, stationary, occluded, dry line,
squall line)
• Cold front (narrow, fast, heavy precipitation), Warm front (wide,
slow, light precipitation)
• The developmental stages and vertical structure of middle
latitude cyclones (boundary between northern cold air and
southern warm air, upper level low to the west of surface low)
• The three regions of cyclogenesis and typical tracks
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