METEOROLOGY

1
METEOROLOGY

• GEL-1370

2
Chapter Five

• Cloud Development Precipitation

3
Goal for this Chapter

• We are going to learn answers to the following
  questions
• Why there any instabilities in the atmosphere?
• How can we make the atmosphere more stable?
• Why cloud droplets seldom reach the ground?
• How rain drops are produced?
• How does the ice crystal process forms
  precipitation?
• What is cloud seeding?
• Difference between freezing rain and sleet?
• How does Doppler radar measure intensity of rain?
• Why heavy showers fall from cumuliform while
  steady precipitation is derived from stratiform
  clouds?

4
Atmospheric stability

• A rising parcel of air expands and cools, while a
  sinking parcel is compressed and warms
• When air is in stable equilibrium, after being
  moved up or down, tends to come back to its
  original position
• Adiabatic Process A process in which there is no
  transfer of heat between the air parcel and its
  surroundings (compression --- warming expansion
  --- cooling)
• Dry adiabatic rate Rate of change of temp in a
  rising or descending unsaturated air parcel
  10C/1000 m in elevation
• Moist adiabatic rate Rate of change of temp in a
  rising or descending saturated air parcel
  6C/1000 m in elevation

5
Concept of equilibrium
6
What happens to a rising air??

• Rising air----- cools ----- RH increases as the
  air temp approaches the dew-point temp-----if air
  cools to its dew point temp, RH 100-----
  further air lifting leads to condensation -----
  cloud forms -----latent heat is released -----
• Stable Air If the rising air is colder than its
  surrounding air, then, it is heavier and will
  sink back to its original position stable air
  strongly resists upward vertical motion. If
  clouds form in rising air, cloud will spread
  horizontally in relatively thin layers
  cirrostratus, altostratus, nimbostratus or
  stratus clouds

7
Dry adiabatic rate unsaturated air cools
_at_10C/1000m
8
Absolute stable atmosphere when rising air parcel
is colder and heavier than surrounding air
9
Stable Air contd.

• Atmosphere is stable when lapse rate is small
• The cooling of surface air could be due to
• Nighttime radiational cooling of the surface
• Influx of cold air from other region brought by
  wind
• Air moving over a colder surface
• The air is generally most stable in the early
  morning around sunrise
• Subsidence Inversion Inversion produced by
  compressional warming the adiabatic warming of
  a layer of sinking air
• Presence of inversion near the ground fog, haze,
  asso-ciated pollutants are kept close to the
  surface

10
Cold surface air produces a stable atmosphere
that inhibits vertical motions fog haze are
kept close to the ground
11
Unstable Air

• When air temperature decreases rapidly as we move
  up, air becomes unstable
• The warming of air may be due to
• Daytime solar heating of the surface
• An influx of warm air brought in by the wind
• Air moving over a warm surface
• As the surface air warms during the day, the air
  becomes more unstable most unstable during
  summer months and when there is much temp
  fluctuation in a day
• Sinking air produces warming and a more stable
  atmosphere while rising air produces cooling and
  unstable atmosphere

12
Unstable atmosphere rising air parcel is warmer
and lighter than the surrounding air
13
How stability of air affects the type of clouds
formed

• Unsaturated Air parcel if forced to rise ----
  expands and cools at the dry adiabatic rate ---
  cools until dew point now RH is 100 ---
  further lifting results in condensation and the
  formation of cloud --- The elevation above which
  the cloud first forms is called condensation
  level
• Conditionally unstable atmosphere (or conditional
  instability) When the environmental lapse rate
  is less than the dry adiabatic rate but greater
  than the moist adiabatic rate, conditional
  instability exists.
• Level of free convection Level at which a lifted
  parcel of air becomes warmer than the surrounding
  in a conditionally unstable atmosphere

14
Unstable Air. Warmth from the forest fire heats
the air, causing insta. near the surface warm,
less dense air bubbles upward, expanding
cooling as it rises rises air cools to dew
point, condensation begins cumulus cloud forms
15
Conditionally unstable air when unsaturated
stable air is lifted to a level where it becomes
saturated and warmer than the air surrounding air
16
Cloud Development and stability

• Some surface heats up quickly --- air in contact
  warms --- hot bubble of air (thermal) rises ---
  undergoes expansion cooling when it rises ---
  Two things can happen i) thermal mixes with
  cooler air and looses its identity and air
  vertical movement slows down ii) air keeps
  cooling until it reaches to its saturation point
  --- moisture will condense --- thermal becomes
  visible as a cumulus cloud
• Outside of a cumulus cloud, there is downward
  movement of air because i) evaporation around the
  outer edge of the cloud makes the air cooler and
  denser ii) completion of the convection
  current started by the thermal

17
How clouds form a) surface heating convection
b) forced lifting along topographic barriers c)
convergence of surface air d) forced lifting
along weather fronts
18
Cumulus cloud formation from the hot air rising
from earths surface around the cloud, air is
sinking
19
Why Cumulus clouds appear-disappear-reappear

• Cumulus clouds grow shuts off surface heating
  and upward convection --- without continual
  supply of air, cloud disappears --- heating and
  upward convection starts again

20
Topography and Clouds

• Large air masses rise when approaching a mountain
  chain --- this leads to cooling if the air is
  cool, clouds form --- Orographic clouds---during
  this condensation, latent heat is released
• Temperature at the leeward side is higher (loss
  of heat in the upwind side) dew point temp on
  the leeward side is lower than the windward side
• Drier air in the leeward side More rain in
  upwind side and rain shadow (low precipitation)
  in the leeward side

21
Rain shadow, Orographic uplift cloud development
22
Formation of lenticular clouds Moist air rises
in the upwind side of the wave, it cools and
condenses, producing cloud in the downwind side,
air sinks and warms the cloud evaporates
23
Precipitation Processes

• Average diameter cloud droplets 0.02 mm
• Typical raindrop size 2 mm
• Growth of cloud droplets by condensation is slow
  to produce rain clouds can develop and begin to
  rain in less than an hour
• 1 million average size cloud droplets will make a
  average size raindrop Other processes??
• Two important processes on how rain is produced
• Collision-Coalescence Process
• Ice-crystal (or Bergeron) process

24
Relative sizes of raindrops, cloud droplets,
condensation nuclei
25
Collision Coalescence a) warm cloud composed
only of small cloud droplets of uniform size b)
different size droplets
26
Collision Coalescence contd.

• In clouds warmer than -15C(5 F), collision
  between droplets play a significant role
• Larger drops may form on larger condensation
  nuclei (salt particles or through random
  collision droplets turbulent mixing between
  cloud and drier environment)
• Amount of air resistance depends on the size of
  the drop and its rate of fall --- speed of falls
  increases until the air resistance gravity
  Terminal velocity Larger drops means less
  evaporation also
• Coalescence Merging of droplets by collision
• Forces that hold together tiny droplet together
  are so strong that if the droplets collide with
  another droplet, they would not stick together

27
How surface area depends on the size
28
Collision coalescence contd.

• Rising air currents slow the rate at which drops
  fall --- thick cloud with strong updrafts will
  maximize the time droplets spend in a cloud ---
  the bigger size droplets
• When the fall velocity of the drop gt updraft
  velocity, droplet slowly descends when it
  reaches the bottom of the cloud, size 5 mm ---
  typically occur in a rain shower originating in
  the warm, convective cumulus clouds
• Factors in the production of raindrops
• Clouds liquid water content (most important)
• Range of droplet sizes, cloud thickness, updrafts
  of the cloud, electric charge of the droplets and
  the electric field in the cloud

29
Cloud droplet rising then falling through a
warm cumulus cloud by growth and coalescence
30
Ice crystal Process

• Bergeron process of rain formation A process
  that produces precipitation involves tiny ice
  crystals in a supercooled cloud growing larger at
  the expense of the surrounding liquid droplets
• Ice crystals and liquid cloud droplets must
  coexist in clouds at below freezing
• Accretion or riming of ice crystals Ice crystals
  grow larger by colliding with the supercooled
  liquid droplets the droplets freeze into ice and
  stick to the ice crystal

31
Distribution of ice and water in a cumulonimbus
cloud
32
Water droplets and ice crystal are in
equilibrium water vapor molecules gt liquid is
saturation vapor pressure over water is greater
than it is over ice
33
(No Transcript)
34
(No Transcript)
35
(No Transcript)
36
Cloud Seeding Precipitation

• Cloud Seeding Inject a cloud with small
  particles that will act as nuclei, so that cloud
  particles will grow large enough to fall to the
  surface as precipitation
• Silver iodide is used has a crystalline
  structure similar to ice crystal, as it acts as
  an effective ice nucleus at temp. of -4C (25 F)
  and lower
• Important factors in cloud-seeding experiment
  Type of cloud, its temperature, moisture content,
  droplet size distribution, and updraft velocities
  in the cloud
• Cloud seeding in certain instances may lead to
  more precipitation in others, to less
  precipitation, and in still others, to no change
  in precipitation amounts
• Can avoid hail storms --- very important use

37
Natural seeding by cirrus clouds may lead to
precipitation downwind
38
Precipitation Types

• Rain (Meteorology definition!) falling drop
  diameter ? 0.5 mm
• Drizzle Water drop diameter lt 0.5 mm
• Most drizzle falls from stratus clouds also,
  rain passing through undersaturated zone and
  undergo evaporation leading to smaller-sized
  droplets drizzle
• Virga Precipitation that falls from a cloud but
  evaporates before reaching the ground
• Raindrops that reach the earths surface are
  seldom larger than 6mm as collision between
  raindrops tend to break them apart into many
  smaller drops

39
Virga Streaks of Falling precipitation
evaporates before reaching the ground
40
Raindrops lt 2mm nearly spherical gt2mm, elliptical
41
Precipitation Types Contd.

• Snow Much of the precipitation reaching the
  ground begins as snow
• During summer, freezing level is usually high
  snowflakes falling from a cloud melt before
  reaching the surface
• During winter, freezing level is much lower, and
  falling snowflakes have a better chance of
  survival
• Snowflakes can fall 300 m below the freezing
  level before completely melting
• Fallstreaks Falling ice crystals that evaporate
  before reaching the ground
• Ice crystals have been observed falling at temp
  -47C

42
Ice crystals beneath cirrus clouds
43
Precipitation Types contd.

• When snowflakes fall through very cold air with a
  low moisture content, they do not readily stick
  together powdery flakes of dry snow
  accumulates on ground
• Flurries Light snow showers that fall
  intermittently for short duration often from
  developing cumulus clouds
• Snow Squall A more intense snow showers
  (comparable to summer rain showers) usually form
  from cumuliform clouds
• Ground Blizzard Drifting Blowing snow after
  snow fall ended
• Blizzard Weather with low temp gt30 knot winds
  bearing large amounts of fine, dry, powdery snow

44
Sleet Freezing Rain

• Sleet Partially snowflake (or cold raindrop)
  passing through warmer air undergoes partial
  melting when it again goes through subfreezing
  surface layer of air, partially melted snowflake
  or cold raindrop turns back into a tiny
  transparent ice pellet, called, sleet
• Freezing Rain Supercooled liquid drops upon
  striking a cold surface, form a thin veneer of
  ice this form of precipitation is called
  freezing rain
• Freezing drizzle If the water droplets are
  small, then, it is called freezing drizzle
• Rime White/Milky granular deposit of ice formed
  by the rapid freezing of supercooled water drops
  when they come in contact with an object in
  below-freezing air

45
Sleet partially snowflake (cold droplet)
freezes into a pellet of ice before reaching the
ground
46
Accumulation of rime on tree branches
47
Ice storm caused tree limbs to break Power
lines to sag
48
Snow grains, pellets and hail

• Snow grains Small, opaque grains of ice
  (equivalent of drizzle) fall from stratus clouds
• Snow Pellet White, opaque grains of ice of the
  size of rain drop
• Hail Pieces of ice either transparent or
  partially opaque, ranging in size from that of
  small peas to that of golf balls or larger
  biggest size in US 757 g 14 cm diam.
• Single hailstorm can damage in minutes annual
  loss hundreds of millions of in US
• Hail is produced in a cumulonimbus cloud when
  large frozen raindrops that grow by accumulating
  supercooled liquid droplets

49
Hail contd.

• Graupel Ice particles between 2-5 mm in diameter
  that form in a cloud often by the process of
  accretion
• For a hail to grow to the size of golf ball, it
  must remain for 5-10 minutes in the cloud
• Ice crystals of appreciable size that cant be
  supported by rising air, begin to fall Hail
• Largest form of precipitation occurs during the
  warmest time of the year (due to strong updraft
  that keeps the crystal to become bigger)
• Preventing hailstorm--- cloud seeding ---
  excessive nuclei prevents from growing

50
Accumulation of small hail after a thunderstorm
51
Coffeyville Hailstone (Sept. 3, 1970), Kansas
Layered structure indicates travel through a
cloud of varying water content and temp.
52
When updrafts are tilted, ice particles are swept
horizontally through the cloud, producing the
optimal trajectory for hailstone growth
53
Measurement of Precipitation

• Rain Gauge Instrument to collect measure
  rainfall
• Tipper Bucket rain gauge Receiving funnel
  leading to two small metal collectors bucket
  below the funnel collects the rain water each
  time a bucket tips (with 1/100), an electric
  contact is made recorded each tip it loses
  some rainfall limitation Automated weather
  stations use this technique
• Weighing-type rain gauge Precipitation is caught
  in a cylinder accumulates in a bucket special
  gears translate weight of rain (or snow) into mm
  or inch of precipitation info can be transmitted
  to satellites or land-based stations

54
Standard rain gauge surface area 10 x area of
the cylinder
55
Tipping bucket rain gauge 1/100 bucket tips
56
Rain/snow conversion Doppler Radar

• 10 cm of snow 1 inch of water
• Fresh snowpack water equivalent 101
• Useful about spring runoff and potential for
  flooding
• Radar (RAdio Detection And Ranging) Gathers info
  about storms and precipitation in previously
  inaccessible regions
• A transmitter sending short, microwaver signals
  --- Fraction of the energy is scattered back by
  the target to the Transmitter detected by a
  Receiver Returning signal provides info about
  targets distance intensity of the rainfall

57
Doppler Radar

• Doppler Radar Provide information on distance,
  amount of rainfall and whether the rain/cloud is
  stationary or moving
• Concept of Doppler Shift
• Doppler Radar allows scientists to peer into a
  tornado-generating thunderstorms and observe its
  wind

58
Doppler radar display of precipitation intensity
Oklahoma, April 24, 1999
59
Doppler radar display of 1-hr rainfall amounts -
Oklahoma, April 24, 1999
60
chapter 5- Summary

• Adiabatic process dry adiabatic moist
  adiabatic rate
• Environmental lapse rate
• Conditions for Stable and unable atmosphere
• What cloud type is formed in stable air
• Condensation nuclei, cloud seeding
• Rain shadow, orographic uplift
• Coalescence, accretion
• Rain, drizzle, virga, shower, fallstreaks,
  flurries, snow squall, sleet, freezing rain
• Blizzard, hailstone, standard rain gauge
• Doppler radar
• Water equivalent