Transcript CHAPTER – 3 - Wayne State

METEOROLOGY
GEL-1370
Chapter Five
Cloud Development &
Precipitation
Goal for this Chapter
We are going to learn answers to the following
questions:
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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?
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; ~10°C/1000 m in
elevation
• Moist adiabatic rate: Rate of change of temp in a rising or
descending saturated air parcel; ~6°C/1000 m in elevation
Concept of equilibrium
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
Dry adiabatic rate; unsaturated air cools
@10°C/1000m
Absolute stable atmosphere when rising air parcel is
colder and heavier than surrounding air
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, & associated pollutants are kept close to the surface
Cold surface air produces a stable atmosphere that
inhibits vertical motions – fog & haze are kept close
to the ground
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
Unstable atmosphere – rising air parcel is warmer
and lighter than the surrounding air
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
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
Conditionally unstable air – when unsaturated
stable air is lifted to a level where it becomes
saturated and warmer than the air surrounding air
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
How clouds form: a) surface heating & convection; b) forced
lifting along topographic barriers; c) convergence of surface air;
d) forced lifting along weather fronts
Cumulus cloud formation from the hot air rising from
earth’s surface – around the cloud, air is sinking
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
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
Rain shadow, Orographic uplift & cloud
development
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
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
Relative sizes of raindrops, cloud droplets, &
condensation nuclei
Collision & Coalescence: a) warm cloud
composed only of small cloud droplets of uniform
size; b) different size droplets
Collision & Coalescence – contd.
• In clouds warmer than -15°C(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
How surface area depends on the size
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 > 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
– Cloud’s 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
Cloud droplet rising & then falling through a warm
cumulus cloud by growth and coalescence
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
Distribution of ice and water in a cumulonimbus cloud
Water droplets and ice crystal are in equilibrium; water vapor
molecules > liquid is saturation vapor pressure over water is
greater than it is over ice
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 -4°C (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
Natural seeding by cirrus clouds may lead to
precipitation downwind
Precipitation Types
• Rain (Meteorology definition!): falling drop diameter 
0.5 mm
• Drizzle: Water drop diameter < 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 earth’s surface are seldom larger
than ~6mm as collision between raindrops tend to break
them apart into many smaller drops
Virga: Streaks of Falling precipitation evaporates
before reaching the ground
Raindrops < 2mm nearly spherical;
>2mm, elliptical
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 ~-47°C
Ice crystals beneath cirrus clouds
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 & >30 knot winds
bearing large amounts of fine, dry, powdery snow
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
Sleet – partially snowflake (cold droplet) freezes into
a pellet of ice before reaching the ground
Accumulation of rime on tree branches
Ice storm caused tree limbs to break &
Power lines to sag
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
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 can’t 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
Accumulation of small hail after a
thunderstorm
Coffeyville Hailstone (Sept. 3, 1970), Kansas:
Layered structure indicates travel through a cloud of
varying water content and temp.
When updrafts are tilted, ice particles are swept
horizontally through the cloud, producing the optimal
trajectory for hailstone growth
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
Standard rain gauge – surface area = 10 x
area of the cylinder
Tipping bucket rain gauge: 1/100” bucket tips
Rain/snow conversion & Doppler Radar
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10 cm of snow ~ 1 inch of water
Fresh snowpack: water equivalent 10:1
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 target’s distance & intensity
of the rainfall
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 tornadogenerating thunderstorms and observe its wind
Doppler radar display of precipitation
intensity –Oklahoma, April 24, 1999
Doppler radar display of 1-hr rainfall
amounts - Oklahoma, April 24, 1999
chapter –5- Summary
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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