Chapter 5: Cloud Chapter 5: Cloud Development and Development and
PrecipitationPrecipitation
Atmospheric StabilityAtmospheric Stability Determining stabilityDetermining stability Cloud development and stabilityCloud development and stability Precipitation processesPrecipitation processes Precipitation typesPrecipitation types Measuring precipitationMeasuring precipitation
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Atmospheric StabilityAtmospheric StabilityQ: Why does the air rise on some occasions and not on others? Why do the size and shape of clouds vary so much when the air does rise? A: because of different atmospheric stability conditions
• Stable and unstable equilibrium stability analysis: giving initial perturbations,
Stable: parcel moves back
Unstable: parcel moves away from the original position 2
Determining StabilityDetermining Stability
Stability analysis: assume a parcel rises following the dry or moist adiabatic process; then compare its Tp with the environmental Te (following environmental lapse rate); colder T means denser air.
Stable condition: If a rising parcel’s Tp < Te, it is denser and would sink back.
Unstable condition: If the rising parcel’s Tp > Te, it is less dense and will continue to rise
• Stability does not control whether air will rise or sink.Stability does not control whether air will rise or sink. Rather, it controls whether rising air will Rather, it controls whether rising air will continuecontinue to rise to rise or whether sinking air will or whether sinking air will continuecontinue to sink. to sink.
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Atmospheric StabilityAtmospheric Stability adiabatic process: no heat exchange of the air parcel with the adiabatic process: no heat exchange of the air parcel with the
environment so that rising air parcel expands and coolsenvironment so that rising air parcel expands and cools dry adiabatic lapse rate for parcels (10 C/km)dry adiabatic lapse rate for parcels (10 C/km) moist adiabatic lapse rate for parcels (taken as 6 C/km)moist adiabatic lapse rate for parcels (taken as 6 C/km) environmental lapse rate for the atmosphere (~ 6.5 C/km)environmental lapse rate for the atmosphere (~ 6.5 C/km)
Q: if a rising parcel’s T decreases at 10 C/km in an adiabatic Q: if a rising parcel’s T decreases at 10 C/km in an adiabatic process, does its T increases at 10 C/km as the parcel process, does its T increases at 10 C/km as the parcel descends? a) yes, b) nodescends? a) yes, b) no
Q: why is the moist lapse rate lower than dry lapse rate?Q: why is the moist lapse rate lower than dry lapse rate?
a)a) because condensation occurs in the moist adiabatic processbecause condensation occurs in the moist adiabatic process
b)b) because evaporation occurs in the dry adiabatic processbecause evaporation occurs in the dry adiabatic process
c)c) because condensation occurs in the dry adiabatic processbecause condensation occurs in the dry adiabatic process4
Q: why is the moist lapse rate lower over tropics (higher T) Q: why is the moist lapse rate lower over tropics (higher T) than over polar regions (lower T)?than over polar regions (lower T)?
because warm saturated air contains more liquid water for because warm saturated air contains more liquid water for condensation; condensation;
because cold saturated air contains more liquid water for because cold saturated air contains more liquid water for condensationcondensation
Q: What does a radiosonde measure?Q: What does a radiosonde measure? dry adiabatic lapse rate; dry adiabatic lapse rate; moist lapse rate; moist lapse rate; environmental lapse rateenvironmental lapse rate
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Absolutely Stable AtmosphereAbsolutely Stable Atmosphere stabilizing processesstabilizing processes
nighttime surface radiational nighttime surface radiational cooling;cooling;
warm air advected to cold warm air advected to cold surface;surface;
air aloft warming (e.g., air aloft warming (e.g., subsidence inversions)subsidence inversions)
• Stable air provides ideal Stable air provides ideal conditions for high pollution conditions for high pollution
levels.levels.
Q: For a rising surface dry Q: For a rising surface dry parcel with dew point T of parcel with dew point T of 20C, its temperature at 2000 20C, its temperature at 2000 m height is: a) 10C, b) 14C, m height is: a) 10C, b) 14C, c) 18 C, d) 20Cc) 18 C, d) 20C 6
Absolutely Unstable AtmosphereAbsolutely Unstable Atmosphere destabilizing processesdestabilizing processes
daytime solar heating of daytime solar heating of surface air;surface air;
cold air advected to cold air advected to warm surfacewarm surface
superadiabatic lapse rates superadiabatic lapse rates (> 10 C/km)(> 10 C/km)
• Unstable air tends to be Unstable air tends to be well-mixed.well-mixed.
Q: For a descending saturated Q: For a descending saturated parcel at 1000 m height parcel at 1000 m height (with T = 24C), its T at (with T = 24C), its T at surface is: a) 28C, b) 30C, c) surface is: a) 28C, b) 30C, c) 32C, d) 34C32C, d) 34C 7
Conditionally Unstable AirConditionally Unstable Air
Conditional Conditional instability:instability:
environmental lapse environmental lapse rate between dry and rate between dry and moist lapse ratesmoist lapse rates
Lifting condensation Lifting condensation level is the cloud level is the cloud basebase
Level of free Level of free convectionconvection
Q: Between what heights in the figure would TQ: Between what heights in the figure would Tpp = T = Tee??a) 0-1 km, b) 1-2 km, c) 2-3 kma) 0-1 km, b) 1-2 km, c) 2-3 km
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Q: If environmental lapse is greater than dry lapse rate, the atmosphere is a) stable, b) unstable, c) conditionally unstable
Q: If environmental lapse is less than moist lapse rate, the atmosphere is a) stable, b) unstable, c) conditionally unstable
Q: The earth’s atmosphere is ordinarily a) stable, b) unstable, c) conditionally unstable
Q: The air T in an unsaturated parcel follows the a) dry lapse rate, b) moist lapse rate, c) environmental lapse rate
Q: If air T increases with height, the air is surely stable. If it decreases with height, the air is: a) stable, b), unstable, c) conditionally unstable, d) undecided9
Q: For stable condition, if clouds exist, they are usuallya)cumuliform clouds, b) stratiform clouds
Q: For conditionally unstable conditions, if clouds exist, they are usuallya)cumuliform clouds, b)stratiform clouds
Q: Do you usually expect to see layered clouds in Tucson during the day in summer? a) yes, b) no
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Four Ways for Cumulus Development Four Ways for Cumulus Development
surface surface heating and heating and free free convectionconvection
uplift along uplift along topographytopography
convergence convergence of airof air
lifting along lifting along weather frontsweather fronts
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Convection and CloudsConvection and Clouds thermalsthermals fair weather fair weather
cumuluscumulus• Fair weather cumulus Fair weather cumulus provide a visual marker of provide a visual marker of thermals.thermals.
• Bases of fair-weather Bases of fair-weather cumulus clouds marks the cumulus clouds marks the lifting condensation levellifting condensation level, , the level at which rising air the level at which rising air first becomes saturated.first becomes saturated.
Q: why are there large clear Q: why are there large clear areas between cumulus areas between cumulus clouds?clouds?
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Topography and CloudsTopography and Clouds rain shadow: Due to frequent westerly winds, the western rain shadow: Due to frequent westerly winds, the western
slope of the Rocky Mountains receives much more slope of the Rocky Mountains receives much more precipitation than the eastern slope.precipitation than the eastern slope.
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Precipitation ProcessesPrecipitation ProcessesThe diameter increases by 2 orders of magnitude from condensation nucleus to cloud droplet, and from cloud droplet to raindrop
Growth of cloud droplets by condensation is too slow (a few days), but rain drops can develop in < 1 hr in nature
Q: 0.0002 mm is: a) 20 μm, b) 2 μm, c) 0.2 μm, d) 0.02 μm
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Collision and Coalescence ProcessCollision and Coalescence Process Warm clouds (with T above freezing)Warm clouds (with T above freezing) terminal velocityterminal velocity
large drops fall faster than large drops fall faster than
small dropssmall drops coalescence:coalescence:
the merging of a large cloudthe merging of a large cloud
droplet with small dropletsdroplet with small droplets
by collisionby collision
Q: Do larger drops fall faster in a vacuum? a) yes, b) no
Q: Does coalescence exist for uniform droplets? a) yes, b) no 15
Ice Crystal ProcessIce Crystal Process cold clouds (ice crystals and liquid droplets coexist) cold clouds (ice crystals and liquid droplets coexist) supercooled water droplets due to lack of ice nucleisupercooled water droplets due to lack of ice nuclei
Q: what are the two reasons for the existence of the anvil?
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Saturation vapor pressures Saturation vapor pressures over liquid water is higher than over liquid water is higher than over iceover iceThis causes water vapor This causes water vapor molecules to diffuse from water molecules to diffuse from water droplet towards the ice crystalsdroplet towards the ice crystalsIce crystals grow at the Ice crystals grow at the expense of water droplets. It is expense of water droplets. It is called ice-crystal (or Bergeron) called ice-crystal (or Bergeron) process.process.
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Snow pellets and snowflakesSnow pellets and snowflakes Accretion: ice crystals grow by colliding with supercooled Accretion: ice crystals grow by colliding with supercooled
water droplets to form snow pellets water droplets to form snow pellets
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Cloud Seeding and PrecipitationCloud Seeding and Precipitation cloud seeding: inject (or seed) a cloud with small particles that cloud seeding: inject (or seed) a cloud with small particles that
will act as nuclei so that the cloud particles will grow large will act as nuclei so that the cloud particles will grow large enough to fall to the surface as precipitationenough to fall to the surface as precipitation
silver iodide (AgI): as cloud-seeding agent because it has a silver iodide (AgI): as cloud-seeding agent because it has a crystalline structure similar to an ice crystal; it acts as an crystalline structure similar to an ice crystal; it acts as an effective ice nucleus at T = -4˚C and lower.effective ice nucleus at T = -4˚C and lower.
Very popular in some countriesVery popular in some countries
• It is very difficult to determine whether a cloud seedingIt is very difficult to determine whether a cloud seedingattempt is successful. How would you know whetherattempt is successful. How would you know whetherthe cloud would have resulted in precipitation if it hadn’tthe cloud would have resulted in precipitation if it hadn’tbeen seeded?been seeded?
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Q: What are the preferable clouds for seeding?A: existence of supercooled liquid droplets; low ratio of ice crystals to droplets.
Q: In warm clouds (with T above freezing), cloud droplets grow to rain droplets primarily through
a) collision/coalescence processb) ice crystal process, c) accretion
Q: Cloud liquid droplet collision is calleda) coalescence, b) accretion
Q: What is the purpose of using silver iodide for the seeding of supercooled clouds?
a) Increase the number of ice nucleib) Increase the number of cloud nuclai
Q: Cloud (liquid) droplet does not exist for temperature:a) warmer than 0˚C, b) warmer than −20˚C, c) warmer than −40˚C, d) colder than −40˚C
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Precipitation TypesPrecipitation Types
RainRain SnowSnow Sleet and freezing rainSleet and freezing rain HailHail
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RainRain
Rain: drop diameter of 0.5 Rain: drop diameter of 0.5 ~ 6mm~ 6mm
Drizzle: drop diameter < Drizzle: drop diameter < 0.5mm0.5mm
Virga: rainfall not Virga: rainfall not reaching surfacereaching surface
Rain drop shapeRain drop shape
#1: tear-shaped#1: tear-shaped
#2: spherical shape for small #2: spherical shape for small raindrops with diameter <2 raindrops with diameter <2 mmmm
#3: For large raindrops with #3: For large raindrops with diameter > 2 mmdiameter > 2 mm
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SnowSnow Fallstreaks: ice crystals and Fallstreaks: ice crystals and
snowflakes from high cirrus snowflakes from high cirrus clouds that usually do not clouds that usually do not reach surfacereach surface
Flurries: light snow falling Flurries: light snow falling from cumulus cloudsfrom cumulus clouds
snow storm: heavy snowfallsnow storm: heavy snowfall Blizzard: low T and strong Blizzard: low T and strong
wind bearing large amounts wind bearing large amounts of snow, reducing visibility of snow, reducing visibility to a few metersto a few meters
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SnowSnow Dendrite: Snowflake shape Dendrite: Snowflake shape
depends on both temperature depends on both temperature and relative humidityand relative humidity
Annual snowfall in U.S. and Annual snowfall in U.S. and CanadaCanada
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Mt. Rainier in Washington receives an annual average snowfall of 17 m!
Sleet and Freezing RainSleet and Freezing Rain
Sleet: frozen raindrop (called Sleet: frozen raindrop (called ice pellet); makes a tapping ice pellet); makes a tapping sound when falling on glass; sound when falling on glass; bounce when striking the bounce when striking the groundground
Freezing rain: supercooled Freezing rain: supercooled liquid drops spread out and liquid drops spread out and freeze on cold surfacefreeze on cold surface
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Q: what is the necessary vertical temperature profile for sleet?A: a below-cloud (above freezing) layer to melt snowflake; deep freezing layer to freeze the raindrop
Q: what is the necessary vertical temperature profile for sleet?A: shallow below-cloud layer (so that supercooled raindrop can not be frozen)
Sleet and Freezing RainSleet and Freezing Rain
Ice storm: substantial Ice storm: substantial accumulation of freezing rainaccumulation of freezing rain
Rime: white granular ice, Rime: white granular ice, formed by freezing small, formed by freezing small, supercooled cloud or fog supercooled cloud or fog dropletsdroplets
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Snow Grains and Snow PelletsSnow Grains and Snow Pellets
snow grains: snow equivalent of drizzle snow grains: snow equivalent of drizzle snow pellets: larger than snow grains and bounce on surface; snow pellets: larger than snow grains and bounce on surface;
formed as ice crystals collide with supercooled water droplets; formed as ice crystals collide with supercooled water droplets; usually from cumulus congestus cloudsusually from cumulus congestus clouds
Graupel: Graupel:
when snow pellets accumulate a heavy coating of rime, when snow pellets accumulate a heavy coating of rime,
they are called graupelthey are called graupel
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HailHail• hail is produced when graupel grows by hail is produced when graupel grows by
accumulating supercooled liquid droplets, a accumulating supercooled liquid droplets, a process called accretion. Strong updrafts are process called accretion. Strong updrafts are needed. needed.
• A hailstone can be sliced open to A hailstone can be sliced open to reveal accretion rings, one for each reveal accretion rings, one for each updraft cycle.updraft cycle.
2818.75 inch in diameter
Q: what is the shape of a small rain droplet?: what is the shape of a small rain droplet? a) tear-shaped, b) spherical, c) mushrooma) tear-shaped, b) spherical, c) mushroom
Q: What is the difference between sleet and freezing rain?Q: What is the difference between sleet and freezing rain?A: sleet refers to raindrops freezing through a deep cold layer A: sleet refers to raindrops freezing through a deep cold layer
below cloud and could bounce on the ground, while below cloud and could bounce on the ground, while freezing rain refers to supercooled liquid drops spreading freezing rain refers to supercooled liquid drops spreading out and freezing on cold surface (ground, trees, …) and out and freezing on cold surface (ground, trees, …) and substantially affect driving conditions (both on the road substantially affect driving conditions (both on the road and on the wind shield)and on the wind shield)
Q: When ice crystals collide with supercooled water droplets, Q: When ice crystals collide with supercooled water droplets, what would be formed?what would be formed?
a) snow pellets, b) graupel, c) hail a) snow pellets, b) graupel, c) hail
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Precipitation Measurement InstrumentsPrecipitation Measurement Instruments standard rain gauge: 0.01 inch interval ; tracestandard rain gauge: 0.01 inch interval ; trace tipping bucket rain gauge: used in ASOStipping bucket rain gauge: used in ASOS
• difficult to capture rain in a bucket when wind blows strongly.difficult to capture rain in a bucket when wind blows strongly.• Tipping bucket underestimates rainfall for heavy eventsTipping bucket underestimates rainfall for heavy events
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Doppler Radar and PrecipitationDoppler Radar and Precipitation Radar: radio detection and ranging Radar: radio detection and ranging Doppler radar:Doppler radar: use Doppler shiftuse Doppler shift (e.g., a higher-pitched(e.g., a higher-pitchedwhistle as a train whistle as a train approaches you); approaches you); Provide precipitationProvide precipitationarea and intensity;area and intensity;Provide horizontal Provide horizontal speed of falling rainspeed of falling rain Polarimetric radar: Polarimetric radar: identify rain from snowidentify rain from snow(by using horizontal (by using horizontal and vertical pulses)and vertical pulses)
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Measuring precipitation from space.
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Q: Can you claim from your rain gauge that precipitation rate is 0.001 in/hour? a) yes, b) no
Q: Surface temperature is above 0C fora)sleet, b) frozen rain, c) hail
Q: how to transform a stable atmosphere near surfaceto an unstable atmosphere?
Q: for a thick nimbostratus cloud with ice crystals and supercooled cloud droplets of about the same size, which precipitation process would be most important in producing rain from the cloud? Why?
