NATS 101 Lecture 13 Precipitation Processes

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NATS 101Lecture 13Precipitation Processes

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Supplemental References for Todays Lecture

• Danielson, E. W., J. Levin and E. Abrams, 1998
  Meteorology. 462 pp. McGraw-Hill. (ISBN
  0-697-21711-6)
• Gedzelman, S. D., 1980 The Science and Wonders
  of the Atmosphere. 535 pp. John-Wiley Sons.
  (ISBN 0-471-02972-6)

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Review Vertical Stability

• Rising and sinking unsaturated (clear) air
• Temp changes at DAR of 10oC/km
• Dew Point (DP) changes at rate of 2oC/km
• Rising and sinking saturated (cloudy) air
• Latent Heating Mitigates Adia. Cooling
• Temp and DP cool at MAR of 6oC/km
• Water Vapor Condenses into Liquid

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Review Vertical Stability

• Vertical Stability Determined by ELR
• Conditionally Unstable
• (MAR lt ELR lt DAR)
• Temp Difference between Environmental Air and Air
  Parcel, and the Depth of Conditionally
  Instability Controls
• Vertical Extent and Severity of Cumulus

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Conditionally Unstable Lower Rock
Ahrens, Fig 5.7
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Environmental Lapse Rate (ELR)
ELR is the Temp change with height that is
recorded by a weather balloon
6.5o C/km
6.0o C/km
ELR is 6.5o C/km, on average, and thus is
conditionally unstable!
10.0o C/km
ELR is absolutely unstable in a thin layer just
above the ground on hot, sunny days
Ahrens, Meteorology Today 5th Ed.
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Lapse Rates and Cumulus Types
Ahrens, Meteorology Today 5th Ed.
The ELR and depth of unstable layer modulates the
type of Cu. As depth increases, the vertical
extent of Cu generally increases. As temp
difference between the air parcel and the
environment increases, the updraft speed and
severity of Cb typically increase.
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Cloud Droplets to Raindrops

• A raindrop is 106 bigger than a cloud droplet
• Several days are needed for condensation alone to
  grow raindrops
• Yet, raindrops can form from cloud droplets in a
  less than one hour
• What processes account for such rapid growth?

106 bigger
106 bigger
Ahrens, Fig. 5.15
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Terminal Fall Speeds (upward
suspension velocity)
Small-Large Raindrops
Cloud Droplets-Drizzle
CCN
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Collision-Coalescence

• Big water drops fall faster than small drops
• As big drops fall, they collide with smaller
  drops
• Some of the smaller drops stick to the big drops
• Collision-Coalescence
• Drops can grow by this process in warm clouds
  with no ice
• Occurs in warm tropical clouds

Area swept is smaller than area of drop
small raindrop
Collection Efficiency 10-50
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Warm Cloud Precipitation

• As cloud droplet ascends, it grows larger by
  collision-coalescence
• Cloud droplet reaches the height where the
  updraft speed equals terminal fall speed
• As drop falls, it grows by collision-coalescence
  to size of a large raindrop

Updraft (5 m/s)
Ahrens, Fig. 5.16
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Mixed Water-Ice Clouds

• Clouds that rise above freezing level contain
  mixture of water-ice
• Mixed region exists where Temps gt -40oC
• Only ice crystals exist where Temps lt -40oC
• Mid-latitude clouds are generally mixed

glaciated region
Ahrens, Fig. 5.17
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SVP over Liquid and Ice

• SVP over ice is less than over water because
  sublimation takes more energy than evaporation
• If water surface is not flat, but instead curves
  like a cloud drop, then the SVP difference is
  even larger
• So at equilibrium, more vapor resides over cloud
  droplets than ice crystals

Ahrens, Meteorology Today 5th Ed.
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SVP near Droplets and Ice
Ahrens, Fig. 5.18
SVP is higher over supercooled water drops than
ice
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Ice Crystal Process

• Since SVP for a water droplet is higher than for
  ice crystal, vapor next to droplet will diffuse
  towards ice
• Ice crystals grow at the expense of water drops,
  which freeze on contact
• As the ice crystals grow, they begin to fall

Effect maximized around -15oC
Ahrens, Fig. 5.19
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Accretion-Aggregation Process
Small ice particles will adhere to ice crystals
Supercooled water droplets will freeze on contact
with ice
snowflake
ice crystal
Ahrens, Fig. 5.17
Accretion (Riming)
Aggregation
Splintering
Also known as the Bergeron Process after the
meteorologist who first recognized the importance
of ice in the precipitation process
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Summary Key Concepts

• Condensation acts too slow to produce rain
• Several days required for condensation
• Clouds produce rain in less than 1 hour
• Warm clouds (no ice)
• Collision-Coalescence Process
• Cold clouds (with ice)
• Ice Crystal Process
• Accretion-Splintering-Aggregation

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Examples of Precipitation Types
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Definitions of Liquid Precipitation
Williams, The Weather Book, p73
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Temp Profiles for Precipitation
Ahrens, Meteorology Today 5th Ed.
Snow - Temp colder than 0oC everywhere (generally
speaking!) Sleet - Melting aloft, deep freezing
layer near ground Freezing Rain - Melting aloft,
shallow freezing layer at ground Rain - Deep
layer of warmer than 0oC near ground
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Weather Conditions Associated with Precipitation
Types
Gedzelman, The Science and Wonders of the
Atmosphere
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Radar Estimates of Precipitation

• Radar emits pulses of EM radiation of wavelength
  between 3-10 cm
• Pulse reflects off raindrops, dust, bugs, chaff,
  etc.
• Distance from radar and intensity of
  precipitation can be determined from radar
  reflectivity

Danielson et al
Object size can be determined from amplitude of
return pulse. Larger objects are more
reflective. Reflections from objects farther away
take longer to return.
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Doppler Radar

• Doppler can detect motion toward or away from
  radar by the frequency of the return beam
• Higher - toward radar
• Lower - away from radar
• Doppler effect explains why pitch of whistle
  changes as a train approaches then moves away

Lower frequency
Higher frequency
Danielson et al
Frequency of return beam changes when reflective
object is moving either toward or away from
radar. Velocity can be determined from frequency
shift.
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Summary Key Concepts

• Precipitation can take many forms
• Drizzle-Rain-Glazing-Sleet-Snow-Hail
• Depending on specific weather conditions
• Radar used to sense precipitation remotely
• Location-Rate-Type (liquid v. frozen)
• Cloud drops with short wavelength pulse
• Wind component toward and from radar

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Assignment for Next Lecture

• Topic Atmospheric Pressure
• Reading - Ahrens pg 141-148
• Problems - 6.1, 6.7, 6.8