NATS 101 Lecture 13 Precipitation Processes. Supplemental References for Today’s Lecture...
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Transcript of NATS 101 Lecture 13 Precipitation Processes. Supplemental References for Today’s Lecture...
Supplemental References for Supplemental References for Today’s LectureToday’s 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)
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
Review: Vertical Stability
Vertical Stability Determined by ELR
Conditionally Unstable
(MAR < ELR < DAR)
Temp Difference between Environmental Air and Air Parcel, and the Depth of Conditionally Instability Controls
Vertical Extent and Severity of Cumulus
Environmental Lapse Rate (ELR)
ELR is the Temp change with height that is recorded by a weather balloon
ELR is absolutely unstable in a thin layer just above the ground on hot, sunny days
Ahrens, Meteorology Today 5th Ed.
ELR is 6.5o C/km, on average, and thus is conditionally unstable!
6.06.0oo C/km C/km
10.010.0oo C/km C/km
6.56.5oo C/km C/km
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
Terminal Fall Speeds (upward suspension velocity)
1.E-061.E-051.E-041.E-031.E-021.E-011.E+001.E+011.E+021.E+03
0.0002 0.02 0.1 0.2 1 2 5Diameter (millimeters)
Terminal Fall Speed (cm/s)
Small-Large RaindropsCloud Droplets-DrizzleCCN
small raindrop
Area swept issmaller than
area of drop
Collision-CoalescenceBig 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 cloudsCollection Efficiency 10-50%
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
Ahrens, Fig. 5.16
Updraft (5 m/s)
Mixed Water-Ice Clouds
Clouds that rise above freezing level contain mixture of water-ice
Mixed region exists where Temps > -40oC
Only ice crystals exist where Temps < -40oC
Mid-latitude clouds are generally mixed
Ahrens, Fig. 5.17
glaciated regionglaciated region
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.
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
Ahrens, Fig. 5.19
Effect maximized around -15oC
Accretion-Aggregation Process
Accretion (Riming)
Aggregation
Supercooled water droplets will freeze on contact with ice
ice crystalice crystal
Small ice particles will adhere to ice
crystals
snowflakesnowflake
Splintering
Ahrens, Fig. 5.17
Also known as the Bergeron Process after the meteorologist who first recognized the importance of ice in the precipitation process
Summary: Key Concepts
Condensation acts too slow to produce rain
Several days required for condensationClouds produce rain in less than 1 hour
Warm clouds (no ice)
Collision-Coalescence Process
Cold clouds (with ice)
Ice Crystal ProcessAccretion-Splintering-Aggregation
Examples of Precipitation Types Type Size Description
Drizzle < 0.5 mm Small uniform drops that fall from stratus clouds
Rain 0.5 - 5 mm Size of drops generally vary from one place to another
Freezing Rain 0.5 - 5 mm Rain that freezes on contact with object
Sleet 0.5 - 5 mm Ice parti cles from raindrops that freeze during descent
Snow 1 - 2 mm Aggregated ice crystals that remain frozen during descent
Hail 5 to 10 cm or larger
Hard pellets of ice from cumulonimbus clouds
Temp Profiles for Precipitation
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
Ahrens, Meteorology Today 5th Ed.
Summary: Key Concepts
Precipitation can take many forms
Drizzle-Rain-Glazing-Sleet-Snow-HailDepending on specific weather conditions
Radar used to sense precipitation remotely
Location-Rate-Type (liquid v. frozen)Cloud drops with short wavelength pulseWind component toward and from radar