1. ATMO 336 Weather, Climate and Society Vertical Stability Precipitation Processes

2. Concept of Stability Stable Rock always returns to starting point
Unstable Rock never returns to starting point
Conditionally Unstable Rock never returns if rolled past top of initial hill
Ahrens, Fig 5.1

3. Archimedes’ Principle
Archimedes' principle is the law of buoyancy.
It states that "any body partially or completely submerged in a fluid is buoyed up by a force equal to the weight of the fluid displaced by the body."
The weight of an object acts downward, and the buoyant force provided by the displaced fluid acts upward. If the density of an object is greater/less than the density of water, the object will sink/float.
Demo: Diet vs. Regular Soda.

4. Absolutely Stable: Top Rock
Stable air strongly resists upward motion
External force must be applied to an air parcel before it can rise
Clouds that form in stable air spread out horizontally in layers, with flat bases-tops
Ahrens, Fig 5.3

5. Absolutely Unstable: Middle Rock
Unstable air does not resist upward motion
Clouds in unstable air stretch out vertically
Absolute instability is limited to very thin layer next to ground on hot, sunny days
Superadiabatic lapse rate
Ahrens, Fig 5.5

6. Conditionally Unstable: Lower Rock
Ahrens, Fig 5.7

7. 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.

8. Summary: Key Concepts II
Vertical Stability Determined by ELR
Absolutely Stable and Unstable
Conditionally Unstable
Temp Difference between ELR and Air Parcel, and Depth of Layer of Conditionally Instability Modulates
Vertical Extent and Severity of Cumulus

9. ATMO 336 Weather, Climate and Society Precipitation Processes

10. 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
Text and caption are self explanatory. A raindrop is 1,000,000 times bigger in volume/mass than a cloud droplet.
What can cause growth to occur in less than one hour?
106 bigger
Ahrens, Fig. 5.15

11. Terminal Fall Speeds (Upward Suspension Velocity)
Diagram gives terminal fall speeds for CCN, droplets and raindrops. NOTE logarithmic ordinate. It also gives the updraft speed required to keep the particle suspended.
CCN are easy to suspend via turbulent motions; just 0.1 microns per second updraft is needed.
Small cloud droplets need 1.0 cm/s, easily achievable by winter storms.
Large raindrops (5mm diameter) require a fast updraft 1.0 m/s.
CCN
1 km in 1010 sec
Cloud Droplets -> Drizzle
1 km in 105 sec
Small-Large Raindrops
1 km in 102 sec

12. 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
small raindrop
Area swept is
smaller than area of drop
Define Coalesence…collection of smaller drops by larger, faster falling drops.
Inefficient mechanism, only 10-50% of smaller drops in path stick.
Important for Warm Clouds…no ice.
Collection Efficiency 10-50%

13. 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)
Size of largest raindrop gives clues as to updraft speed (within limits).
Ahrens, Fig. 5.16

14. 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
glaciated region
Most midlatitude clouds contain a mixture of ice, liquid water, and SUPERCOOLED water.
Such clouds are called MIXED CLOUDS
Ahrens, Fig. 5.17

15. Ahrens, Meteorology Today 5th Ed.
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
THIS IS AN IMPORTANT POINT…take time to explain the
SVP over ice is LESS THAN SVP over liquid because it takes more energy for water to go from solid to vapor phase than liquid to vapor phase.
Difference is largest around -15 to -20 C
Ahrens, Meteorology Today 5th Ed.

16. SVP near Droplets and Ice
Higher SVP over liquid means more vapor around droplets than around ice crystals.
Ahrens, Fig. 5.18
SVP is higher over supercooled water drops than ice

17. Effect maximized around -15oC
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
High vapor levels around droplets and lower concentrations over ice will tend to homogenize by random molecular motions…DIFFUSION.
Thus water vapor molecules over liquid will move towards ice crystals.
Vapor pressure over ice becomes larger than SVP over ice, smaller than SVP over liquid droplet.
Droplets will shrink as liquid evaporates to replace evacuated vapor molecules.
Supercooled water droplets will be attracted to ice crystal, where they freeze/adhere on contact.
Effect maximized around -15oC
Ahrens, Fig. 5.19

18. Accretion-Aggregation Process
Small ice particles will adhere to ice crystals
Supercooled water droplets will freeze on contact with ice
snowflake
ice crystal
Accretion-supercooled droplets freeze on contact with ice
Splintering-ice crystals shatter into smaller crystals.
Aggregation-small crystals adhere to other crystals to from SNOWFLAKES
Ahrens, Fig. 5.17
Accretion (Riming)
Splintering
Aggregation
Also known as the Bergeron Process after the meteorologist who first recognized the importance of ice in the precipitation process

19. 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
Make certain that you get this far.

20. Examples of Precipitation Types
Table is self explanatory.

21. 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

22. 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