1. Class #24: Wednesday, March 4
Clouds, fronts, precipitation processes, upper-level waves, and the extratropical cyclone
Class #24: Wednesday, March 4, 2009

2. Lifting, fronts and cloud formation
At fronts, one, two, three or all four lifting processes can be acting at the same time
Frontal lifting forces the warmer air over the colder air, and an upslope enhances lifting
Convergence occurs because the wind direction changes at the front
Convection can occur with surface heating
Class #24: Wednesday, March 4, 2009

3. The generic front: convergence and frontal lifting
Class #24: Wednesday, March 4, 2009

4. Cold front: convergence, frontal lifting, often convection
Class #24: Wednesday, March 4, 2009

5. Warm front: Convergence and frontal lifting
Class #24: Wednesday, March 4, 2009

6. Class #24: Wednesday, March 4, 2009

7. Class #24: Wednesday, March 4, 2009

8. Cross section through a warm front and cold front
Class #24: Wednesday, March 4, 2009

9. Cross sections at a later time: convection in afternoon
Class #24: Wednesday, March 4, 2009

10. Review of the basic cloud types
Class #24: Wednesday, March 4, 2009

11. Frontal lifting and cloud types
Frontal lifting is weaker at warm fronts than cold fronts
Convergence is weaker at warm fronts than cold fronts
Convection is rare at warm fronts, common with cold fronts
Layer clouds are common with fronts
Class #24: Wednesday, March 4, 2009

12. How clouds produce precipitation
Clouds produce precipitation with two different mechanisms
Both mechanisms can be active in the same cloud
First, the collision--coalescence process, also called the warm rain process
Second, the ice crystal process, also called the Bergeron—Wegner process
Class #24: Wednesday, March 4, 2009

13. The collision—coalescence process
Cloud droplets are not all exactly the same size
Statistically speaking, there is a spectrum of cloud droplet sizes
Condensation alone is too slow to produce precipitation-sized particles (it would take days)
Cloud droplets fall at different speeds
Class #24: Wednesday, March 4, 2009

14. Collision—coalescence (continued)
Terminal velocity in a cloud is the velocity of a droplet relative to the surrounding air
Dropping an object in a rising elevator, it will fall to the floor of the elevator
Cloud droplets can fall relative to the air around them, even as they and the air rises with respect to the ground
Larger cloud droplets have a greater terminal velocity than smaller cloud droplets
Class #24: Wednesday, March 4, 2009

15. Collision—coalescence (continued)
Larger drops have a greater terminal velocity than smaller drops because they are less buffeted by turbulent eddies.
The larger drops, falling faster, collide with some smaller drops.
Some collisions result in sticking together of the two drops, or coalescence.
The result of coalescence is a larger drop
Class #24: Wednesday, March 4, 2009

16. Collision and coalescence: smallest drops can escape
Class #24: Wednesday, March 4, 2009

17. Class #24: Wednesday, March 4, 2009
Warm rain
Repeated collisions favor the largest droplets, which continue to collide and grow most quickly while they fall fastest.
This process can produce raindrop-sized drops in about 20 minutes, many times faster than condensation.
One typical raindrop contains about 1 million cloud droplets
Class #24: Wednesday, March 4, 2009

18. Warm rain isn’t the entire story
The collision—coalescence process explains how rain can form in clouds with no ice, or in the lower (above-freezing) portions of deeper/colder clouds
Near mid-latitude fronts and in extratropical cyclones, another process is at work—the ice crystal process. It depends on the presence of ice crystals
Class #24: Wednesday, March 4, 2009

19. Class #24: Wednesday, March 4, 2009
Ice crystal formation
The ice crystal process begins with the formation of ice crystals
At temperatures below -40ºC, ice crystals can form spontaneously (deposition)
At higher temperatures, small particles called ice nuclei form surfaces for water vapor to freeze.
There are lots less ice nuclei than CCN
Class #24: Wednesday, March 4, 2009

20. Mixed clouds have water droplets and ice crystals
At temperatures just below freezing, few substances can act as ice nuclei
At lower temperatures (higher in the cloud) more substances can act as ice nuclei
Ice nuclei have molecular structures similar to the ice crystal
Condensation of supercooled water occurs for T<0º without an ice nucleus
Class #24: Wednesday, March 4, 2009

21. Ice crystals can also act as ice nuclei
Class #24: Wednesday, March 4, 2009

22. Ice crystal types depend on temperature
Class #24: Wednesday, March 4, 2009