class #24: wednesday, march 4, 2009 1 class #24: wednesday, march 4 clouds, fronts, precipitation...
TRANSCRIPT
Class #24: Wednesday, March 4, 2009
1
Class #24: Wednesday, March 4
Clouds, fronts, precipitation processes, upper-level waves, and the extratropical cyclone
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
2
The generic front: convergence and frontal lifting
Class #24: Wednesday, March 4, 2009
3
Cold front: convergence, frontal lifting, often convection
Class #24: Wednesday, March 4, 2009
4
Warm front: Convergence and frontal lifting
Class #24: Wednesday, March 4, 2009
5
Class #24: Wednesday, March 4, 2009
6
Class #24: Wednesday, March 4, 2009
7
Cross section through a warm front and cold front
Class #24: Wednesday, March 4, 2009
8
Cross sections at a later time: convection in afternoon
Class #24: Wednesday, March 4, 2009
9
Review of the basic cloud types
Class #24: Wednesday, March 4, 2009
10
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
11
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
12
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
13
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
14
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
15
Class #24: Wednesday, March 4, 2009
16
Collision and coalescence: smallest drops can escape
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
17
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
18
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
19
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
20
Ice crystals can also act as ice nuclei
Class #24: Wednesday, March 4, 2009
21
Ice crystal types depend on temperature
Class #24: Wednesday, March 4, 2009
22