Class #24: Wednesday, March 4, 2009 1 Class #24: Wednesday, March 4 Clouds, fronts, precipitation...

Class #24: Wednesday, March 4, 2009

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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


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The generic front: convergence and frontal lifting


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Cold front: convergence, frontal lifting, often convection


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Warm front: Convergence and frontal lifting


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Cross section through a warm front and cold front


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Cross sections at a later time: convection in afternoon


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Review of the basic cloud types


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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


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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


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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


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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


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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


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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


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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


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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


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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


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Ice crystals can also act as ice nuclei


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Ice crystal types depend on temperature


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Class #24: Wednesday, March 4, 2009 1 Class #24: Wednesday, March 4 Clouds, fronts, precipitation...

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