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Cyclogenesis and Cyclolysis. ATS 113. What We’ll Learn:. The Polar Front causes the Midlatitude Jet Stream (by the Thermal Wind Relationship). The MLJ has troughs and ridges. Troughs and ridges lead to patterns of divergence aloft, causing low pressure at the surface. Midlatitude Jet Stream.
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Cyclogenesis and Cyclolysis ATS 113
What We’ll Learn: • The Polar Front causes the Midlatitude Jet Stream (by the Thermal Wind Relationship). • The MLJ has troughs and ridges. • Troughs and ridges lead to patterns of divergence aloft, causing low pressure at the surface.
Was found directly above the Polar Front due to the Thermal Wind Relationship. 400mb A 500mb 300mb B 400mb 600mb 500mb POLAR FRONT North COLD South WARM
L Vorticity • The “spin of the wind”. • Defined so that CYCLONIC is POSITIVE.
What is PVA? • The process by which the winds are bringing air that has higher vorticity values. Location A has positive vorticity. Location B has no vorticity but does have “positive vorticity advection”. B A
What is PVA? • As air moves from location A to location B, it needs to lose its spin. B A
Losing spin • How does a spinning figure skater lose his spin?
Losing spin • How does a spinning figure skater lose his spin? • Spreading his arms!
So… • Spreading out is a way of losing spin.
A region with PVA is a location in which the air must lose its spin! PVA
…upper level divergence… divergence
…means low pressure at the surface! divergence L
Notice where the low is: East of the trough West of the ridge In the region of positive vorticity advection! divergence L
Notice where the low is: East of the trough West of the ridge In the region of positive vorticity advection! L
Notice where the low is: L East of the trough West of the ridge In the region of positive vorticity advection!
Why was the midlatitude jet here in the first place? POLAR FRONT!
Why was the midlatitude jet here in the first place? POLAR FRONT!
“Cyclogenesis” creates a low along this front at the surface: L
…by Thermal Wind Relationship, no longer a reason for the jet to be here! L
Now what kind of vorticity advection is above the low? NONE! L
So how much upper level divergence is above the low? NONE! L
No “support” for the low– it begins to die… CYCLOLYSIS! L
Birth and Death • Cyclogenesis: • Strengthening a cyclone by lowering its pressure. • Happens when PVA in the jet stream causes upper level divergence. • Cyclolysis: • Weakening a cyclone by increasing its pressure. • Happens when the jet stream is no longer directly above the low due to the formation of an occluded front.
Real Life Examples: Jet stream has a very sharp trough in the midwest.
Real Life Examples: Positive vorticity in trough; negative vorticity in ridge.
Real Life Examples: Positive Vorticity Advection near Detroit
Real Life Examples: A strong low near Detroit!
Another Example: A huge trough in the Rockies.
Another Example: Positive Vorticity in the trough.
Another Example: Positive Vorticity Advection ahead of the trough.
Another Example: L A low and precipitation at the surface!