Chapter10

Extratropical Cyclones and AnticyclonesChapter 10: 

Extratropical Cyclones and Anticyclones Chapter 10

Extratropical Cyclones: 

Extratropical Cyclones A cyclone is any circulation around low pressure “Extratropical” means outside of the tropics An extratropical cyclone is a large low pressure system that often forms in the mid-latitudes

Norwegian Cyclone Model: 

Norwegian Cyclone Model Bjerknes developed the typical life cycle of an extratropical cyclone by looking at weather maps Step 1: The polar front separates warm air and cold air Step 2: A “frontal wave” develops Step 3: Counterclockwise circulation intensifies causing a warm front to the east and a cold front to the west of the wave apex Step 4: The circulation intensifies until an occlusion happens (now at its strongest) Step 5: Surrounded by cold air, the cyclone dissipates as a “cut-off cyclone”

Step 1: Polar Front: 

Step 1: Polar Front

Step 2: Open Wave Develops: 

Step 2: Open Wave Develops

Step 3: Cyclonic Circulation: 

Step 3: Cyclonic Circulation

Step 4: Occlusion: 

Step 4: Occlusion

Step 5: Cut-Off Cyclone: 

Step 5: Cut-Off Cyclone

Slide9: 

Fig. 10.1

Slide10: 

Fig. 10.2

Slide11: 

Fig. 10.5

What Causes Step 2?!: 

What Causes Step 2?! Why does that initial wave form along the polar front? Bjerknes didn’t know because he only had surface weather maps The answer lies in the upper levels Cyclogenesis – the formation of a cyclone Key Ingredients for cyclogenesis: Surface temperature gradients (front) Strong jet stream Presence of mountains or other surface boundaries

Baroclinic Instability: 

Baroclinic Instability Tilted pattern of rising air and sinking air that liberates energy for a cyclone Warm air rising can lead to the formation of clouds and precipitation Typical regions of cyclogenesis and paths of cyclones varies from season to season

Slide14: 

Fig. 10.6

Cyclone Types: 

Cyclone Types Panhandle Hooks – forms in the panhandle region and curves northeastward Nor’Easters – winter cyclones that move up the eastern Atlantic states Pineapple Express – jet stream from Hawaii causing storms in California Alberta Clipper – storm that moves quickly out of Canada in the winter

Role of Mountains: 

Role of Mountains Mountains provide a barrier to the air in the troposphere Wind blowing over a mountain range finds its height shrinking due to the terrain below The shrinking of the air column causes divergence and the air to spin slowly (conservation of angular momentum) After passing the mountain range, the air column is stretched, resulting in convergence and the air to spin faster

Slide17: 

Box 10.1

Slide18: 

Box 10.1

Slide19: 

Fig. 10.7

Slide20: 

Fig. 10.9

Slide21: 

Fig. 10.10

Appearance of a Mature Cyclone: 

Appearance of a Mature Cyclone On a satellite picture, a mature cyclone can have the shape of a comma (“comma cloud”) The tail of the comma is produced along the cold front The head of the comma is produced by clouds circling the low pressure The cloudless region between the head and tail is produced by dry air descending from aloft that causes evaporation (called the “dry slot”)

Slide23: 

Box 10.3

Structure of Low Pressure: 

Structure of Low Pressure Recall that the structure of low pressure has convergence at low levels, rising air above the center, and divergence aloft How can low pressure get even lower? The divergence aloft must be greater than the convergence at the surface

Means of Divergence: 

Means of Divergence Divergence is the air moving apart This can happen in two ways: Speed divergence: Air speeding up downstream will cause the air parcels to spread out (like cars leaving a toll booth) Directional divergence: Air flowing away from each other will also cause air parcels to spread out (like two cars taking different paths at a Y-intersection)

Slide26: 

Fig. 10.17

Slide27: 

Box 10.3

Slide28: 

Fig. 10.18

Anticyclones: 

Anticyclones After the cold front of an extratropical cyclone, the next large weather system to appear is typically an anticyclone (high pressure) Lows form and grow along fronts (boundaries between air masses) Highs are the air masses themselves Lows – short-lived, cloudy, wet, and stormy with strong pressure gradients Highs – longer-lived, clear, dry, and calm with weak pressure gradients

Slide30: 

Fig. 10.24

Anticyclone Characteristics: 

Anticyclone Characteristics Highs have divergence at the surface, with sinking air at the center and convergence aloft Sinking air is compressed and warmed As a result, the atmosphere is stable and a temperature inversion may exist aloft The inversion, along with typically weak winds, can combine to produce pollution episodes

Slide32: 

Fig. 10.26