Chapter 13 Extratropical Cyclones

Chapter overview:

• Cyclone characteristics • Cyclone evolution • Cyclone tracks • Lee cyclogenesis • Stacking and tilting: Vertical structure of extratropical cyclones • Cyclogenesis

o Boundary layer processes o Diabatic heating o Upper level processes - Troughs and ridges o Upper level processes - The jet streak

• Divergence, convergence, cyclogenesis, and cyclolysis • Overview of a mature extratropical cyclone

Cyclone Characteristics Extratropical: Refers to something outside of the tropics (i.e. in the middle or high latitudes) Extratropical cyclone: A cyclone that occurs in the middle or high latitudes

Cyclogenesis: Development or intensification of a cyclone. What causes surface pressure to decrease and cyclogenesis to occur? Cyclolysis: Weakening or dissipation of a cyclone.

Extratropical cyclone characteristics: - pressure - wind - size - temperature distribution - fronts - clouds and precipitation

The airflow in a cyclone occurs in three dimensions. Ahead of the cold front, in the warm sector, warm air moves towards and over the warm front. This is referred to as a warm air conveyor belt. Behind the cold front, cool, dry air descends.

Cyclone Evolution

Panel a One condition that favors cyclogenesis is the presence of a baroclinic zone. Initially a stationary front is located in the mid-latitudes. Panel b An area of lower pressure forms (cyclogensis) along a kink on the polar front. This is referred to as a frontal wave. What causes this area of lower pressure to form? How does the stationary front change once the frontal wave develops? What causes clouds and precipitation to form at this stage in the cyclone life cycle? Panel c The incipient cyclone typically moves towards the east or northeast and grows into an open wave in 12 to 24 hours. How has the pressure of the cyclone changed from panel b to panel c? How can you tell this from the weather maps shown on the previous page? How do the winds change in response to the change in cyclone pressure? What is the spatial distribution of precipitation along the warm and cold fronts? Warm sector: The region of warm air between the advancing cold and warm fronts. What types of weather occur in the warm sector?

Panel d As the cyclone continues to move northeastward the pressure continues to fall, the winds continue to increase, and the wave develops into a mature cyclone. How does the size of the warm sector change from panels c to d? What causes this change in size of the warm sector? What type of weather occurs northwest of the cyclone center? Panels e and f What type of front forms as the cold front catches up to the warm front? The cyclone is referred to as being occluded at this time.

What temperature air surrounds the cyclone at this point in time? How does this differ from earlier periods in the cyclone life cycle? The cool, dry air that wraps around the cyclone at this time is referred to as a dry tongue.

At this point in time the cyclone begins to weaken (cyclolysis). Triple point: The point where the occluded front, warm front, and cold front meet. Sometimes a new low may form at the triple point. This low is referred to as a secondary low.

The typical life cycle of an extratropical cyclone is shown to the left. Over what period of time does the cyclone life cycle described above / to the left occur? During its lifetime the cyclone transports cold air equatorward and warm air poleward. Thus cyclones act to reduce the temperature gradient between the tropics and the polar regions.

Cyclone Tracks

Cyclogenesis is favored: - in areas of strong baroclinicity / along fronts - east of upper level troughs - east of mountain ranges (m) - where cold air moves over warm water (w and kw) Regions of cyclolysis are shown by G on the map above.

The map below shows preferred locations for cyclogenesis and associated cyclone tracks over the United States.

Lee Cyclogenesis

What happens to westerly flow that passes over a north-south oriented mountain range? Lee side low: A low pressure center that develops on the downwind (lee) side of a mountain range Lee cyclogenesis: Development of a cyclone on the lee side of a mountain range

Stacking and Tilting: Vertical Structure of Extratropical Cyclones How does a thermal low (shown in Ch 10) change with height? Extratropical cyclones are dynamic lows that deepen with height.

In an intensifying extratropical cyclone the area of low pressure (or height) will tilt westward with increasing height. What causes this westward tilt with height?

Where is cold air located relative to the surface cyclone and why is it found in this location? What is the pressure at the surface (aloft) in this cold column of air? Cyclogenesis

Cyclogenesis is associated with: - decreasing surface pressure - increased rotation (vorticity) - upward motion

Net divergence in a column of the atmosphere will cause the surface pressure to decrease and cyclogenesis to occur.

What processes result in divergence or convergence in a column of the atmosphere? - boundary layer effects - diabatic heating - upper level processes

• Troughs / ridges • Jet streaks

Boundary layer effects Friction in the boundary layer causes wind to spiral in towards a low pressure center causing convergence and thus weakens the low. What type of vertical motion does this induce? What types of weather are associated with this vertical motion? Diabatic heating

What processes can cause diabatic heating to occur? How will diabatic heating alter the pressure at the surface (aloft)?

Upper level processes - Troughs and ridges

Why are upper level heights low over the polar regions and high in the tropics? The waviness of upper level height contours is a result of uneven heating of the atmosphere and the rotation of the planet. Wavelength: The distance between adjacent troughs (or ridges)

Longwave: A wave with a wavelength of several thousand kilometers. Longwaves are also known as planetary waves or Rossby waves. Shortwave: A small ripple or disturbance imbedded within a longwave

The westward tilt with height of developing extratropical cyclones results in upper level troughs being located to the west of the surface cyclone.

How does the gradient wind speed change as air moves from an upper level trough to an upper level ridge? Does this result in upper level divergence or convergence?

When an extratropical cyclone becomes occluded the upper level trough is located above the surface cyclone and the system is referred to as being vertically stacked. Once a cyclone becomes vertically stacked how will the location where upper level divergence occurs relative to the surface cyclone position change? Upper level processes - The jet streak

A jet streak is an area of stronger winds embedded in the jet stream. How does the wind speed change as an air parcel enters a jet streak? What impact does this have on the balance between pressure gradient and Coriolis forces acting on the air parcel?

How does the atmosphere respond to this imbalance between the pressure gradient and Coriolis forces?

In the entrance of the jet streak an ageostrophic flow is directed from the right to the left side of the jet streak. In the exit of the jet streak an ageostrophic flow is directed from the left to the right side of the jet streak.

In a straight jet streak divergence occurs in the right entrance and left exit regions and convergence occurs in the left entrance and right exit regions. The figure below is a vertical slice through the exit region of the jet streak shown above.

Where does cyclogenesis (cyclolysis) occur relative to jet streak? Where does rising (sinking) motion occur relative to the jet streak?

For a curved jet streak divergence and convergence only occur on the left side of the jet.

Divergence, convergence, cyclogenesis, and cyclolysis

Panel a A stationary front is located at the surface, with a strong temperature gradient. What does this temperature gradient imply about how the wind speed will change with height? The jet stream will be located above the stationary front.

Panel b A shortwave trough moves into the region disturbing the flow. Where does divergence (convergence) occur relative to this trough? The surface pressure will decrease ahead of the upper level trough. What causes sinking (rising) motion in the column at location 1 (2)? How does the temperature and relative humidity change as air is forced to rise? What type of weather is associated with this rising motion? Latent heat released as clouds form will warm the atmosphere and result in lower surface pressure in the column at location 2. As low pressure develops at the surface at location 2 what happens to the surface winds and how does the stationary front change in response to these winds? Cold air advection occurs behind the surface low and warm air advection occurs ahead of the surface low. How does the intensity of the upper level trough (ridge) change in response to this pattern of temperature advection? What impact does this have on the upper level divergence and convergence? Panel c As cold air wraps all the way around the low pressure center at the surface the cyclone occludes. Once the cyclone has occluded the upper level trough is located above the surface cyclone and the system becomes vertically stacked. Where is upper level divergence occurring relative to the location of the surface low pressure center at this time?

Where is surface convergence occurring relative to the location of the surface low pressure center at this time? How will the intensity of the surface low pressure system change in response to this upper level divergence and surface convergence? Once the cyclone becomes occluded cyclolysis will occur.

The figure at the left summarizes the processes causing convergence and divergence throughout the life cycle of an extratropical cyclone. Cyclogenesis will occur if there is net divergence in the column. Cyclolysis will occur if there is net convergence in the column.

Overview of a Mature Extratropical Cyclone