1. Cyclones and Anticyclones in the Mid-Latitudes
Val Bennington
November, 2008

2. Air pressure and atmospheric motion
Q: What makes the wind blow?
A: Air pressure differences. Winds blow from high toward low pressure.

3. Air pressure
Force exerted by molecules in atm due to gravity and temperature
July 11, 2006
NY Times
As the World Wobbles
Late last November, as a big low-pressure system built over Europe and Asia and high pressure settled in over the Pacific and Atlantic Oceans, the shifts in the atmosphere caused the earth to jiggle ever so slightly, like a hiker adjusting to a shifting load in a backpack.
As a result, the North Pole and its southern counterpart moved about four inches by one measure. (There are several ways to define the poles.)
Despite its diaphanous appearance, the atmosphere weighs about 5,000 trillion metric tons, and its mass is unevenly distributed. All those ridges and troughs on a weather map reflect differences of billions of tons of gases.
Scientists have long known that as the atmosphere shifts, it influences the earth’s rotation. The recent advent of satellites’ global positioning systems made it possible to confirm even the tiniest movements.
There were well-known, regularly occurring wobbles in the earth’s rotation that could shift the poles 30 feet over a year or more. These shifts blocked the detection of subtler, quicker movements caused by day-to-day changes in the atmosphere and the oceans.
Now, these small shifts are being measured by institutions devoted to tracking the planet’s behavior, including the earth orientation department of the United States Naval Observatory and the “time, earth rotation and space geodesy section” of the Royal Observatory of Belgium.
Experts at the Belgian observatory and the Paris Observatory found the November polar shift and a series of other little loops by looking particularly closely at a period from last November through February, when two of the larger regular wobbles in the axis canceled each other out.
They reported their analysis in the July 1 issue of Geophysical Research Letters.

4. Air pressure

5. Mercury barometer
1013 mb = inches
Aneroid barometer

7. Pressure systems Two types: high and low
Low: associated with clouds and instability.
High: associated with clear conditions and stability

8. Low pressure systems Cyclone Converging rising air at surface
Diverging air aloft
Winds rotate counterclockwise in NH
Areas of “light” atmosphere; air is forced into these locations
Unstable surface conditions

9. High pressure system Anticyclone Converging air aloft
Diverging sinking air at surface
Winds rotate clockwise in NH
Areas of “heavy” atmosphere; air is forced out of these locations
Stable surface conditions

11. Is the location for these pressure systems the northern or the southern hemisphere?

12. Surface Chart
Over surface maps the points with same pressure are connected with curves (Isobars).
Isobars characterize the area with lower and higher pressure relative the neighboring points.
These areas are named low pressure (or cyclone) and high pressure (or anticyclone) respectively.

13. Surface Chart The solid dark lines are isobars (millibars or hPa).
The surface winds tend to blow across the isobars toward regions of lower pressure.

14. Anticyclones High pressure systems
Just air masses with temperature and moisture varying slightly over large area
Clear, calm, pretty dry
Blob-like, with small pressure gradients and slower winds

15. Anticyclone
The large blue H on the map indicate the center of high pressure (anticyclone).
Low pressure gradient around the high center.

16. Anticyclone (High) Which way does the wind blow?
Does air diverge or converge at the surface?
Does air converge or diverge above the high?

17. Anticyclone (High)
Which way does the wind blow? > anti-cyclonic = clockwise!
Does air diverge or converge at the surface? >Diverges!
Does air converge or diverge above the high? -->Converges!

18. Anticyclones (Highs)
Due to friction, air is always diverging near surface anticyclones

19. Anticyclones (Highs) Generally boring weather - clear, calm
Linger for a while, but can be nice
Trap air near surface (sinking motion)
Blob-like air masses
Air mass stays long can take on characteristics of land it is over

20. Cyclones

21. What is a Cyclone?
A cyclone is simply an area of low pressure around which the winds flow counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
Cyclones form and grow near the front.
Cyclones (lows) are cloudy, wet, stormy.

22. Cyclones have converging air at surface that rises!
Due to friction, air is always converging near surface cyclones

23. Cyclone
The large red L on the map indicate the center of low pressure (cyclone).
High pressure gradient around the low center.
Norwegian meteorologists discovered that extratropical cyclones are associated with fronts.
Cyclones: growing from birth as a frontal wave, to maturity as an occluded cyclone, and to death as a cut-off cyclone over the course of several days

24. Locating Fronts
Fronts are associated with . . .
Strong temperature gradients
Positive vorticity (counter-clockwise rotation)
Lower pressure
Regions of convergence of the winds
Often precipitation and clouds (regions of ascent)

25. Locating Fronts
Here, the winds are rapidly changing counterclockwise across this temperature gradient.
The winds are blowing warm air from the south.
This is a warm front.

26. Locating Fronts
In this case, the winds are also rapidly changing counterclockwise across this temperature gradient, indicating positive vorticity.
The winds are blowing cold air from the northwest.
This is a cold front.

27. Locating Fronts To find the cyclone:
Find the center of cyclonic circulation
To find the fronts:
Find large temperature gradients
Identify regions of wind shifts
Look for specific temperature advection (warm/cold)
Look for kinks in the isobars (regions of slightly lower pressure)

28. Locating Fronts To find the cyclone:
Find the center of cyclonic circulation
To find the fronts:
Find large temperature gradients
Identify regions of wind shifts
Look for specific temperature advection (warm/cold)
Look for kinks in the isobars (regions of slightly lower pressure)
Consider the reported phenomenon

29. What about Vertical Structure?

30. Pressure…
If we have converging air at the surface, must have divergence aloft!
Otherwise, air would “fill up” the low and the pressure would rise

31. Review
Winds converge at a surface low pressure center
Winds diverge from a surface high pressure center
(this is because of the frictional force at the surface)
This Convergence/Divergence suggests that there must be movement of air in the vertical (can’t lose air parcels)
Flow in the upper troposphere is generally in geostrophic balance, so we do not get divergence/convergence high up caused by friction
How do we get divergence/converge up high?

32. Upper level maps
Over upper level maps, the points with equal elevation above see level are connected with contours.
The height contours characterize the areas with higher (anticyclonic circulation) and lower (cyclonic circulation) height relative the neighboring points.

33. The line of constant temperature
Upper air charts
The contour lines are not straight.
They bend and turn, indicating ridges (elongated highs) where the air is warmer and indicating depressions, or troughs (elongated lows) where the air is colder.
Contours of 500-hPa level
Isotherm
The line of constant temperature

34. Upper Tropospheric Flow
Typical 500 hPa height pattern
Notice the troughs (dotted line) and ridges
The troughs and ridges are successive
In the northern hemisphere, lower pressure is generally to the north of higher pressure

35. Relative Vorticity
If the wind has counterclockwise spin, it has positive vorticity (left)
If the wind has clockwise spin, it has negative vorticity (right)
Vorticity can be directional (top), or speed shear vorticity (bottom)

36. Vorticity in the Upper Troposphere
Vorticity advection: where and what kind?
Pinpoint vorticity minima and maxima.
Negative vorticity advection (NVA) occurs just “downstream” from a ridge axis (vorticity minimum)
Positive vorticity advection (PVA) occurs just “downstream” from a trough axis (vorticity maximum) - +

37. Vorticity Advection and Vertical Motion
Positive vorticity advection (PVA) results in divergence at that level
* Negative vorticity advection (NVA) results in convergence at that level

38. Vorticity Advection and Vertical Motion
Remember that convergence at upper levels is associated with downward vertical motion (subsidence), and divergence at upper levels is associated with upward vertical motion (ascent).
Then, we can make the important argument that . . .

39. Upper Tropospheric Flow and Convergence/Divergence
Downstream of an upper tropospheric ridge, there is convergence, resulting in subsidence (downward motion).
Likewise, downstream of an upper tropospheric trough, there is divergence, resulting in ascent (upward motion).

40. Upper Tropospheric Flow and Convergence/Divergence
Downstream of an upper tropospheric ridge axis is a favored location for a surface high pressure.
Downstream of an upper tropospheric trough axis is a favored location for a surface low pressure center.

41. Upper Tropospheric Flow and Convergence/Divergence
Surface cyclones move in the direction of the upper tropospheric flow!
The storm speed and direction can also be identified on the 500 mb map. Cyclones move in the direction of the 500 mb flow, the 500 mb flow is also called the steering flow. The cyclone also moves at about half the speed of the 500 mb flow.
The surface low pressure center in diagram above will track to the northeast along the upper tropospheric jet (along the surface temperature gradient)

42. Vertical Structure of Cyclones
What else do these diagrams tell us?
Surface cyclone is downstream from the upper tropospheric (~500 hPa) trough axis
Mid-latitude cyclones generally tilt westward with height!

43. Vertical Structure of Cyclones
500 hPa positive vorticity advection causes divergence and ascent
This induces a surface cyclone
Cyclone formation occurs because of this upper-level divergence!

44. Longwaves and Shortwaves
The flow in the upper troposphere is characterized as having . . .
Longwaves: There are typically 4-6 of these around the planet. The longwave pattern can last for as long as 2-3 weeks on occasion, and can result in long periods of anomalous weather
Shortwaves: Embedded in the longwave pattern are smaller scale areas of high vorticity (lots of curvature). They move quickly east within the longwaves, and generally strengthen when they hit a longwave trough. Often, shortwaves result in huge “cyclogenesis” events such as nor-easters or midwest snowstorms.

45. Longwaves vs. Shortwaves
To the left is a North Pole projection of 300 mb heights (contoured) and wind speed (colors)
North Pole is at the center, equator is at the edges
Note the prominent longwave troughs and ridges---especially over North America
LONGWAVE TROUGH

46. Longwaves vs. Shortwaves
Notice two longwave troughs in this 500 mb height (contour) and vorticity (colored) map: One over the NW U.S., and one over eastern Canada.
Also, note a very subtle shortwave over Montana/Wyoming (you can see this in the vorticity field as a strip of anomalously large vorticity.
LONGWAVE TROUGH
SHORTWAVE

47. Vertical Structure of Cyclones
700mb
Downstream from troughs are favorable locations for ascent (red/orange)
Downstream from ridges are good locations for descent (purple/blue)

48. Cyclone Intensification/Weakening
How do we know if the surface cyclone will intensify or weaken?
If upper tropospheric divergence > surface convergence, the cyclone will intensify (the low pressure will become lower)
If surface convergence > upper tropospheric divergence, the cyclone will weaken, or “fill.”
Think of an intensifying cyclone as exporting mass, and a weakening cyclone as importing mass.

49. Pressure…
If we have converging air at the surface, must have divergence aloft!
Otherwise, air would “fill up” the low and the pressure would rise

50. Example of Cyclone Development Forced by Upper Flow
Red line: TROUGH AXIS
Example 300 mb flow which resulted in a massive cyclone development over the midwest.

51. Example of Cyclone Development Forced by Upper Flow
Surface cyclone (over NW Oklahoma) is positioned just downstream of the trough axis in the previous image.
Same time as the previous image.

52. Example of Cyclone Development Forced by Upper Flow
12 hours later, the jet speed maximum has shifted downstream with the trough, and there appear to be two trough axes.
The trough is “negatively tilted,” (NW-SE in orientation) often a sign of very strong PVA and forced ascent.
TROUGH AXIS

53. Example of Cyclone Development Forced by Upper Flow
Now, the surface cyclone has deepened to a very low 977 mb.
In general, it is still located downstream of the trough axis, but the trough axis appears to be catching up to the surface cyclone.

54. Example of Cyclone Development Forced by Upper Flow
12 hours later:
300 mb upper tropospheric low hasn’t moved too much
Upper low is situated over eastern Lake Superior.
TROUGH AXIS

55. Example of Cyclone Development Forced by Upper Flow
SFC at same time:
Surface cyclone is also over eastern Lake Superior!
This means that the surface cyclone is no longer in a favorable position for PVA (or upper divergence and ascent)
At this point, the surface cyclone will weaken!
Cyclone is “vertically stacked.”

56. The full picture

57. Midlatitude cyclones
Strong, “deep” interaction between surface and upper levels
May travel large distances around the globe
Midlatitude cyclone

58. High and low pressure systems
Occur on a variety of spatial and temporal scales
Some pressure systems may be stationary for a long period of time, others may migrate rapidly around the planet
Some pressure systems are closed, others are more belt-like and open

59. Low pressure systems
Types of low pressure systems: tornados, thunderstorms, hurricanes….

60. Low pressure systems
….., midlatitude cyclones, the ITCZ, thermal lows

61. Dust devil versus ITCZ

62. Lee Cyclogenesis

63. ATMS 316- Background Conservation of potential vorticity
conserved for adiabatic frictionless motion
Ratio of absolute vorticity and depth of vortex
Isentropic Potential Vorticity
(Holton 2004, p. 96)

64. ATMS 316- Background Conservation of potential vorticity
for a homogeneous incompressible fluid
z evaluated at constant height
Potential Vorticity
(Holton 2004, p. 96)

65. ATMS 316- Background Conservation of potential vorticity
When the depth of the vortex changes following motion, its absolute vorticity must change to maintain conservation of potential vorticity
(Holton 2004, p. 98)

66. ATMS 316- Background Conservation of potential vorticity
For westerly flow impinging on an infinitely long mountain range…
(a) upstream, zonal flow is uniform (du/dy = 0, v=0), z = 0
(b) deflection of upper q surface upstream of barrier  increases h  absolute vorticity must increase  air column turns cyclonically
(Holton 2004, p. 98)

67. ATMS 316- Background Conservation of potential vorticity
For westerly flow impinging on an infinitely long mountain range…
poleward drift in (b) also causes increase in f
(c) as column crosses mountain, h decreases  absolute vorticity must decrease  z becomes negative  air column drifts equatorward
(Holton 2004, p. 98)

68. ATMS 316- Background Conservation of potential vorticity
For westerly flow impinging on an infinitely long mountain range…
equatorward drift in (c) also causes decrease in f
(d) as column crosses mountain, h increases  absolute vorticity must increase  z becomes positive  air column drifts poleward

69. ATMS 316- Background Conservation of potential vorticity
For westerly flow impinging on an infinitely long mountain range…
(e) alternating series of ridges and troughs downstream of mountain range
cyclonic flow pattern immediately to the east of the mountains (lee side trough)

70. ATMS 316- Background
Lee cyclogenesis
Strong cross-mountain flow alone, however, is not a sufficient cause of cyclogenesis
Lee cyclones function in a similar manner to ordinary cyclones, deriving their kinetic energy from the APE of the baroclinic atmosphere
Lee cyclogenesis is favored where there is a strong cross-mountain flow in the jet stream
(a)
(b)
(c)
(d)
(e)
- Also must have low static stability in the lee of the mountain range

71. ATMS 316- Background Lee cyclogenesis
Preferred regions of cyclogenesis
Rocky Mountains
(Ahrens 2005, p. 222)

72. ATMS 316- Background Lee cyclogenesis
Preferred regions of cyclogenesis
Alps
Narrow mountain range
Theory that applies to Alps lee cyclogenesis is slightly different from that used to describe lee cyclogenesis of the Rockies
Ageostrophic effects dominate and the modification of baroclinic instability by the Alps is more difficult to analyze