1. Mid-Latitude Cyclones and Fronts
Lecture 12
AOS 101

2. Homework 4
COLDEST TEMPS
GEOSTROPHIC BALANCE

3. Homework 4 L
FASTEST WINDS

4. Rising Air Consider an air parcel rising through the atmosphere
The parcel expands as it rises
The expansion, or work done on the parcel causes the temperature to decrease
As the parcel rises, humidity increases and reaches 100%, leading to the formation of cloud droplets by condensation

5. Rising Air
If the cloud is sufficiently deep or long lived, precipitation develops.
The upward motions generating clouds and precipitation can be produced by:
Convection in unstable air
Convergence of air near a cloud base
Lifting of air by fronts
Lifting over elevated topography

6. Lifting by Convergence
Convergence exists when there is a horizontal net inflow into a region
When air converges along the surface, it is forced to rise

7. Background on Cyclones
A cyclone is: An area of low pressure around which the winds flow counter-clockwise in the northern hemisphere, and clockwise in the southern hemisphere
Hurricane (tropical cyclone)
Mid-latitude cyclone
Today, we’ll focus on mid-latitude, or extra-tropical cyclones, which have a life cycle and frontal structures. Hurricanes, which we discussed earlier, have no fronts.

8. Background on Cyclones
Midlatitude cyclones are crucial in maintaining a temperature equilibrium on our planet. This is because in the northern hemisphere . . .
. . . They advect warm air northward
. . . And they advect cold air southward
This helps maintain the radiative equilibrium on our planet!

9. Background on Cyclones
We already know that friction near the surface causes convergence into a low pressure center and that flow is counterclockwise around the low in the N. Hemisphere.
So we end up with the cold air moving south and east and the warm air moving north and west
Likewise, lifting by convergence forces parcels upward so we get clouds and precipitation in the vicinity of the low pressure L

10. Background on Cyclones
The figure to the right represents a typical midlatitude cyclone:
Cold, dry air is advected eastward behind the cold front
Warm, moist air is advected north behind the warm front
The fronts move in the direction the “teeth” point

11. Background on Cyclones
Definition - boundary, transition zone between two different air masses
The two air masses have different densities. Frequently, they are characterized by different temperatures and moisture contents
Front has horizontal and vertical extent
Frontal boundary/zone can be km wide
Types of synoptic-scale fronts:
warm fronts
cold fronts
stationary fronts
occluded fronts

12. Warm Front
A transition zone where a warm air mass replaces a cold air mass
Drawn as a red line with red semi-circles pointing in the direction of the front’s movement

13. Warm Front Again, warm air is less dense than cold air.
As the warm air moves north, it slides up the gently sloping warm front.
Because warm fronts have a less steep slope than cold fronts, the precipitation associated with warm fronts is more “stratiform” (less convective), but generally covers a greater area.

14. Common Characteristics Associated with Warm Fronts
Before Passing
While Passing
After Passing
Winds
South-southeast
Variable
South-southwest
Temperature
Cool-cold, slowly warming
Steady rise
Warmer, then steady
Pressure
Usually falling
Leveling off
Slight rise, followed by fall
Clouds
Cirrus, Cirrostratus, Nimbostratus
Stratus-type
Clearing with scattered Stratocumulus
Precipitation
Light to moderate rain, snow, sleet or drizzle
Drizzle or none
Usually none, sometimes light rain in showers
Visibility
Poor
Poor, but improving
Fair in haze
Dew Point
Steady
Rise, then steady

15. Cold Fronts
A transition zone where a cold air mass replaces a warm air mass
Drawn as a blue line with blue triangles pointing in the direction of the front’s movement

16. Cold Fronts Cold air is more dense than warm air.
As the dense, cold air moves into the warm air region, it forces the warm air to rapidly rise just ahead of the cold front.
This results in deeper clouds and precipitation than we saw with a warm front. The clouds that form can be convective and can be associated with more intense precipitation and thunderstorms
Often, the precipitation along a cold front is a very narrow line of thunderstorms

17. Common Characteristics Associated with Cold Fronts
Before Passing
While Passing
After Passing
Winds
South-southwest
Gusty; shifting
West-northwest
Temperature
Warm
Sudden drop
Steadily dropping
Pressure
Falling steadily
Minimum, then sharp rise
Rising steadily
Clouds
Increasing: Cirrus, Cirrostratus, Cumulonimbus
Cumulonimbus
Cumulus
Precipitation
Short periods of showers
Heavy rains, sometimes with hail, thunder, lightning
Showers, then clearing
Visibility
Fair to poor in haze
Poor, followed by improving
Good, except in showers
Dew Point
High; remains steady
Sharp drop
lowering

18. Occluded Fronts
A region where a faster moving cold front has caught up to a slower moving warm front.
Generally occurs near the end of the life of a cyclone
Drawn with a purple line with alternating semicircles and triangles

19. Cold Occlusion The type most associated with mid-latitude cyclones
Cold front "lifts" the warm front up and over the very cold air
Associated weather is similar to a warm front as the occluded front approaches
Once the front has passed, the associated weather is similar to a cold front
Vertical structure is often difficult to observe

20. Warm Occlusion
Cold air behind cold front is not dense enough to lift cold air ahead of warm front
Cold front rides up and over the warm front
Upper-level cold front reached station before surface warm occlusion

21. Stationary Front Front is stalled
No movement of the temperature gradient
But, there is still convergence of winds, and forcing for ascent (and often precipitation) in the vicinity of a stationary front.
Drawn as alternating segments of red semicircles and blue triangles, pointing in opposite directions

22. How to Locate a Cyclone Find the region of lowest sea level pressure
Find the center of the cyclonic (counter-clockwise) circulation

23. How to Locate a Cyclone L Find the region of lowest sea level pressure
Find the center of the cyclonic (counter-clockwise) circulation L

24. How to Locate a Front
We know that we need to look for low pressure and a boundary of cold and warm air.
To pinpoint the parts of our cyclone, look for specifics in the observation maps
Find the center of cyclonic rotation
Find the large temperature gradients
Identify regions of wind shifts
Identify the type of temperature advection
Look for kinks in the isobars

27. Polar Front Theory - Development and Evolution of a Wave Cyclone Also, referred to as Norwegian Cyclone Model (NCM)
The wave cyclone (often called a frontal wave) develops along the polar front
When a large temperature gradient exists across the polar front - the atmosphere contains a large amount of Available Potential Energy

28. NCM cont.
Northward moving warm air and southward moving cold air are forced around each other, forming a bend in the temperature gradient (b). This forms the warm front and the cold front. Now with a counter-clockwise spin, winds converge at the newly formed low pressure minimum at the center of rotation.

29. NCM cont.
(c)- A fully-developed cyclone is seen hours from its inception. It consists of:
a warm front moving to the northeast
a cold front moving to the southeast
region between warm and cold fronts is the "warm sector"
central low pressure (low, which is deepening with time)
wide-spread precip. ahead of the warm front
narrow band of precip. along the cold front
Wind speeds continue to get stronger as the low deepens
The production of clouds and precip. also generates energy for the storm as Latent Heat is released

30. NCM cont.
(d) - As the cold front moves swiftly eastward, the systems starts to occlude.
Storm is most intense at this stage
have an occluded front trailing out from the surface low
triple point/occlusion - is where the cold, warm, and occluded fronts all intersect

31. Final Stage
(e) - the warm sector diminishes in size as the systems further occludes.
The storm has used most all of its energy and dissipates
cloud/precip production has diminished
The warm sector air has been lifted upward
The cold air is at the surface - stable situation.
The temperature contrast which drove this whole situation from the surface perspective is no longer near the center of the wave of low pressure

32. Another view

33. Weather associated with a typical late fall to early spring mid-latitude cyclone
Figure courtesy of Jon Martin

34. Precipitation Around a Cyclone and its Fronts
To the right is a major cyclone that affected the central U.S. on November 10, 1998.
Around the cold front, the precipitation is more intense, but there is less areal coverage.
North of the warm front, the precipitation distribution is more “stratiform”: Widespread and less intense.

35. Precipitation Around a Cyclone and its Fronts
Again, in this radar and surface pressure distribution from December 1, 2006, the precipitation along the cold front is much more compact and stronger.
North of the warm front, the precipitation is much more stratiform.
Also note the kink in the isobars along the cold front!

36. We have all seen cyclones on weather maps, but how do we know if it will strengthen or weaken? The key to cyclone development is in the upper level flow

37. So let’s look at Upper Levels…
Typical 500 mb height pattern
Similarly to lower levels, at upper levels of the atmosphere, there is often a series of high pressures (high heights) and low pressures (low heights)

38. Upper Levels
Ridge Trough Ridge

39. Why Do These Patterns Occur?
The patterns of convergence and divergence have to do with vorticity advection
If there is positive vorticity advection, divergence occurs
If there is negative vorticity advection, convergence occurs
Let’s explain vorticity …

40. Vorticity
Vorticity is simply a measure of how much the air rotates on a horizontal surface
Positive vorticity is a counterclockwise (i.e. cyclonic) rotation
Negative vorticity is a clockwise (i.e. anticyclonic) rotation
Therefore, troughs contain positive vorticity, and ridges contain negative vorticity
Trough
Ridge

41. Let’s Revisit …
Vorticity < 0
Vorticity < 0
Vorticity > 0

42. Diagnosing Vorticity Advection
To determine vorticity advection, first find the locations of maximum (positive) vorticity and minimum (negative) vorticity
Then, determine what direction the wind is moving
Areas of negative vorticity advection (NVA) will be just downstream of vorticity minima, and areas of positive vorticity advection (PVA) will be just downstream of vorticity maxima

43. Negative vorticity advection Positive vorticity advection

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

45. 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 . . .

46. 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).

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

48. Upper Tropospheric Flow and Convergence/Divergence
Surface cyclones also move in the direction of the upper tropospheric flow!
The surface low pressure center in the diagram above will track to the northeast along the upper level flow