1. Thundersnow Apr 1, 2010

2. Winds, Fronts, and Cyclogenesis
ATS 351
Lecture 10

3. Outline Atmospheric pressure Forces that affect the wind
PGF, Coriolis, Centripetal, Friction
Vertical Motion
Fronts
Mid-Latitude Cyclogenesis

4. Atmospheric Pressure Ideal Gas Law p = RT
p: pressure; : density; R: constant (287 J/kg/K); T: temperature
It takes a shorter column of dense, cold air to exert the same pressure as a taller column of less dense, warm air
Warm air aloft is normally associated with high atmospheric pressure and cold air aloft with low atmospheric pressure
At a given level, more molecules exist above warm air than cold air = higher pressure
Hold Pressure constant: as temperature increases, the gas will become LESS dense
Hold temperature constant: linear relationship so as pressure increases, so does density
air at a higher pressure is more dense than air at a lower pressure
Air above a region of surface HIGH is more dense than air above a surface LOW

5. Surface and Upper-Level Charts
Sea-level pressure chart: constant height
Upper level or isobaric chart: constant pressure surface (i.e. 500mb)‏
High heights correspond to higher than normal pressures at any given latitude and vice versa

6. Cold air aloft: low heights or low pressure
Warm air aloft: high heights or high pressures
Ridge: where isobars bulge northward
Trough: where isobars bulge southward

7. In Northern Hemisphere:
Notice wind direction around L and H on surface map
Point out location of trough and ridge on 500mb map
Point out that surface map is ISOBARS (pressure lines) whereas the upper-level 500mb map is HEIGHTS at 500mb (average value of 500mb is 5600)‏
Point out how winds are flowing on the upper-level chart = blow parallel to contour lines (more on this later), whereas surface winds cross isobars (more on this later too)‏
In Northern Hemisphere:
High pressure: anticyclone (winds blow clockwise and outward from center)‏
Low pressure: mid-latitude cyclone (winds blow counter clockwise and inward towards center)‏

8. Newton’s 2nd Law of Motion
F = ma
F: net force; m: mass of object; a: acceleration
At a constant mass, the force acting on the object is directly related to the acceleration that is produced.
The object accelerates in the direction of the net force acting on it
Therefore, to identify which way the wind will blow, we must identify all the forces that affect the movement of air
Object accelerates in the direction of the NET FORCE acting on it. This is important when we begin to talk about winds and which direction they are going to blow

9. Forces that Affect Wind
Pressure gradient force (PGF)‏
Coriolis force
Centripetal force
Friction

10. Pressure Gradient Force
Pressure gradient = ∆p/d
∆p: difference in pressure
d: distance
PGF has direction & magnitude
Direction: directed from high to low pressure at right angles to isobars
Magnitude: directly related to pressure gradient
Tight lines (strong PGF)  stronger wind
PGF is the force that causes the wind to blow
MAYBE DO WATER LEVEL EXPERIMENT: Shows that the greater the pressure difference, the stronger the force and the faster the water moves = same happens with air

11. Point out direction and magnitude of PGF vectors
Basic run-down of how to calculate PGF. Take one pressure (1024) minus the second (1016) and divide it by the distance using the scale

13. Coriolis Force Apparent deflection due to rotation of the Earth
Right in northern hemisphere and left in southern hemisphere
Stronger wind = greater deflection
No Coriolis effect at the equator greatest at poles.
Only influence direction, not speed
Only has significant impact over long distances
Coriolis = 2vΩsin
M = mass ; Omega = earth’s angular spin rate; V = velocity of object; phi = latitude

14. Geostrophic Winds When the force of PGF and Coriolis are balanced
Travel parallel to isobars at a constant speed
An approximation since isobars are rarely straight in real atmosphere but close enough to understand winds aloft
Spacing of isobars indicates speed
Close = fast, spread out = slow
The wind starts to blow at point 1 and the Coriolis begins to deflect it right (in the NH) while PGF stays the same. When the strength of the Coriolis eventually equals the PGF = GEOSTROPHIC WINDS that blow parallel to the isobars.
Point out how the vectors of PGF and Coriolis are the same at step 5

15. Gradient Winds & Centripetal Force
Gradient wind parallel to curved isobars above the level of frictional influence (winds aloft)‏
An object accelerates when it is changing speed and/or direction.
Therefore, gradient wind blowing around a low pressure center is constantly accelerating
Centripetal acceleration: directed at right angles to the wind, inward toward center of low
Centripetal force: inward-directed force
Results from an imbalance between the Coriolis force and the PGF
Since gradient wind is constantly acceleration - centripetal acceleration is produced

16. Cyclonic flow: PGF > CF Anticyclonic flow: PGF < CF
The NET force associated with the low is greater towards the center (PGF) due to the inward centripetal force.
Why is the PGF going in towards the center of the low?? Because PGF is always going from H to L pressure and therefore is directed TOWARDS the L.
Therefore, the PGF in anticyclonic flow is outward because it is going from H to L. The Coriolis is pointed inwards because it is to the RIGHT of the flow which is clockwise.
Cyclonic flow: PGF > CF
Anticyclonic flow: PGF < CF

17. Zonal & Meridional Winds
Zonal winds: oriented in the W-E direction (parallel to latitude)‏
Moves clouds, storms, surface anticyclones rapidly from west to east
Meridional winds: oriented in a N-S trajectory
Surface storms move slowly and often intensify major storm systems

18. Surface and Upper-Level Winds
Winds on Upper-level Charts
Winds parallel to contour lines and flow west to east
Heights decrease from north to south
Surface Winds
Winds normally cross isobars and blow more slowly than winds aloft
Friction: reduces the wind speed which in turn decreases the Coriolis effect
Friction layer: surface to about 1000m (3300ft)‏
Winds cross the isobars at about 30° into low pressure and out of high pressure
The reason winds are slower and cross isobars at the surface is FRICTION

19. PGF at surface is balanced by the sum of friction and Coriolis force
Surface winds into low and outward from high
The effect of surface friction is to slow down the wind so that, near the ground, the wind crosses the isobars and blows toward lower pressure.
Less Coriolis means wind is not deflected as much to right……so therefore wind is going to slightly turned at an angle - ALPHA. And cross isobars. It will also slow down.
This phenomenon at the surface produces an inflow of air around a low and an outflow of air around a high.
Aloft, away from the influence of friction, the winds blow parallel to the lines, usually in a wavy west-to-east pattern.

20. Winds & Vertical Motions
Since surface winds blow into the center of a low, they are converging and that air has to go somewhere  slowly rises
Vice versa for winds blowing outward from H
Therefore, a surface low has convergence at surface and divergence aloft and a surface high has the opposite.

21. Winds and air motions associated with surface
highs and lows in the Northern Hemisphere.

22. Hydrostatic Balance There is always a strong PGF directed upward
Gravity balances the upward PGF
When they are equal, hydrostatic equilibrium exists
Good approximation for atmosphere with slow vertical movements
Is not valid for violent thunderstorms and tornadoes.
Why is there a strong PGF directed upward??? Because there is higher pressure at the surface than aloft - so therefore wind will want to move from surface upwards.

23. Air Masses of North America
Continental Polar (cP) & Arctic (cA)‏
Cold, dry, stable air in winter
In summer, cP air mass usually brings relief from oppressive heat in central and eastern US
Maritime Polar (mP)‏
In winter, cP/cA air mass is carried over Pacific Ocean where moisture and warmth is added
mP at Pacific Coast is cool, moist, and conditionally unstable
East of Rockies - brings fair weather and cooler temperatures (moisture has been removed by mountains)‏
East coast mP air mass: originates in N. Atlantic
Storms may develop (heavy rain or snow, coastal flooding)‏
Late winter, early spring

24. Air Masses and Fronts
A front is a transition zone between two air masses of different densities
Fronts extend both horizontally and vertically

25. Cold Front
Cold, dry stable polar air (cP) is replacing warm, moist, conditionally unstable subtropical air (mT)
Steep vertical boundary due to surface friction slowing down the surface front
Has strong vertical ascent along the surface front
Strong upper level westerlies push ice crystals far ahead of the front, creating Ci and Cs in advance of the front.
Cold, dense air wedges under warm air, forcing the warm air upward, producing cumuliform clouds
Can cause strong convection, severe weather, and squall lines.
Air cools quickly behind the front

26. Cold Front
Rising motion causes decreased surface pressure ahead of the front
On a surface pressure map, frontal location can be seen by “kinks” in the isobars, changes in wind direction from a southwesterly to a northwesterly wind, and decreases in temperature.
Pressure is lowest at the surface front.
On weather maps, cold fronts are indicated by blue lines with triangles pointing in the direction of frontal motion (towards warmer air)‏

27. Cold Front Before While After Winds S-SW Gusty, shifting W-NW
Temperature
Warm
Sudden drop
Steady drop
Pressure
Steady fall
Min, then sharp rise
Steady rise
Clouds
Increasing, Ci, Cs, Cb Cb Cu
Precipitation
Brief showers
Heavy rains, severe weather
Showers, then clearing
Visibility
Fair to poor
Poor
Good
Dew Point
High, remains steady
Sharp drop
lowering

28. Warm Front
Occurs at the leading edge of an advancing warm, moist, subtropical air mass (mT) from the Gulf replacing a retreating cold, maritime, polar air mass from the North Atlantic (mP)‏
Slowly advances as cold air recedes; moves at about half the speed of an average cold front
Speed may increase due to daytime mixing
Speed may decrease due to nighttime radiational cooling
Smaller vertical slope than cold front

29. Warm Front
Warmer, less-dense air rides up and over the colder, more- dense surface air
“Overrunning”
Produces clouds and precipitation well in advance of the front

30. Warm Front Before While After Winds S-SE Variable S-SW Temperature
Cool-cold
Slowly warming
Steady rise
Warmer, then steady
Pressure
Falling
Leveling off
Slight rise, followed by fall
Clouds
(in order) Ci, Cs, As, Ns, St, fog (Cb in summer)‏
Stratus-type
Clearing with scattered Sc
Precipitation
Light-to- moderate
Drizzle or none
Usually none
Visibility
Poor
Improving
Fair
Dew Point
Steady
Rise, then steady

31. Stationary Front Essentially no movement
Surface winds blow parallel to front, but in opposite directions on either side of it
Separates two air masses
Seen often along mountain ranges when cold air cannot make it over the ridge

32. Hourly surface observations at Gage, Oklahoma showing the passage of a primary and secondary cold front (left) and at Bowling Green, Kentucky showing the passage of a warm front (right).
Source: Wallace and Hobbs, 2006.

33. Occluded Fronts Cold fronts generally move faster than warm fronts
Occlusion occurs when cold front catches up to and overtakes a warm front
Occlusions can be warm or cold

34. Dry Lines Think of a dry line as a moisture boundary
Separates warm, humid mT air in the southern Great Plains from warm, dry cT air
Drier air behind dry lines lifts the moist air ahead of it, triggering storms along and ahead of it
Induces lifting along front
Often produces severe thunderstorms in OK & TX
Unique to southern great plains of US because of the Rocky mountains and the Gulf of Mexico

35. A guide to the symbols for weather fronts that may be found on a weather map:
#1 cold front
#2 warm front
#3 stationary front
#4 occluded front
#5 surface trough
#6 squall/shear line
#7 dry line
#8 tropical wave

38. Features of a Mid-latitude Cyclone
Deep low pressure area with attached cold and warm fronts
Often an occlusion forms, the triple point lending to the formation of severe weather
Precipitation associated with the cold and warm fronts organizes in typical “comma cloud” structure

39. Stages in Wave Cyclone Development

40. Polar Front Theory
Initially, there is a stationary front that acts as the boundary separating cold, continental polar air from warm, maritime tropical air
Winds blow parallel to this front on either side
Polar Fronts are discontinuous

41. Cyclogenesis A wave forms on the front due to a shortwave disturbance
Central Pressure
A wave forms on the front due to a shortwave disturbance
Frontal Wave
Incipient Cyclone
The front develops a "kink" where the wave is developing
Precipitation will begin to develop along the front
Overrunning and lifting

42. Strengthening
The cyclonic circulation around the low becomes more defined
The central pressure intensifies
The cold front and warm front have more organized motion
Cyclone usually pushed east or northeast by the winds aloft

43. Mature Cyclone
The cold front catches up with the warm front and an occlusion forms
The cyclone is at its strongest at this point
Severe weather often develops near the “triple point”

44. Dissipation The occlusion grows with time
Eventually, the occlusion is so great that the supply of warm, moist air into the low is cut off
Cold air on both sides
When this happens, the system starts to dissipate