1. Geog 166: weather systems
Prof. Leila M. V. Carvalho

2. objectives
The weather analysis will be focused first on the formation and evolution of extratropical cyclones
We will discuss a case-study that caused strong winds and heavy precipitation to parts of the central United States
It was unusually intense but typifies many features of winter storms in middle and high latitudes

3. Plotting conventions synoptic charts

4. Extratropical cyclones: case study example: November 1998
500hPa chart for 00UTC
Polar Vortex is split into two regional cyclonic features (Russia and Northern Canada)
Two ridges separate the two troughs
Typical distance between two troughs : 50o of longitude or ~ 4000km
Solid red lines denote axes of ridges and dashed red lines are axes of troughs

5. 500hPa charts (contours 60m intervals (dKm) and relative vorticity (blue shading 10-4s-1). Right sea level pressure 4hPa interval and hPa thickness (colors)
Baroclinic waves travel ~ 10m/s: wintertime climatological wind at 700Pa (steering level)
Successive ridges (or troughs typically ass at a fixed point on Earth at roughly 4 days (but it can be sometimes 1 or 2 days apart depending on the steering flow)
Baroclinic waves tend to be stronger over the oceans (as we have seen many times) but they can develop over land
Frontal surfaces

6. Amplification of the wave
Note the intensification of the Low pressure system and how it “splits off the westerly to form a CUTOFF LOW (ISOLATED MINIMUM OF GEOPOTENTIAL HEIGHT FIELD)
That implies the existence of a closed cyclonic circulation
Note the tightening of the contours that imply the intensification of the winds around the low
Note the intensification of the low pressure at the surface

7. Advection of temperature, amplification of the wave and the extratropical cyclone
The surface low is the EXTRATROPICAL CYCLONE
Note that pressure gradients increase with time showing the intensification of the low as the cold air moves southward and the warm air moves northward
In the beginning the surface Low is eastward of the trough but as the 500hPa and low level cyclone amplify they come into vertical alignment

8. Vorticity analysis
indicates cyclonic vorticity related to the horizontal wind shear (u varies rapidly with y and v varies rapidly with x)
these maxima move eastward 9 hours later
Surface Low pressure system drops from 998 hPa at 00UTC to 978 hPa at 18UTC (bottom) to 968 at 00UTC (not shown) Nov 11=> 30hPa in 24h!!! 3x more rapid than the typical cyclone

9. 500 hPa (thick black), 1000hPa height (thin black), hPa thickness (dashed) Arrows: sense of the geostrophic wind
Amplification of the wave in the thickness field: advection of temperature by the wind
Note that the strengthening of the east-west gradient of temperature leads to the weakening of the North-south temperature gradient as the wave is growing
Surface low aligned with the 500hPa trough: weakening of the horizontal temperature advection
Bottom: from text book: Atmospheric Circulation Systems: Palmen and Newton (p. 326)

10. In summary
The geostrophic wind field evolves from a highly baroclinic pattern (high gradients and strong vertical wind shear) into a more equivalent varotropic pattern (less directional shear of the lower tropospheric geostrophic wind field in fully developed baroclinic waves
Strong winds but weaker temperature gradients marks the end of the intensificaiton phase in the life cycle of the cyclone

11. Vertical velocity and 500hPa charts
Rising of the northward air in the region of warm advection
sinking air in the region of southward cold advection to the rear of the cyclone
Rising of warm air and sinking of cold air is indicative of a conversion of potential energy into kinetic energy
In baroclinic waves the potential energy is associated with the east-west temp gradients and the kinetic energy with the meridional wind component
Rapid Intensification

12. Note that the juxtaposition of this inward-spiraling rising and subsiding air (“yin-yang” pattern) influences the pattern of rain and clouds

13. Frontal Zones
Narrow bands marked by sharp horizontal gradients and sometimes outright discontinuities in wind and temperature
Frontogenesis: development of frontal zones (initiated by large-scale horizontal deformation fields
Mesoscale circulations in the plane perpendicular to the fronts are instrumental in increasing temperature contrasts and organizing the distribution of precipitation into bands parallel to the fronts

14. Winds and pressure: cold front Below: sea level pressure, and surface winds
Pronounced wind shift, winds exhibit strong westerly component. To the east southerly wind component are dominant. Isobars bend sharply along the front
As a cold front pass in the NH there will be a shift in an anticyclonic sense (veering) from southerly to westerly

15. Winds and pressure: warm front Below: sea level pressure, and surface winds
Is a more subtle feature
Shows signal of advection around the surface low
As a warm front pass in the NH there will be a shift in an anticyclonic sense (veering) from southeasterly to southerly
Late stage there will be a junction of the two fronts: occluded front extends from the center of the surface low to a triple point when it meets the warm and cold front
When an occluded front pass a station the surface wind veers from southeasterly to southwesterly

16. Cause also winds to veer as it pass over a station
Secondary cold front
A front which forms behind a frontal cyclone and within a cold air mass, characterized by an appreciable horizontal temperature gradient
Cause also winds to veer as it pass over a station

17. Temperature structure
The zone of relatively uniform temperature to the southeast of the surface low is referred to as the warm sector of a cyclone
The cold front marks the leading edge of the advancing colder air from the west

18. Close-up in the temperature and winds
To the east of the cold front the temperatures are relatively homogeneous
Proceeding westward from the front, temperatures drop by 10oC within the first few hundred of km
A cold front can be defined as the boundary of a frontal zone that is advancing in the direction of the warmer air.
A cold front at a station marks the beginning of a period of falling temperatures due to the wind shift
Note that both fronts are embedded within troughs of low passage and their passage is marked by wind shifts and further decrease in temperature.
The passage of a warm front is associated with a local increase in temperature

19. Stationary fronts
Fronts that exhibit little movement in either direction and are indicated on synoptic charts as dashed lines with alternating read and blue line segments
When an occluded front approach a station, surface air temperature rises, and after the front passes the station, the temperature drops (for a stationary observed it is like he/she was experiencing a back to back warm and cold fronts)

20. Occluded Front:
Cold air moves faster than the warm air

21. Frontal systems have a tridimensional feature

22. What is the relationships Among Frontal Boundaries, Troughs and Ridges, Vorticity and Low and High Pressure Systems at the surface?

23. Northern Hemisphere
Positive vorticity
Negative vorticity
Ascending Air
Descending air
Cold
Warm
Low pressure systems are observed east of the upper level Troughs
High pressure systems are observed east of the upper level Ridges
Cold and warm fronts are related to low pressure systems at the surface. Frontal boundaries extend upward as we will see in the next slides

24. Vertical Cross Section:
Div
Cold air:
denser than warm air
Conv.
As a cold Front moves toward a warmer region it lifts warmer air ahead of the front
If the atmosphere is unstable or conditionally unstable cumulonimbus clouds and heavy precipitation may occur

25. Vertical Cross Section:
Div
Cold
Conv.
As a warm Front moves toward a colder region warmer air is lifted and shallow clouds are formed

26. Difficulties to locate fronts on the basis of gradients of surface air temperatures
Over Oceans: air temperature is strongly influenced by sea surface temperature
In mountainous terrain, large differences in station elevations mask the temperature gradients
Terrain effects, patchy nocturnal inversions, convective storms, urban heat islands effects can raise or lower the temperature and create gradients that can confuse the location of frontal systems

27. Moisture
Frontal zones tend to be marked by strong gradients in dew point and equivalent potential temperature, specially when the cold air is of continental origin and the warmer air is of marine origin (e.g. eastern USA).
During spring and summer, the moisture gradient is often a more reliable indicator of frontal positions
During summer, under conditions of southerly low level flow, there often exists a sharp contrast between humid air advected northward from the Gulf of Mexico and much drier air that has subsided along the eastern slopes of the Rockies: the boundary between these marine and continental air masses is referred to as Dry Line

28. Example of Dry line observed in a Td map: http://www. tornadochaser
Tornados can form along a dry line, in the presence of other triggering such as strong high level jets

29. Vertical Cross Section:
Div
Dry air is:
denser than
moist air
Conv.
As dry air moves toward a region with moist air it lifts humid air ahead and may form intense storms
If the atmosphere is unstable or conditionally unstable cumulonimbus clouds and heavy precipitation may occur, including tornados

30. Hourly observations: cold front
Secondary
Front
Cold
Front
Gage,Oklahoma
rain

31. Hourly observations: warm front
Bowling Green, Kentucky

32. Hourly observations: occluded front
975hPa!
veering
rain
snow
Marquette, Michigan

33. Surface weather
rain
Snow
00:00UTC
09:00UTC
18:00UTC

34. Snow
rain
00:00UTC
09:00UTC
18:00UTC

35. Snow
rain
WV 18:00UTC
WV 00:00UTC
WV 09:00UTC

36. Precipitation as seen by radar
0620UTC
Composite: 15:35 UTC

37. Some additional ideas about the evolution of frontal systems, cyclones and baroclinic waves:

38. Surface high- and low-pressure centers can migrate
relative to the Rossby wave aloft.
In (a), the surface low exists below the area of upper-level divergence, and the resultant uplift maintains or strengthens the cyclone.

39. Surface systems are generally guided by upper-level winds,
so eventually the center of the low might be displaced to the
zone where upper-level convergence occurs (b).
The sinking air fills into the low, causing its demise.
This process occurs as the cyclone undergoes the
transition from its mature to occluding stages.