1. Instability Baroclinic instability (needs vertical shear,
i.e., horizontal T gradient)
Kinetic Energy
through the lifting of relatively warm, light air and the sinking of relatively cold, dense air
Barotropic instability (needs horizontal shear)
What kind of instability is required for mid-latitude
cyclone to develop?
Baroclinic instability!

2. L Cyclone Cyclone (mid-latitude cyclone or extratropical cyclone):
A region of low pressure, which has a scale of ~ 1000 km and is surrounded by strong wind rotating in the counterclockwise direction in NH and clockwise in SH.
1006
cold air mass
1002
direction of
propagation
998
accompanying with cold front and warm front (and maybe with occluded front as well) L
cold front
warm air mass
warm front
Low pressure center at surface is around 1000 mb
Life cycle is about several days and moves following steering flow.

3. Cyclogenesis
Cyclogenesis – A fall in pressure of greater than a certain magnitude
One potential formation of a cyclogenesis - Lee cyclogenesis
Another potential formation of a cyclogenesis C
e.g., continental polar air mass W
e.g., maritime warm tropical air mass L W C
Stationary front W C
Cyclogenesis

4. Cyclone evolution
Norwegian model (developed during and shortly after World War I)
By T. Bergeron and H. Solberg, based on surface observations

5. Cyclone and Comma-shaped Cloud
Comma cloud
(mature stage)

6. Mature Cyclone
By Richard Grotjahn

7. Cyclogenesis
preferred locations of cyclone formation in US (i) east of the Rockies (ii) off the east coast of N America (iii) Gulf of Mexico (iv) mid-west initiated in continental polar mass from Canada
(Lee cyclogenesis)

8. Symmetrical thermal structure
low

9. Asymmetrical thermal structure
Cyclones slant back into colder air masses and strengthen with height, sometimes exceeding 9 km in depth

10. Cyclone Development Favorable conditions for cyclone to develop:
Low level warm temperature advection (WTA)
Low level moisture advection (often come with WTA)
Positive vorticity advection at 500 mb or upper level diffluent flow (so downstream of a diffluent trough is really good for surface cyclone to develop!)
Note: The wave should not be too long (beta effect is too strong) or too short. The static stability should not be too large (suppress w)
4. The right side of the entrance or the left side of the exit of the 200/300 mb Jet streak

11. Cyclone Development

12. Cyclone Development
Most cyclones deepen underneath downstream of a diffluent trough
The rest deepen underneath downstream of a regular trough
Very few deepen underneath downstream of a confluent trough
- This may have other mechanisms at different level to help.
Enhance
divergence
Divergence +
diffluence
500 mb
surface L

13. Cyclone Development 1002.7 mb Surface map 300 mb map Divergence +
diffluence X X
Surface map
300 mb map

14. Cyclone Development
24h forecast
1000 mb
48h forecast
999 mb

15. Latent Heat & Sensible Heat
temperature
gradient in vertical
Latent heat :
moisture gradient
in vertical
Height tendency

16. L Bomb Bomb – Explosive cyclogenesis
(slightly smaller in size than regular cyclones)
Average pressure deepening rate : 1 mb/hr for at least 24 h!
Most bombs occur over the ocean during the
season, downstream from & near the strongest
cold
diffluent upper level troughs
sea surface temperature gradient L
500 mb
surface
Divergence +
diffluence

17. Bombs

18. Ice Storms
By Richard Grotjahn

19. Cold-Air Outbreaks
By Richard Grotjahn

20. Cold-Air Outbreaks
By Richard Grotjahn