Baroclinic Instability: Key to Cyclone Development

1. Kinetic Energy Instability Baroclinic instability (needs vertical shear, i.e., horizontal T gradient) 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. 1002 1006 998 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. cold air mass direction of propagation 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. C C L W W Stationary front 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 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. L 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

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

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

15. Latent Heat & Sensible 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 Divergence + diffluence 500 mb surface

17. Bombs

18. Ice Storms By Richard Grotjahn

19. Cold-Air Outbreaks By Richard Grotjahn

20. Cold-Air Outbreaks By Richard Grotjahn