1. Niels Woetmann Nielsen Danish Meteorological Institute
Application of quasigeostrophic arguments in interpretation of extratropical cyclogenesis
Niels Woetmann Nielsen
Danish Meteorological Institute

2. ● Extratropical cyclones in a global perspective
● Energy conversions
● Simplified quasigeostrophic equations
● Typical flow patterns in extratropical cyclogenesis
● Example
● Exercise

3. Why bother about extratropical cyclones and anticyclones?
because they dominate the weather at middle and high latitudes.

4. Why use QG-theory to explain their development?
because it is the simplest way to explain how they develop
because QG-theory, when combined with a few carefully selected NWP fields, can explain the dynamics behind the major cloud patterns seen in satellite images.

5. Extratropical cyclones and anticyclones develop as result of what is called baroclinic instability.
Baroclinic instability can not occur everywhere on the globe.
In a global perspective baroclinicity is generated where the northward relatively warm and moist branch of the Ferrell circulation meets the southward relatively cold and dry branch of the polar circulation
– and baroclinic instability can only occur in this zone.

6. Global meridional circulation and surface winds

7. During extratropical cyclogenesis mean available potential energy is converted to eddy available potential energy on the cyclone scale and part of the eddy available potential energy is converted to eddy kinetic energy in the developing extra-tropical cyclone.

8. Conversion from mean available potential energy to eddy available potential energy occurs in a process where at the same latitude and height cold air is advected southward and warm air is advected northward.
Conversion from eddy available potential energy to eddy kinetic energy occurs in a process where at the same latitude and height cold air sinks and warm air rises.

9. Energy cycle for Northern Hemisphere winter

10. “Kinematical” south-north eddy transport of momentum
Upper: trough and ridge axes with positive tilt
Middle: trough and ridge axes without tilt
Bottom: trough and ridge axes with negative tilt

11. A developing extratropical cyclone have sinking cold air penetrating into a warmer air mass and rising warm air penetrating into a colder air mass.
How can this be explained by QG-theory?

13. Typical flow structures in extratropical cyclogenesis

17. Baroclinic instability
Upper-tropospheric wave amplification by differential temperature advection (cold (warm) advection below upper trough (ridge) – this process converts mean available potential energy (APE) into eddy APE.
Upper-tropospheric wave amplification intensifies the secondary circulation (ageostrophic winds and vertical velocity) in such a way that ascending (descending) motion intensifies in warm (cold) air downstream of the upper-level trough (ridge)
- this process converts eddy APE into eddy kinetic energy (KE)

18. A positive feed-back occurs since the intensified low level circulation (increase in eddy KE) leads to an increase in the conversion from mean to eddy APE, the latter promoting increased conversion from eddy APE to eddy KE and so on.
The feedback occurs through the secondary circulation (vertical velocity and ageostrophic wind).
Relative vorticity advection is the most important process in the upper troposphere.
In the lower troposphere the most important process is temperature advection and diabatic heating.

19. Baroclinic instability can not occur if the wave length is too short and static stability too high.
Under these conditions the secondary circulation (SC) induced by upper-tropospheric processes (relative vorticity advection) does not reach the lower troposphere and the SC induced by low-level temperature advection does not reach the upper troposphere. No interaction between the processes can take place.

20. “Small-scale” extratropical cyclogenesis case
18 November 2009

21. “Small-scale cyclogenesis 18 November 2009
DMI-HIRLAM analyses of mslp and 300 hPa pot. temp. (K)
Left: 00UTC and right: 12UTCr

22. “Small-scale” cyclogenesis example (1)
Incipient stage of cyclogenesis 00UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and 300 hPa wind

23. “Small-scale” cyclogenesis example (1)
Incipient stage of cyclogenesis 00UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and rel. vorticity and wind at 300 hPa

24. “Small-scale” cyclogenesis example (2)
Deepening phase of cyclone 06UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and 300 hPa wind

25. “Small-scale” cyclogenesis example (2)
Deepening phase of cyclone 06UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and rel. vorticity and wind at 300 hPa

26. “Small-scale” cyclogenesis example (3)
Mature stage of cyclone 12UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and 300 hPa wind

27. “Small-scale” cyclogenesis example (3)
Mature stage of cyclone 12UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and rel. vorticity and wind at 300 hPa

28. “Small-scale” cyclogenesis example (4)
Filling stage 18UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and 300 hPa wind

29. “Small-scale” cyclogenesis example (4)
Filling stage 18UTC 18 November 2009
Left: MSG channel 5; Right: DMI-HIRLAM analysis of mslp and rel. vorticity and wind at 300 hPa

30. Exercise
Suppose that a forecaster has only access to nwp-analyses of mean sea level pressure (mslp), wind and relative vorticity (rvor) at 300 hPa and a MSG channel 5 image valid at analysis time.
The forecaster is asked to consider the surface low (L) west of Spain (see s1) and is given the following tasks: In connection with L
● identify regions with significant positive and negative rvor advection at 300hPa and infer qualitatively their contribution to vertical velocity (w)
● identify regions with significant cold and warm advection (mean from surface to 300hPa) and infer qualitatively their contribution to w
● Identify a region with both warm advection and positive rvor advection and a region with both cold advection and negative rvor advection
● Which way will L most likely move? - and is it likely that L will intensify?
● What is the sign of the correlation between rvor at 300 hPa and bright(+) and dark (-) colours on the MSG image – is the correlation weak or strong?

31. DMI-HIRLAM analysis of mslp and wind at 300 hPa valid 12UTC 27 Feb
DMI-HIRLAM analysis of mslp and wind at 300 hPa valid 12UTC 27 Feb (s1)

32. DMI-HIRLAM analysis of mslp and rvor and T at 300 hPa valid 12UTC 27 Feb. 2010 (s2)

33. MSG channel-5 12UTC 27 February 2010

34. s3 s3 s3

35. s4

36. s5

37. s6