Understanding Frontogenesis: Formation & Decay of Weather Fronts

1. Frontogenesis • Frontogenesis: • The generation of intensity of a front • Warm air merged onto colder air • Temperature gradient amplified at least one order of magnitude Frontogenesis: the formation of a front Frontolysis: the decay of a front A good example of non-frontal zone boundary is . dryline Mesoscale fronts: land-sea breeze, storm outflow (a few hours) Synoptic scale fronts: fronts on the weather maps (many days)

2. Frontogenesis Kinematics and thermodynamics of Frontogenesis: 2D frontogenesis (F): Frontogenesis function First law of thermodynamics Diabatic heating (e.g., latent heat, radiation)

3. q - Dq y q q + Dq x Frontogenesis Assume that winds do no vary along the front and x axis // q lines, 0 [ ] [ ] [ ] [ ] [ ] [ ] Inhomogeneous diabatic heating Confluence/diffluence Tilting effect

4. Frontogenesis (1) Confluence/diffluence Frontogenesis, F> 0 Frontolysis. F < 0 q - Dq q - Dq y q y q q + Dq q + Dq x x q - Dq q - Dq y q y q q + Dq q + Dq x x

5. Frontogenesis (1) Confluence/diffluence

6. Frontogenesis (2) Tilting effect Frontogenesis, F> 0 Frontolysis. F < 0 q + Dq q + Dq q q z z q - Dq q - Dq y y N E q + Dq q + Dq q q q - Dq z z q - Dq y y

7. Frontogenesis (3) Quasi-horizontal variation due to diabatic heating Frontogenesis, F> 0 Frontolysis. F < 0 Day Night Cold, cloudy Cold, cloudy, less cooling q - Dq q - Dq y y q q q + Dq q + Dq Warm side, longwave radiative cooling, stronger cooling Warm solar heating x x

8. Front Passing

9. Thunderstorm Frequency Thunderstorm frequency map for the United States

10. Thunderstorms The upper part usually composes ice and is spread out as anvils. Types: • Short-lived cell • Multicell • Suepercell or split cell (can have hails and tornados) • Short-lived cell : when shear is weak, shear < 10 ms-1 below 6 km, • Multicell : moderate shear, 10 ~ 20 ms-1, • Supercell : strong shear, shear > 20 ms-1. Storms propagation speed = mean wind speed + propagation due to new formation of cell.

11. Thunderstorms Life time: short-lived cell: ~ 30 min multicell: ~ 10-15 min for each cell supercell: ~ nearly steady state (several hours) Storm dissipates because of: water loading, cut of energy supply, dry air entrainment, mixing, etc.

12. Reference for what type of storms but not their severity. Parameters: Bulk Richardson number ( ) Thunderstorms Storm types are strongly related to the Bulk Richardson number (an overestimated w)

13. Thunderstorms Why CAPE? Need energy to develop a storm (no help from large scales, like upper level trough to winter storms) Why shear? • The ability of a gust front to trigger a new cell (for multicell) • The ability of an updraft to interact with environment wind shear to produce an enhanced quasi-steady storm structure. (supercell)

14. Shear and Storm Types

15. Supercell • Isolated convetive storms (life time - several hours) • Usually requires large CAPE and strong wind shear • Low level moist, upper level dry ( - strong downdraft) • Shear too strong is not good either (destroy the storm structure) • Can potentially produce tornados

16. Supercell

17. Supercell

18. Supercell

19. Shear and Storm Splitting Uni-directional shear Multi-directional shear

20. Shear and Storm Moving Uni-directional shear Multi-directional shear

21. Supercell

22. Supercell Note: The wind vectors in the middle latitude of the northern hemisphere usually turn clockwise with height (Coriolis force effect). So, usually the split right-moving storm survives.

23. Supercell

24. Supercell Anticyclonic circulation Cyclonic circulation Uni-directional shear Survival Multi-directional shear

25. Storms and Floods • For multicell and supercell, if the system is quasi-stationary or slowly moving, • Produce heavy rainfall • Flashflood can occur