ATMO 251

1. ATMO 251 Fronts and Frontogenesis, Part 2

2. Deformation • There are three important properties that a vector wind field can have • Divergence (tendency to move apart) • Vorticity (tendency to induce spin) • Deformation (tendency to change shape) “Stretching deformation” “Shearing deformation”

3. Let the second term be zero for now • Let du/dx equal -dv/dy • This means the divergence (du/dx+dv/dy) = 0

4. Pure deformation field • Zero divergence everywhere • A “col” or “saddle point” in the middle – calm winds • An object anywhere in this field will be “deformed” by the winds

5. Pure deformation field • Zero divergence everywhere • A “col” or “saddle point” in the middle • An object anywhere in this field will be “deformed” by the winds H L L H

6. Pure deformation field • Zero divergence everywhere • A “col” or “saddle point” in the middle • An object anywhere in this field will be “deformed” by the winds

7. Pure deformation field • Zero divergence everywhere • A “col” or “saddle point” in the middle • An object anywhere in this field will be “deformed” by the winds

8. Pure deformation field • Zero divergence everywhere • A “col” or “saddle point” in the middle • An object anywhere in this field will be “deformed” by the winds

9. Pure deformation field • Zero divergence everywhere • A “col” or “saddle point” in the middle • An object anywhere in this field will be “deformed” by the winds Axis of dilatation (axis along which object is stretched)

10. Another example

11. 700-mb temperature, heights, winds

12. 700-mb heights and streamlines Axis of dilatation?

13. Water vapor loop

14. Water vapor loop \ \

15. Frontogenesis • Now, imagine that the pure deformation we looked at earlier is happening to a “box” with a temperature gradient – cold on one side and warm on the other • What happens to the temperature gradient?

16. If it’s warm on the west side and cold on the east side, the deformation makes the isotherms spread out – the temperature gradient gets weaker

17. BUT, if it’s cold on the north side and warm on the south side, the isotherms get closer together.

18. Frontogenesis • This has made the temperature gradient stronger, and is called “frontogenesis” – the creation of a front

19. Frontolysis • When the temperature gradient gets weaker, it is called “frontolysis” – the death of a front • As we just saw, the orientation of the temperature gradient in relation to the axis of dilatation determines whether deformation strengthens or weakens the gradient frontolysis frontogenesis

20. Frontogenesis • How else can frontogenesis/frontolysis occur? • Convergence/divergence • But at the synoptic scale, divergence is usually small, so deformation is more often important

21. Frontogenesis and thermal wind • Frontogenesis puts the atmosphere out of thermal wind balance – if the temperature gradient is changing, the wind shear must be changing too • If T gradient changes, P gradient also must change

22. Frontogenesis and thermal wind

23. Frontogenesis and thermal wind

24. Frontogenesis and thermal wind

26. Why does this all matter? • Frontogenesis actually causes rising and sinking motion! • This vertical motion doesn’t happen just because a front exists, but because the front is strengthening or weakening • Because the vertical motion acts to weaken the front, if we see a strong front with storms ahead of it, it’s almost certain that frontogenesis is going on • We don’t see it plotted on TV, but it is very common to see frontogenesis used in research and forecasting

27. 950-mb winds and potential temperature

28. White contours = frontogenesis

29. White contours = frontogenesis