Lab 4 – Atms 4320

1. Lab 4 – Atms 4320 The Nocturnal Jet and Severe Weather.

2. The Nocturnal Jet and Severe Weather. • This is a low-level jet found over the southern plains and midwest (East of the Rocky Mountains).

3. The Nocturnal Jet and Severe Weather. • It may be east or northeast of a low-level trough or cyclone, and ahead of a cold front. This feature is associated with copious low-level moisture transport and can feed into convective systems, MCCs and MCSs.

4. The Nocturnal Jet and Severe Weather. • Northerly Jet? • The LL “Cool” J

5. The Nocturnal Jet and Severe Weather. • These low-level jets, called the “southern plains low-level jet” is driven by topography and has a distinct diurnal oscillation (strongest at night and weakest during the day). • Typically associated with the “dry line”, thus “dry-line” convection can also demonstrate a distinct diurnal variation.

6. The Nocturnal Jet and Severe Weather. 

7. The Nocturnal Jet and Severe Weather. • The dryline: • sharp moisture gradients between moist Gulf air and dry desert air. • get strong convergence along this line, and there’s a distince windshift. • diurnal oscillation of in the fair weather ridge (most convection that fires up, fires up within 200 mi of the dry line and become a squall line. • the dry line moves eastward during the day (toward the Gulf), and westward at night (toward the Rockies)

8. The Nocturnal Jet and Severe Weather. • Moist air acts like a pool hanging on the continent, thus the density differences between the moist and dry air drive the circulations.

9. The Nocturnal Jet and Severe Weather. • Sea breeze type transverse circulations develop in association with the dry line, driving convection.

10. The Nocturnal Jet and Severe Weather. • Soil moisture plays a role in the development of a strong dry line, that’s why this phenomena is common in the spring and early summer.

11. The Nocturnal Jet and Severe Weather. • Modelling studies have shown that: • soil moisture is important in determining the strength of the dry line. • wind shear is important also, but • without the sloping terrain, little or no development of a dryline.

12. The Nocturnal Jet and Severe Weather. • Density differences between the dry air and moist air can lead to a favorable environment for severe weather. • Generally studied using numerical models, and generally studied as a geostrophically balanced system. The equations used are the two-dimentional linearized basic equations, or a Boussinesq fluid.

13. The Nocturnal Jet and Severe Weather. • Boussinesq fluid (pp 197 - 198 Holton): (especially useful for boundary layer studies) • Non divergent • Small Rossby Number • Barotropic (ie density a function of pressure only) and incompressible system

14. The Nocturnal Jet and Severe Weather. • Rotation effects can be neglected. • “Shallow water” system of equations. Based on assumptions above, this system is valid for disturbances that have a depth shallower than the e-folding depth of the atmosphere (H) which is roughly 8 km.

15. The Nocturnal Jet and Severe Weather. • Linearized equations where :

16. The Nocturnal Jet and Severe Weather. • Holton (2004) pp. 197ff combines these:

17. The Nocturnal Jet and Severe Weather. • Can solve analytically (for “c”) by assuming some “wave type” solution. Where;

18. The Nocturnal Jet and Severe Weather. • m = wave number in the vertical • k = wave number in the horizontal • c = phase speed (propagation speed of the principal wave components)

19. The Nocturnal Jet and Severe Weather. • Example (in x,t coordinates): • (1) (2)

20. The Nocturnal Jet and Severe Weather. • Operation 1:

21. The Nocturnal Jet and Severe Weather. • Operation 2

22. The Nocturnal Jet and Severe Weather. • Then, from above: • Divide both sides by: • And we get:

23. The Nocturnal Jet and Severe Weather. • The End!!!!!