Lab 4 – Atms 4320 / 7320 The Nocturnal Jet and Severe Weather.

 This is a low-level jet found over the southern plains and midwest (East of the Rocky Mountains).

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.

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

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.

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)

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.

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

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.

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.

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.

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

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.

The Nocturnal Jet and Severe Weather.  Linearized equations where :

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

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

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)

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

The Nocturnal Jet and Severe Weather.  Operation 1:

The Nocturnal Jet and Severe Weather.  Operation 2

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

The Nocturnal Jet and Severe Weather.  The End!!!!!