Lab 4 Atms 4320

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Lab 4 Atms 4320

• The Nocturnal Jet and Severe Weather.

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The Nocturnal Jet and Severe Weather.

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

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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.

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The Nocturnal Jet and Severe Weather.

• Northerly Jet?
• The LL Cool J

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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.

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The Nocturnal Jet and Severe Weather.

• ?

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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
  theres 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)

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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.

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The Nocturnal Jet and Severe Weather.

• Sea breeze type transverse circulations develop
  in association with the dry line, driving
  convection.

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The Nocturnal Jet and Severe Weather.

• Soil moisture plays a role in the development of
  a strong dry line, thats why this phenomena is
  common in the spring and early summer.

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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.

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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.

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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

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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.

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The Nocturnal Jet and Severe Weather.

• Linearized equations where

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The Nocturnal Jet and Severe Weather.

• Holton (2004) pp. 197ff combines these

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The Nocturnal Jet and Severe Weather.

• Can solve analytically (for c) by assuming some
  wave type solution. Where

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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)

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The Nocturnal Jet and Severe Weather.

• Example (in x,t coordinates)
• (1) (2)

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The Nocturnal Jet and Severe Weather.

• Operation 1

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The Nocturnal Jet and Severe Weather.

• Operation 2

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The Nocturnal Jet and Severe Weather.

• Then, from above
• Divide both sides by
• And we get

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The Nocturnal Jet and Severe Weather.

• The End!!!!!