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

Lab 4 – Atms 4320 / 7320Lab 4 – Atms 4320 / 7320

The Nocturnal Jet and Severe The Nocturnal Jet and Severe Weather.Weather.


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


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


Northerly Jet?Northerly Jet?

The LL “Cool” JThe LL “Cool” J


These low-level jets, called the “southern These low-level jets, called the “southern plains low-level jet” is driven by topography plains low-level jet” is driven by topography and has a distinct diurnal oscillation and has a distinct diurnal oscillation (strongest at night and weakest during the (strongest at night and weakest during the day).day).

Typically associated with the “dry line”, thus Typically associated with the “dry line”, thus “dry-line” convection can also demonstrate a “dry-line” convection can also demonstrate a distinct diurnal variation.distinct diurnal variation.



The dryline:The dryline:

sharp moisture gradients between moist Gulf air and dry desert sharp moisture gradients between moist Gulf air and dry desert air.air.

get strong convergence along this line, and there’s a distince get strong convergence along this line, and there’s a distince windshift.windshift.

diurnal oscillation of in the fair weather ridge (most convection 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 that fires up, fires up within 200 mi of the dry line and become a squall line.squall line.

the dry line moves eastward during the day (toward the Gulf), the dry line moves eastward during the day (toward the Gulf), and westward at night (toward the Rockies)and westward at night (toward the Rockies)


Moist air acts like a pool hanging on the Moist air acts like a pool hanging on the continent, thus the density differences continent, thus the density differences between the moist and dry air drive the between the moist and dry air drive the circulations.circulations.


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


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


Modelling studies have shown that:Modelling studies have shown that:

soil moisture is important in determining the soil moisture is important in determining the strength of the dry line.strength of the dry line.

wind shear is important also, butwind shear is important also, but

without the sloping terrain, little or no development without the sloping terrain, little or no development of a dryline.of a dryline.


Density differences between the dry air and Density differences between the dry air and moist air can lead to a favorable moist air can lead to a favorable environment for severe weather.environment for severe weather.

Generally studied using numerical models, Generally studied using numerical models, and generally studied as a geostrophically and generally studied as a geostrophically balanced system. The equations used are balanced system. The equations used are the two-dimentional linearized basic the two-dimentional linearized basic equations, or a Boussinesq fluid.equations, or a Boussinesq fluid.


Boussinesq fluid (pp 197 - 198 Holton): (especially Boussinesq fluid (pp 197 - 198 Holton): (especially useful for boundary layer studies)useful for boundary layer studies)

Non divergent Non divergent

Small Rossby Number Small Rossby Number

Barotropic (ie density a function of pressure only) Barotropic (ie density a function of pressure only) and incompressible system and incompressible system

0 VPGFCo

dtVd

Po


Rotation effects can be neglected.Rotation effects can be neglected.

““Shallow water” system of equations. Shallow water” system of equations. Based on assumptions above, this system Based on assumptions above, this system is valid for disturbances that have a depth is valid for disturbances that have a depth shallower than the e-folding depth of the shallower than the e-folding depth of the atmosphere (H) which is roughly 8 km.atmosphere (H) which is roughly 8 km.


Linearized equations where : Linearized equations where :

gRTHuUu ,

0

0

01

01

zw

t

zw

xu

gzp

tw

xp

tu

o

o


Holton (2004) pp. 197ff combines these:Holton (2004) pp. 197ff combines these:

02

22

2

2

2

2

2

2

xwN

zw

xw

t


Can solve analytically (for “c”) by assuming Can solve analytically (for “c”) by assuming some “wave type” solution. Where;some “wave type” solution. Where;

ctmzkxAw sin


m = wave number in the vertical m = wave number in the vertical

k = wave number in the horizontal k = wave number in the horizontal

c = phase speed (propagation speed of the c = phase speed (propagation speed of the principal wave components)principal wave components)

zLm 2

xLk 2


Example (in x,t coordinates):Example (in x,t coordinates):

(1)(1) (2)(2)

2

22

2

2

sin,

,

xuu

tu

andctkxAtxu

andtxuu


Operation 1:Operation 1:

ctkxAcctkxAct

ctkxAct

ctkxAt

sincos

cossin

2

2

2


Operation 2Operation 2

ctkxAkctkxAkx

ctkxAkx

ctkxAx

sincos

cossin

2

2

2


Then, from above:Then, from above:

Divide both sides by:Divide both sides by:

And we get:And we get:

ctkxAcctkxAk sinsin 22

ctkxAtxu sin,

kcck 22


The End!!!!!The End!!!!!

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