low-level jet

(1)

The stable boundary layer and nocturnal

low-level jet

Roger K. Smith

Nocturnal low-level jet

¾ The low-level jet (LLJ) is a thin stream of fast moving air, with maximum wind speeds of 10 to 20 ms

⁻¹

usually located 100 to 300 m above the ground.

¾ Peak speeds up to 30 ms

⁻¹

have been reported and altitudes of the peak were occasionally as high as 300 m above ground.

¾ The LLJ can have a width of hundreds of kilometres and a length of thousands of kilometres, making it more like a sheet than a narrow ribbon, in some cases.

¾ In many cases the LLJ reaches its peak during the night and reaches its peak during the predawn hours.

¾ LLJs occur on 10% of winter nights in parts of Australia, with

peak speeds between 00 and 05 local time (Brook, 1985).

(2)

The profile of wind component in the direction of the geostrophic wind (u

_g

) showing a nocturnal jet, compared with the profile the previous afternoon.

1430 0814

0001 1548

θ

^o

C 1000

500 z m

1500

day

night night day

5 0

0 u

_g

u ms

⁻¹

10 1000

10 15 20

1500

500 z m

Nocturnal stable layer evolution during during Wangara Night 6-7

Stull, 1983

∆θ (

^o

K)

15 10 5 0 100

z (k m )

200 300 400 500 600

0 03 06 00

21

18 Local time

(3)

Nocturnal jet evolution during during Wangara Night 13-14

Malcher and Kraus, 1983 0 5 10 15

0.2 z (k m )

0.4 0.6 0.8 1.0

0 Local time 06 18 00 21

m/s

Boundary-layer evolution during Wangara Night 13-14

Malcher and Kraus, 1983

Local time (h) Local time (h)

12 16 20 0 4 8 12 12 16 20 0 4 8 12

0 0.5 1.0 1.5

0.5 1.0 1.5

z (k m ) z (k m )

θ = 294 K

291 K

288 K 285 K 282 K

279 K 8 m/s

10 12 14

(4)

¾ Consider a homogeneous layer of inviscid fluid on an f-plane confined between rigid horizontal boundaries.

z

y x

f A simple theory for the nocturnal LLJ

u

_g

= (u

_g

,0) LO u

HI

SH

g

u fv u

t

v fu fu v

t

∂ − = −µ

∂

∂ + = − µ

∂

Mathematical formulation

Momentum equations assuming a linear drag law

(5)

g

u v

fv u, fu fu v

t t

∂ − = −µ ∂ + = − µ

∂ ∂

g

u fv 0 t

v fu fu t

∂ − =

∂

∂ + = −

∂

Daytime Nighttime

g

fv u

fu fu v

− = −µ

= − µ

u v

, 0

t t

 ∂ ∂  =

 ∂ ∂ 

  µ = 0

Steady No friction

g g g

2 2 2

u u u

u , v , | u |

1 1 1

= = ε =

+ ε + ε + ε

f ε = µ

2

2 2

2

f (u, v) f (u ,0)

g

t

 ∂ +  =

 ∂ 

 

Daytime

Nighttime

g

v u

u u v

= ε

= − ε

g g g

2 2 2

u u u

u , v , | u |

1 1 1

= = ε =

+ ε + ε + ε

f ε = µ

g

g 2

u u u ( cosft sin ft) 1

v u (cosft sin ft) 1

= + ε −ε +

+ ε

= − ε + ε

+ ε

2

2 2

2

f (u, v) f (u ,0)

g

t

 ∂ +  =

 ∂ 

 

u u

g

A cosft Bsin ft u Ccosft Dsin ft

= + +

= +

u fv 0 t

∂ − =

∂

B C

D A

=

= −

g 2

u u 1 v u

1 = + ε

= ε

+ ε

t = 0

(6)

−4

−2 2 4

0 Geostrophic wind

5 15

0 1 2 3 4

5 6 7 8 9 11 10

12 13 14

Initial condition Daytime steady state wind in BL

Wind 13 h after sunset

Wind 4 h after sunset 16

15 Wind 7 h after sunset

v

u Inertial turning of the LLJ

10 Southern Hemisphere

(7)

Vector difference of the maximum wind below 1500 m above ground level at 0300 EST from that at 2100 EST, across Australia for the night of 27/28 May 1991. Isobars are of Mslp at 1000 EST on 28 May.

The End