Section 8 Vertical Circulation at Fronts 1.Structural and dynamical characteristics of mid-latitude...
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Transcript of Section 8 Vertical Circulation at Fronts 1.Structural and dynamical characteristics of mid-latitude...
Section 8 Vertical Circulation at Fronts
1. Structural and dynamical characteristics of mid-latitude fronts
2. Frontogenesis
3. Semi-geostrophic equations
4. Symmetric instability
1. Structure and dynamical characteristics of mid-latitude fronts
EXAMPLES OF FRONTS
A front is a transition zone between different air masses. It is characterized by:
1. Larger than background horizontal temperature (density) contrasts ( strong vertical shear)
2. Larger than background relative vorticity
3. Larger than background static stability
4. a quasi linear structure (length >> width)
Let’s for the moment consider a zero-order front
We will assume that: 1) front is parallel to x axis 2) front is steady-state 3) pressure is continuous across the front 4) density and T are discontinuous across the front
dzz
pdy
y
pdp
Warm side of front dzz
pdy
y
pdp
ww
w
Cold side of front
€
dpc =∂p
∂y
⎛
⎝ ⎜
⎞
⎠ ⎟c
dy +∂p
∂z
⎛
⎝ ⎜
⎞
⎠ ⎟c
dz
Substitute hydrostatic equation and equate expressions:
gdzdyy
p
y
pwc
wc
0
Solve for the slope of the front
wc
wc
g
yp
yp
dy
dz
€
dpc = dpwWe have
€
∂p
∂z= −ρgand
wc
wc
g
yp
yp
dy
dz
For cold air to underlie warm air, slope must be positive
1) Across front pressure gradient on the cold side must be larger that the pressure gradient on the warm side
y
p
fug
1
Substituting geostrophic wind relationship
wc
gcgw
g
uuf
dy
dzcw
cw gg uu
2) Front must be characterized by positive geostrophic relative vorticity
0dy
dug
The stronger the density (T) contrast becomes, the stronger is the vorticity at the front.
First-order fronts
1) Larger than background horizontal temperature (density) gradient
2) Larger than background relative vorticity
3) Larger than background static stability
Working definition of a cold or warm front
The leading edge of a transitional zone that separates advancing cold (warm) air from warm (cold) air, the length of which is significantly greater than its width. The zone is characterized by high static stability as well as larger-than-background temperature gradient and relative vorticity.
2. Frontogenetic Function
the Lagrangian rate of change of the magnitude of potential temperature gradient
Move to the whiteboard and talk about 1D frontogenesis
dt
dF
Expanding the total derivative
€
d
dt=
∂
∂t+ u
∂
∂x+ v
∂
∂y+ w
∂
∂z
expanding the term involving the magnitude of the gradient
2/1222
zyx
3D Frontogenesis
(
€
F =1
∇θ
∂θ
∂x
1
Cp
p0
p
⎛
⎝ ⎜
⎞
⎠ ⎟
κ∂
∂x
dQ
dt
⎛
⎝ ⎜
⎞
⎠ ⎟
⎡
⎣ ⎢
⎤
⎦ ⎥−
∂u
∂x
∂θ
∂x
⎛
⎝ ⎜
⎞
⎠ ⎟−
∂v
∂x
∂θ
∂y
⎛
⎝ ⎜
⎞
⎠ ⎟−
∂w
∂x
∂θ
∂z
⎛
⎝ ⎜
⎞
⎠ ⎟
⎧ ⎨ ⎪
⎩ ⎪
⎫ ⎬ ⎪
⎭ ⎪
€
+∂∂y
1
Cp
p0
p
⎛
⎝ ⎜
⎞
⎠ ⎟
κ∂
∂y
dQ
dt
⎛
⎝ ⎜
⎞
⎠ ⎟
⎡
⎣ ⎢
⎤
⎦ ⎥−
∂u
∂y
∂θ
∂x
⎛
⎝ ⎜
⎞
⎠ ⎟−
∂v
∂y
∂θ
∂y
⎛
⎝ ⎜
⎞
⎠ ⎟−
∂w
∂y
∂θ
∂z
⎛
⎝ ⎜
⎞
⎠ ⎟
⎧ ⎨ ⎪
⎩ ⎪
⎫ ⎬ ⎪
⎭ ⎪
zz
w
yz
v
xz
u
dt
dQp
zC
p
z p
0
The Three-Dimensional Frontogenesis Function
)
dt
dF
The solution
becomes
€
F1D =d
dt(∂θ
∂x) =
∂
∂x(dθ
dt) −
∂u
∂x
∂θ
∂x−
∂v
∂x
∂θ
∂y−
∂ω
∂x
∂θ
∂pCompared to
• Confluence terms (or stretching deformation): with
• Shearing terms (or shearing deformation):
involved with
• Tilting terms: with derivative of omega
€
∂u
∂x,
∂v
∂y
€
∂v
∂x,
∂u
∂y
€
F =1
∇θ
∂θ
∂x
1
Cp
p0
p
⎛
⎝ ⎜
⎞
⎠ ⎟
κ∂
∂x
dQ
dt
⎛
⎝ ⎜
⎞
⎠ ⎟
⎡
⎣ ⎢
⎤
⎦ ⎥−
∂u
∂x
∂θ
∂x
⎛
⎝ ⎜
⎞
⎠ ⎟−
∂v
∂x
∂θ
∂y
⎛
⎝ ⎜
⎞
⎠ ⎟−
∂w
∂x
∂θ
∂z
⎛
⎝ ⎜
⎞
⎠ ⎟
⎧ ⎨ ⎪
⎩ ⎪
⎫ ⎬ ⎪
⎭ ⎪(
zy
w
yy
v
xy
u
dt
dQ
yp
p
Cy p
01
zz
w
yz
v
xz
u
dt
dQp
zC
p
z p
0 )
The terms in the yellow box all contain the derivativewhich is the diabatic heating rate. These terms arecalled the diabatic terms.
dt
dQ
zx
w
yx
v
xx
u
dt
dQ
xp
p
CxF
p
011 (
zy
w
yy
v
xy
u
dt
dQ
yp
p
Cy p
01
zz
w
yz
v
xz
u
dt
dQp
zC
p
z p
0 )
€
F =1
∇θ
∂θ
∂x
1
Cp
p0
p
⎛
⎝ ⎜
⎞
⎠ ⎟
κ∂
∂x
dQ
dt
⎛
⎝ ⎜
⎞
⎠ ⎟
⎡
⎣ ⎢
⎤
⎦ ⎥
⎧ ⎨ ⎪
⎩ ⎪
⎫ ⎬ ⎪
⎭ ⎪
Temperature gradient Horizontal gradient in diabatic heating or cooling rate
If and have the same sign, it means the diabatic heating will increase the temperature gradient.
€
∂∂x
dQ
dt
⎛
⎝ ⎜
⎞
⎠ ⎟
€
∂∂x
dt
dQp
zC
pzF
p
0/
Vertical cross section of potential temperature
zx
w
yx
v
xx
u
dt
dQ
xp
p
CxF
p
011 (
zy
w
yy
v
xy
u
dt
dQ
yp
p
Cy p
01
zz
w
yz
v
xz
u
dt
dQp
zC
p
z p
0 )
The terms in this yellow box represent the contributionto frontogenesis due to horizontal deformation flow.
zx
w
yx
v
xx
u
dt
dQ
xp
p
CxF
p
011 (
zy
w
yy
v
xy
u
dt
dQ
yp
p
Cy p
01
zz
w
yz
v
xz
u
dt
dQp
zC
p
z p
0 ) €
−∂u
∂x
∂θ
∂x
⎛
⎝ ⎜
⎞
⎠ ⎟−
∂v
∂x
∂θ
∂y
⎛
⎝ ⎜
⎞
⎠ ⎟
€
−∂u
∂y
∂θ
∂x
⎛
⎝ ⎜
⎞
⎠ ⎟−
∂v
∂y
∂θ
∂y
⎛
⎝ ⎜
⎞
⎠ ⎟
Stretching deformation
Shearing deformation
yy
vy
xx
uxF
//
Stretching Deformation
Deformation acting on
temperature gradient
Deformation acting on
temperature gradient
x
y
x
Time = t
T
T-
T- 2T
T- 3T
T- 4T
T- 5T
T- 6T
T- 7T
T- 8T
y
Time = t + t
TT- T- 2TT- 3T
T- 4TT- 5T
T- 6TT- 7TT- 8T
yy
vy
xx
uxF
//
Stretching Deformation
zx
w
yx
v
xx
u
dt
dQ
xp
p
CxF
p
011 (
zy
w
yy
v
xy
u
dt
dQ
yp
p
Cy p
01
zz
w
yz
v
xz
u
dt
dQp
zC
p
z p
0 ) €
−∂u
∂x
∂θ
∂x
⎛
⎝ ⎜
⎞
⎠ ⎟−
∂v
∂x
∂θ
∂y
⎛
⎝ ⎜
⎞
⎠ ⎟
€
−∂u
∂y
∂θ
∂x
⎛
⎝ ⎜
⎞
⎠ ⎟−
∂v
∂y
∂θ
∂y
⎛
⎝ ⎜
⎞
⎠ ⎟
Stretching deformation
Shearing deformation
xy
uy
yx
vxF
//
Shearing Deformation
Deformation acting on
temperature gradient
Deformation acting on
temperature gradient
x
y
TT-
T- 2TT- 3T
T- 4TT- 5T
T- 6TT- 7T
T- 8T
x
y
TT-
T- 2TT- 3T
T- 4TT- 5T
T- 6TT- 7T
T- 8T
xy
uy
yx
vxF
//
Shearing Deformation
zx
w
yx
v
xx
u
dt
dQ
xp
p
CxF
p
011 (
zy
w
yy
v
xy
u
dt
dQ
yp
p
Cy p
01
zz
w
yz
v
xz
u
dt
dQp
zC
p
z p
0 )
The terms in this yellow box represent the contributionto frontogenesis due to tilting.
zx
w
yx
v
xx
u
dt
dQ
xp
p
CxF
p
011 (
zy
w
yy
v
xy
u
dt
dQ
yp
p
Cy p
01
zz
w
yz
v
xz
u
dt
dQp
zC
p
z p
0 )
Tilting terms
Weighting factor
Magnitude of gradient in one directionMagnitude of total gradient
TiltingOf vertical Gradient
(E-W direction)
zy
wy
zx
wxF
//
TiltingOf vertical Gradient
(N-S direction)
Tilting terms
zy
wy
zx
wxF
//
Before
x or y
z
After
x or y
z
zx
w
yx
v
xx
u
dt
dQ
xp
p
CxF
p
011 (
zy
w
yy
v
xy
u
dt
dQ
yp
p
Cy p
01
zz
w
yz
v
xz
u
dt
dQp
zC
p
z p
0 )
The terms in this yellow box represent the contributionto frontogenesis due to vertical shear acting on a horizontal temperature gradient.
zx
w
yx
v
xx
u
dt
dQ
xp
p
CxF
p
011 (
zy
w
yy
v
xy
u
dt
dQ
yp
p
Cy p
01
zz
w
yz
v
xz
u
dt
dQp
zC
p
z p
0 )
yz
v
xz
uzF
/
Vertical shear acting on a horizontal temperature gradient(also called vertical deformation term)
Vertical shear of E-W windComponent acting on
a horizontal temp gradient in xdirection
Vertical shear of N-S windcomponent acting on
a horizontal temp gradient in ydirection
yz
v
xz
uzF
/
Vertical shear acting on a horizontal temperature gradient
Before
x
z z
x
After
zx
w
yx
v
xx
u
dt
dQ
xp
p
CxF
p
011 (
zy
w
yy
v
xy
u
dt
dQ
yp
p
Cy p
01
zz
w
yz
v
xz
u
dt
dQp
zC
p
z p
0 )
The term in this yellow box represents the contributionto frontogenesis due to divergence.
zz
wzF
/
Compressionof vertical Gradient
by differential vertical motion
Differential vertical motion
zz
wzF
/
Before
x or y
z
After
x or y
z
zx
w
yx
v
xx
u
dt
dQ
xp
p
CxF
p
011 (
zy
w
yy
v
xy
u
dt
dQ
yp
p
Cy p
01
zz
w
yz
v
xz
u
dt
dQp
zC
p
z p
0 )
2D Frontogenetic Function
x
y
T
T-
T- 2T
T- 3T
T- 4T
T- 5T
T- 6T
T- 7T
T- 8T
x
y
TT-
T- 2TT- 3T
T- 4TT- 5T
T- 6TT- 7T
T- 8T
The stretching and shearing deformations “look like” one another:
yy
v
xy
u
yyx
v
xx
u
xF D
12
Another view of the 2D frontogenesis function
y
v
x
uD
€
ζ ∂v
∂x−
∂u
∂y
€
F1 =∂u
∂x−
∂v
∂y
21FD
x
u
21FD
y
v
22F
x
v
ζ
Recall the kinematic quantities: divergence (D)vorticity (ζ)
stretching deformation (F1)shearing deformation (F1).
y
u
x
vF
2
and note that:
22 ζ
F
y
u
Substituting:
y
FD
x
F
yy
F
x
FD
xF D
ζζ 2222
1 12212
y
FD
x
F
yy
F
x
FD
xF D
ζζ 2222
1 12212
This expression can be reduced to:
yxF
yxF
yxDF D
2
22
1
22
2 22
1
x
y
x
y Shearing and stretching
deformation“look alike” with
axes rotated
We can simplify the 2D frontogenesis equation by rotating our coordinate axes to align with the axis of dilatation of the flow (x´)
€
F2D =1
2∇θD ∇θ
2( ) + F
∂θ
∂ ′ x
⎛
⎝ ⎜
⎞
⎠ ⎟2
−∂θ
∂ ′ y
⎛
⎝ ⎜
⎞
⎠ ⎟
2 ⎡
⎣ ⎢ ⎢
⎤
⎦ ⎥ ⎥
⎧ ⎨ ⎪
⎩ ⎪
⎫ ⎬ ⎪
⎭ ⎪
€
F = (F12 + F2
2)1/ 2where F is the total deformation
This equation illustrates that horizontal frontogenesis is only associated with divergence and deformation, but not vorticity€
F2D =1
2∇θD ∇θ
2( ) + F
∂θ
∂ ′ x
⎛
⎝ ⎜
⎞
⎠ ⎟2
−∂θ
∂ ′ y
⎛
⎝ ⎜
⎞
⎠ ⎟
2 ⎡
⎣ ⎢ ⎢
⎤
⎦ ⎥ ⎥
⎧ ⎨ ⎪
⎩ ⎪
⎫ ⎬ ⎪
⎭ ⎪
€
F2D =∇θ
2(F cos2β − D)
Where F is the total deformation of the flow, β is the angle between the isentropes and the dilatation axis of the total deformation field, and D is divergence (D <0 for convergence)
€
F
∇θ2
∂θ
∂ ′ x
⎛
⎝ ⎜
⎞
⎠ ⎟2
−∂θ
∂ ′ y
⎛
⎝ ⎜
⎞
⎠ ⎟
2 ⎡
⎣ ⎢ ⎢
⎤
⎦ ⎥ ⎥= F cos2α = −F cos2βNote that
€
F2D =∇θ
2D +
F
∇θ2
∂θ
∂ ′ x
⎛
⎝ ⎜
⎞
⎠ ⎟2
−∂θ
∂ ′ y
⎛
⎝ ⎜
⎞
⎠ ⎟
2 ⎡
⎣ ⎢ ⎢
⎤
⎦ ⎥ ⎥
⎧ ⎨ ⎪
⎩ ⎪
⎫ ⎬ ⎪
⎭ ⎪
x
y
TT-
T- 2TT- 3T
T- 4TT- 5T
T- 6TT- 7T
T- 8T
x
y
TT-
T- 2TT- 3T
T- 4TT- 5T
T- 6TT- 7T
T- 8T
€
F2D =∇θ
2(F cos2β − D)
Frontogenesis occurs 1)if non-zero is coincident with convergence (D<0)2)if the total deformation field (F) acts on isentropes that are between 0 and 45° of the dilatation axis of the total deformation. deformation. €
∇
3. S.G. vs. Q.G. Approximations
• Q.G.: S.G.
€
u = ug, v = vg + va
dug
dt= fva
€
u = ug + ua, v = vg + va , f = const
du
dt= fv −
∂φ
∂xdv
dt= − fu −
∂φ
∂y
⎧
⎨ ⎪ ⎪
⎩ ⎪ ⎪
€
ug >> ua , vg >> va
dgug
dt= fva
dgvg
dt= − fua
⎧
⎨ ⎪ ⎪
⎩ ⎪ ⎪
€
dg
dt=
∂
∂t+ ug
∂
∂x+ vg
∂
∂ywhere
Right side of equation represent the forcing (known from measurements or in model solution)
Geostrophic deformation Diabatic heating
, the streamfunction, is the response. V and ω can be derived from
€
−γ∂∂p
⎛
⎝ ⎜
⎞
⎠ ⎟∂2ψ
∂y 2 + 2∂M
∂p
⎛
⎝ ⎜
⎞
⎠ ⎟∂2ψ
∂p∂y+ −
∂M
∂y
⎛
⎝ ⎜
⎞
⎠ ⎟∂2ψ
∂p2 = Qg −γ∂
∂y
dθ
dt
⎛
⎝ ⎜
⎞
⎠ ⎟
Sawyer-Eliassen Equation
Questions: 1) How is the thermal wind balance maintained by the transverse circ.? 2) Where should we expect upward motion (precipitation)?
€
dug
dt= fva
coldwarm
Cold
warm
Nature of the solution of the Sawyer-Eliassen Equation:
A direct circulation (warm air rising and cold air sinking) will result with positive forcing.
An indirect circulation (warm air sinking and cold air rising) will result with negative forcing.
Cold air Warm air
Isentrope
Cold air Warm air
Isentrope
€
−∂∂y
dθ
dt
⎛
⎝ ⎜
⎞
⎠ ⎟> 0
(heating in the warm sideand cooling in the cold side) will produceA thermally direct circulation and promoteFrontogenesis.
yy
V
xy
UQ gg
g
γ2
Geostrophic shearing
deformation
Geostrophic stretching
deformation
yy
VQ g
STg
γ2 Geostrophic stretching deformation
Note in this figure that both and are negative, implying
frontogenesis and a direct circulation in which warm air is rising and
cold air sinking.
y
Vg
y
Entrance region of jet
xy
UQ g
sHg
γ2 Geostrophic shearing deformation
Note in this figure that both and are positive, implying
frontogenesis and a direct circulation in which warm air is rising and
cold air sinking.
y
U g
x
confluent flow along front
Why does a spinning top stay upright?
• Buoyancy tends to stabilize air parcels against vertical displacements, and rotation tends to stabilize parcels with respect to horizontal displacements.
• If ordinary static and inertial stabilities are satisfied, is the flow always stable?
4. Symmetric Instability
• hydrostatic instability
• Inertial instability
0dz
d v stable 0dz
d v neutral 0dz
d v unstable
€
f f −∂ug
∂y
⎛
⎝ ⎜
⎞
⎠ ⎟> 0 stable
€
f f −∂ug
∂y
⎛
⎝ ⎜
⎞
⎠ ⎟= 0 neutral
€
f f −∂ug
∂y
⎛
⎝ ⎜
⎞
⎠ ⎟< 0 unstable
MSI: an intuitive explanation
M = fy-ug
70
60
4030
dM/dy>0
M = absolute zonal momentum
see also: Jim Moore’s meted module on frontogenetic circulations & stability)
-
-
-
-
-
-
-
-
--
Dash: e
Solid: Mg
Potential Symmetric Stability PotentialPotential Symmetric INstability
e ee 2y
z
gM
z z
y y
gg MM 2
e ee 2
gM
gg MM 2
Stable Neutral Unstable
e ee 2
gM
g
g
M
M
2
€
red :θ e or θ v gMblue :
Symmetric instability evaluation
The flow satisfies and0dz
d v
€
f f −∂ug
∂y
⎛
⎝ ⎜
⎞
⎠ ⎟z
> 0