AOSS 321, Winter 2009 Earth System Dynamics Lecture 11 2/12/2009
description
Transcript of AOSS 321, Winter 2009 Earth System Dynamics Lecture 11 2/12/2009
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AOSS 321, Winter 2009 Earth System Dynamics
Lecture 112/12/2009
Christiane Jablonowski Eric Hetland
[email protected] [email protected]
734-763-6238 734-615-3177
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Today’s lecture
• Derivation of the potential temperature equation (Poisson equation)
• Dry adiabatic lapse rate• Static stability, buoyancy oscillations • Derivation of the Brunt-Väisälä frequency
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Thermodynamic equation
€
c p
DT
Dt−α
Dp
Dt= J
c p
T
DT
Dt−
α
T
Dp
Dt=
J
T(Divide by T)
€
αT
=Rd
pUse equation of state(ideal gas law)
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Thermodynamic equation
€
c p
T
DT
Dt−
Rd
p
Dp
Dt= 0
c p
D(lnT)
Dt− Rd
D(ln p)
Dt= 0
For conservative motions (no heating, dry adiabatic: J = 0):
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Derivation of Poisson’s Equation (1)
€
c p
D(lnT)
Dt= Rd
D(ln p)
Dt⇔ c pD(lnT) = Rd D(ln p)
c p D(lnT) = Rd D(ln p)p0
p
∫T0
T
∫
⇔ c p lnT
T0
⎛
⎝ ⎜
⎞
⎠ ⎟= Rd ln
p
p0
⎛
⎝ ⎜
⎞
⎠ ⎟
⇔ − c p lnT0
T
⎛
⎝ ⎜
⎞
⎠ ⎟= −Rd ln
p0
p
⎛
⎝ ⎜
⎞
⎠ ⎟
(integrateover Dt)
(integrate)
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€
c p lnT0
T
⎛
⎝ ⎜
⎞
⎠ ⎟= Rd ln
p0
p
⎛
⎝ ⎜
⎞
⎠ ⎟
lnT0
T
⎛
⎝ ⎜
⎞
⎠ ⎟c p
= lnp0
p
⎛
⎝ ⎜
⎞
⎠ ⎟
Rd
T0 = T p0
p
⎛
⎝ ⎜
⎞
⎠ ⎟
Rd
c p
≡ Θ
is called potential temperature!
Derivation of Poisson’s Equation (2)
Poisson’s Equation
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Definition of the potential temperature
€
=T p0
p
⎛
⎝ ⎜
⎞
⎠ ⎟
Rd
c p
with p0 usually taken to be constant withp0 = 1000 hPa
The potential temperature is the temperature a parcel would have if it was moved from some pressure level and temperature down to the surface.
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Definition of potential temperature
Does it makes sense that the temperature T would change in this problem? We did it adiabatically. There was no source and sink of energy.€
=T p0
p
⎛
⎝ ⎜
⎞
⎠ ⎟
Rd
c p
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Annual mean zonal mean temperature T
300
200
230
210220
Source: ECMWF, ERA40
260
260 260
Equator South PoleNorth Pole
Pressure
(hPa)
100
1000
1
10
Kelvin
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Annual mean zonal mean potential temperature
300
330
350
285285
Source: ECMWF, ERA40
Equator South PoleNorth Pole
Pressure
(hPa)
100
1000
Kelvin
How does the temperature field look?
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• For a dry adiabatic, hydrostatic atmosphere the potential temperature does not vary in the vertical direction:
• In a dry adiabatic, hydrostatic atmosphere the temperature T must therefore decrease with height.
Dry adiabatic lapse rate
€
∂∂z
= 0
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• An air parcel that has a temperature of 17 ºC at the 850 hPa pressure level is lifted dry adiabatically. What is the temperature and density of the parcel when it reaches the 500 hPa level?
Class exercise
850 hPa
500 hPa
T = 17 ºC, ?
T?, ?
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Dry adiabatic lapse rate: Derivation
Start with Poisson equation:
Take the logarithm of :
Differentiate with respect to height€
=T p0
p
⎛
⎝ ⎜
⎞
⎠ ⎟
Rd
c p
€
lnθ = lnT +Rd
cp
ln p0 − ln p( )
€
∂ lnθ
∂z=
∂ lnT
∂z+
Rd
cp
∂ ln p0
∂z−
∂ ln p
∂z
⎛ ⎝ ⎜
⎞ ⎠ ⎟
⇔1
θ
∂θ
∂z=
1
T
∂T
∂z−
Rd
cp p
∂p
∂z = 0 (p0 constant)
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Dry adiabatic lapse rateUse hydrostatic equation
Plug in ideal gas law for p, then multiply by T:
For dry adiabatic, hydrostatic atmosphere with
€
T
θ
∂θ
∂z=
∂T
∂z+
g
c p
€
−∂T
∂z=
g
c p
≡ Γd
d: dry adiabatic lapse rate (approx. 9.8 K/km)
€
1
θ
∂θ
∂z=
1
T
∂T
∂z+
Rdρg
cp p
€
∂∂z
= 0
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Static stability• We will now assess the static stability characteristics of the atmosphere.
• Static stability of the environment can be measured with the buoyancy frequency N.
• N is also called Brunt-Väisälä frequency. The square of this buoyancy frequency is defined as
€
N 2 =g
Θ
∂Θ
∂z
We will derive this equation momentarily, but first let’sdiscuss some static stability/instability conditions.
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Static stabilityWe will now assess the static stability characteristics of the atmosphere.
€
∂Θ∂z
> 0 statically stable
∂Θ
∂z= 0 statically neutral
∂Θ
∂z< 0 statically unstable
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Stable and unstable situations
Check out this marble in a bowl:http://eo.ucar.edu/webweather/stablebowl.html
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
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Stable and unstable air masses
Stable air: A rising parcel that is cooler than the surrounding atmosphere will tend to sink back to its original position (why?).
Unstable air: A rising parcel that is warmer than the surrounding atmosphere will continue to rise (why?).
Neutral air:The parcel remains at the new location after being displaced, its temperature varies exactly as the temperature of the surrounding atmosphere.
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Unstable air
Unstable air: makes thunderstorms possible. Here: visible since clouds rise to high elevations!
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Stable air
Stable air: makes oscillations (waves) in the atmosphere possible, visible due to the clouds!
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Stable air
Stable air: makes oscillations (waves) in the atmosphere possible, what is the wave length?
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Stable air
Stable air: temperature inversions suppress rising motions. Here: stratiform clouds have formed.
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Let’s take a closer look: Temperature as function of height
z
Warmer
Coolerz
T
- ∂T/∂z is defined as lapse rate
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Let’s take a closer look: Temperature as function of height
z
Warmer
Coolerz
T
- ∂T/∂z is defined as lapse rate
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Let’s take a closer look: Temperature as function of height
z
Warmer
Coolerz
T
- ∂T/∂z is defined as lapse rate
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Let’s take a closer look: Temperature as function of height
z
Warmer
Coolerz
T
- ∂T/∂z is defined as lapse rate
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The parcel method
• We are going displace this parcel – move it up and down.
– We are going to assume that the pressure adjusts instantaneously; that is, the parcel assumes the pressure of altitude to which it is displaced.
– As the parcel is moved its temperature will change according to the adiabatic lapse rate. That is, the motion is without the addition or subtraction of energy. J is zero in the thermodynamic equation.
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Parcel cooler than environmentz
Warmer
Cooler
If the parcel moves up and finds itself cooler than the environment then it will sink. (What is its density? larger or smaller?)
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Parcel cooler than environmentz
Warmer
Cooler
If the parcel moves up and finds itself cooler than the environment then it will sink. (What is its density? larger or smaller?)
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Parcel warmer than environmentz
Warmer
Cooler
If the parcel moves up and finds itself warmer than the environment then it will go up some more. (What is its density? larger or smaller?)
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Parcel warmer than environmentz
Warmer
Cooler If the parcel moves up and finds itself warmer than the environment then it will go up some more. (What is its density? larger or smaller?)
This is our first example of “instability” – a perturbation that grows.
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Let’s quantify this:Characteristics of the environment
€
Tenv = Tsfc − Γz with constant Γ = -∂Tenv
∂z
⎛
⎝ ⎜
⎞
⎠ ⎟ lapse rate
So if we go from z to z + Δz,
then the change in Tenv of the environment is
ΔTenv = Tsfc − Γ(z + Δz) − (Tsfc − Γz) = −ΓΔz
We assume that the temperature Tenv of the environment changes with a constant linear slope (or lapse rate ) in the vertical direction.
Tsfc: temperature at the surface
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Let’s quantify this:Characteristics of the parcel
€
So if we go from z to z + Δz,
then the temperature change ΔT of the parcel is
ΔTparcel = Tparcel ,z − ΓdΔz − Tparcel ,z = −ΓdΔz
Γd =g
c p
≡ dry adiabatic lapse rate
We assume that the temperature Tparcel of the parcel changes with the dry adiabatic lapse rate d.
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Stable:Temperature of parcel cooler than
environment
€
Tparcel < Tenv
⇔
Γ < Γd
environmentparcel
environment parcel
compare the lapse rates
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Unstable: Temperature of parcel greater than
environment.
€
Tparcel > Tenv
⇔
Γ > Γd
environmentparcel
environment parcel
compare the lapse rates
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Stability criteria from physical argument
€
>d unstable
Γ = Γd neutral
Γ < Γd stable
Lapse rate of the environment
Dry adiabatic lapse rate of the parcel
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Hydrostatic balance
€
environment in hydrostatic balance
0 = −1
ρ env
∂penv
∂z− g
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But our parcel experiences an acceleration, small displacement z
€
Dw
Dt=
D2δz
Dt 2= −
1
ρ parcel
∂penv
∂z− g
Assumption of immediate adjustment of pressure.
€
∂pparcel
∂z=
∂penv
∂z
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Solve for pressure gradient
€
environment in hydrostatic balance
0 = −1
ρ env
∂penv
∂z− g
⇔ ∂penv
∂z= −gρ env
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But our parcel experiences an acceleration
€
Dw
Dt=
D2δz
Dt 2=
−1
ρ parcel
∂penv
∂z− g
D2δz
Dt 2=
−1
ρ parcel
−gρ env( ) − g =gρ env
ρ parcel
− g
D2δz
Dt 2= g
ρ env
ρ parcel
−1 ⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟
D2δz
Dt 2= g
penv
RdTenv
⎛
⎝ ⎜
⎞
⎠ ⎟RdTparcel
pparcel
⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟−1
⎡
⎣ ⎢ ⎢
⎤
⎦ ⎥ ⎥= g
Tparcel
Tenv
−1 ⎛
⎝ ⎜
⎞
⎠ ⎟
use ideal gas law:
€
penv = pparcelRecall:
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Rearrange:
€
D2δz
Dt 2= g
Tparcel
Tenv
−1 ⎛
⎝ ⎜
⎞
⎠ ⎟
=g
Tenv
Tparcel − Tenv( )
=g
Tenv
δTparcel
δz−
δTenv
δz
⎛
⎝ ⎜
⎞
⎠ ⎟ δz
=g
Tenv
−δTenv
δz
⎛
⎝ ⎜
⎞
⎠ ⎟− −
δTparcel
δz
⎛
⎝ ⎜
⎞
⎠ ⎟
⎡
⎣ ⎢
⎤
⎦ ⎥ δz
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Back to our definitions of temperature change
€
Recall : Γ = −∂Tenv
∂z and Γd = −
∂Tparcel
∂z
€
D2δz
Dt 2+
g
Tenv
(Γd − Γ)δz = 0Second-order, ordinary differential equation:
€
D2δz
Dt 2=
g
Tenv
−δTenv
δz
⎛
⎝ ⎜
⎞
⎠ ⎟− −
δTparcel
δz
⎛
⎝ ⎜
⎞
⎠ ⎟
⎡
⎣ ⎢
⎤
⎦ ⎥ δz
⇔D2δz
Dt 2=
g
Tenv
(Γ − Γd ) δz
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Recall: Dry adiabatic lapse rate
Taking the logarithm of , differentiating with respect to height, using the ideal gas law and hydrostatic equation gives:
€
d − Γ =g
c p
+∂Tenv
∂z=
Tenv
θ
∂θ
∂z
€
d =g
c p
= −∂Tparcel
∂zdry adiabatic lapse rate (approx. 9.8 K/km)
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Rearrange:
€
D2δz
Dt 2+
g
Tenv
(Γd − Γ) δz = 0
D2δz
Dt 2+
g
Tenv
Tenv
θ
∂θ
∂z δz = 0
D2δz
Dt 2+
g
θ
∂θ
∂z δz = 0
D2δz
Dt 2+ N 2 δz = 0
With the Brunt-Väisälä frequency
€
N =g
θ
∂θ
∂z
⎛
⎝ ⎜
⎞
⎠ ⎟
1
2
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Buoyancy oscillations
€
D2δz
Dt 2+ N 2δz = 0
1) stable, the solution to this equation
describes a buoyancy oscillation with period 2/N
2) unstable, corresponds to growing
perturbation, this is an instability
3) neutral
€
with N =g
θ
∂θ
∂z
⎛
⎝ ⎜
⎞
⎠ ⎟
1
2= g
∂lnθ
∂z
⎛
⎝ ⎜
⎞
⎠ ⎟
1
2
€
∂∂z
> 0,N 2 > 0 :
€
∂∂z
< 0,N 2 < 0 :
€
∂∂z
= 0,N 2 = 0 :
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Solution to the differential equation
The general solution can be expressed via the exponential function with a complex argument:
€
z = Aexp(±iNt)
with A: amplitude, N: buoyancy frequency,
If N2 > 0 the parcel will oscillate about its initial level with a period = 2/N.Average N in the troposphere N ≈ 0.01 s-1
€
D2δz
Dt 2+ N 2δz = 0
€
i = −1
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Remember Euler’s formula
with x: real number
€
z = Re(Aexp(±iNt))
If N2 > 0 (real) the solution is a wave with period = 2/N (more on waves in AOSS 401)
€
exp(ix) = cos(x) + isin(x)
Physical solution:
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Stable solution:Parcel cooler than environmentz
Warmer
Cooler
If the parcel moves up and finds itself cooler than the environment then it will sink (and rise again). This is a buoyancy oscillation.
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Stable and unstable air masses
Picture an invisible box of air (an air parcel). If we compare the temperature of this air parcel to the temperature of air surrounding it, we can tell if it is stable (likely to remain in place) or unstable (likely to move).http://eo.ucar.edu/webweather/stable.html
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
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Temperature soundings
Sounding of the environment:T inversion
Sounding of the parcel
z