richard rotunno national center for atmospheric research, usa dynamical mesoscale mountain...
TRANSCRIPT
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Richard RotunnoRichard Rotunno
National Center for Atmospheric Research , USA
Dynamical Mesoscale Mountain Meteorology
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• Dynamic Of or pertaining to force producing motion
• Meso-(intermediate)scale Length ~ 1-100km Time ~1h – 1day
• Mountain Meteorology Science of atmospheric phenomena caused by mountains
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Topics
Lecture 1 : Introduction, Concepts, Equations
Lecture 2: Thermally Driven Circulations
Lecture 3: Mountain Waves
Lecture 4: Mountain Lee Vortices
Lecture 5: Orographic Precipitation
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Thermally Driven Circulations
Whiteman (2000)Whiteman (2000)
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Jane
Eng
lish
Mt.
Sha
sta
Mountain Waves
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Haw
aii
Mountain Lee Vortices East
Spa
ce S
hutt
le
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Orographic Precipitation
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What do all these phenomena have in common?
Buoyancy
par
parenvgBρ
ρρ −=
δenvρ
parρg
Displacement
= density“env” = environment“par” = parcel
ρ
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Buoyancy is acceleration
gz
ppar
parpar ρδρ −
∂∂
−=&&
gz
p
z
penv
envpar ρ−=∂
∂≈
∂∂
Bgpar
parenv ≡−
=ρ
ρρδ&&
forcesofsumonacceleratimass
=×
To a goodapproximation...
= pressure = vertical coordinatezp
δenvρ
parρg
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δρ
ρρ 2NgBpar
parenv −≡−
=
( ) ( ) δρρρδρρρpar
envparenv
envenv zz ∂
∂+≅
∂
∂+≅ 0;0
⎟⎟⎠
⎞⎜⎜⎝
⎛
∂∂
−∂∂
−=parenv zz
gN
ρρ
ρ2
Stability
= “static stability” = “Brunt-Väisälä Frequency”NN 2
= 0 for incompressibledensity-stratifed fluid
(unstable)(stable) 0,0 22 <> NN
(unstable)(stable) 0,0 >×<× BB δδ
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Air is a compressible fluid…
0≠∂∂
parzρ
RT
p=ρGas Law
⎟⎟⎠
⎞⎜⎜⎝
⎛
∂∂
−∂∂
=parenv z
T
z
T
T
gN 2
1st Law of Thermo (adiabatic)
18.91 −−≅
∂∂
⇒−≈∂
∂=
∂∂ kmC
zTg
zp
zTc
par
par
parparp
o
ρ
in terms of Temperature2,NB
= specific heat at constant pressure , R = gas constant for dry airpc
env
envpar
T
TTgB
−=
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Air Parcel Behavior in a Stable Atmosphere
)(zTenv
)(δparT
00 2 <−==⇒> δδδ NB&&
δ
Temperature
z
02 >N
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Air Parcel Behavior in an Unstable Atmosphere
)(zTenv
)(δparT
δ
Temperature
z
02 <N
00 2 >−==⇒> δδδ NB&&
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env
envpargBθ
θθ −=
pcR
mb
pT⎟⎠⎞
⎜⎝⎛≡≡1000
; ππ
θ
in terms of potential temperature BN ,2 θ
⎟⎟⎠
⎞⎜⎜⎝
⎛
∂∂
−∂∂
=parenv zz
gN
θθ
θ2
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Air Parcel Behavior in Stable or Unstable Atmosphere
δ
Potential Temperature
)()(
stablezenvθ
)()(
unstablezenvθ
.ctpar =θ
z
envz
gN
∂∂
=θ
θ2
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Dynamic Mesoscale Mountain Meteorology
Governing Equations
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In terms of and …..
1st Law of Thermodynamics
Dt
Dp
Dt
DQ
Dt
DTcp ρ
1+=
Dt
DQ
cDt
D
pπθ 1
=π
Dt
DQ
Dt
cDT p =
)ln( θWith previous definitions
Common form…
θ
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Newtons 2nd Law
FkgcDt
uDp
rr+−∇−= ˆπθ
xx
y
z
),,( wvuu =r
zyx kji ∂+∂+∂=∇ ˆˆˆ
zyxt wvuDt
D∂+∂+∂+∂=
pcp ∇=∇ρ
πθ 1
With previous definitions
= frictional force/unit massFr
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01
=⋅∇+ uDtD rρ
ρ
),( θπρρ =
Mass Conservation
uDt
D r⋅∇=⎟⎟
⎠
⎞⎜⎜⎝
⎛⎟⎠⎞
⎜⎝⎛ −+ π
κθ ln
11ln
With previous definitions
pcR≡κ
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Summary of Governing Equations
FkgcDt
uDp
rr+−∇−= ˆπθ
Dt
DQ
cDt
D
pπθ 1
=
uDt
D r⋅∇=⎟⎟
⎠
⎞⎜⎜⎝
⎛⎟⎠⎞
⎜⎝⎛ −+ π
κθ ln
11ln
Conservation of
momentum
energy
mass
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Simplify Governing Equations I
kgcDt
uDp
ˆ−∇−= πθr
0=DtDθ
uDt
D r⋅∇=⎟
⎠⎞
⎜⎝⎛ −
πκ
ln11
Conservation of
momentum
energy
mass
Neglect molecular diffusion 0,0 == FQr
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Simplify Governing Equations II
0000 1)( θππθ
pzp c
gzzgc −=⇒−=∂
Conservation of momentum
Boussinesq Approximation
1~
,1
~~,
~
0000 <<<<+=+= π
πθ
θπππθθθ
kBkg
cDt
uDp
ˆˆ~
)~(0
0 +−∇=+−∇≅ ϕθθπθ
r
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Simplify Governing Equations III
0=DtDB
Conservation of energy
θθθ ~0 +=With
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Simplify Governing Equations IVConservation of mass
uDt
D r⋅∇=⎟
⎠⎞
⎜⎝⎛ −
πκ
ln11
)1000/ln(ln mbpκπ =By definition
uDt
Dp
c
r⋅∇=− 2
1ρ
0≅⋅∇ ur
1,1,122
22
2
2
<<<<<<cgL
cL
cU ω3 conditions for effective
incompressibility(Batchelor 1967 pp. 167-169)
= speed of soundc
= velocity, length, frequency scales
ω,,LU
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Summary of Simplified Governing Equations
kBDt
uD ˆ+−∇= ϕr
0=DtDB
0=⋅∇ ur
Conservation of
momentum
energy
mass
Still nonlinear (advection)
Filtering equations for mean / turbulent fluxes ofB and u (see Sullivan lectures)
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Summary
- Buoyancy is a fundamental concept for dynamical mountain meteorology
- Boussinesq approximation simplifies momentum equation
- For most mountain meteorological applications, velocity field approximately
solenoidal ( )0=⋅∇ ur