Gravity waves: introduction and global view · Gravity waves: introduction and global view Peter...
Transcript of Gravity waves: introduction and global view · Gravity waves: introduction and global view Peter...
Gravity waves:
introduction and global view
Peter Preusse
ECMWF, 2 September 2015 – p. 1
Structure of middle atmosphere
✇ dudt − fv + 1
ρ∂p∂x = X
✇ steady state, zonal mean: ⇒ −fv = X
✇ X by planetary waves and gravity waves
Decelleration of westerlies induces poleward circulation
ECMWF, 2 September 2015 – p. 2
GW impact on middle atmosphere
QBO
summer winter
surface temperatures,
precipitation(via QBO, circulation, NAO)
Brewer-Dobson
circulation-trend
cold summermesopause
Brewer-Dobson
circulation
tro
po
sp
here
str
ato
sp
here
meso
sp
here
radiative forcing
PSC
mesospheric circulation
cirrus, dehydration(direct and indirect)
Red: Processes which are driven to >50 % by GWsPurple: Indirect effects
ECMWF, 2 September 2015 – p. 3
General circulation → polar troposphere
Sigmond and Scaife, J. Clim., 2010
Sensitivity on the strength of GW momentum flux
ECMWF, 2 September 2015 – p. 4
QuasiBiennialOscillationQBOAfter:
B. Naujokat
ECMWF, 2 September 2015 – p. 5
QBO → tropospheric weather
from Marshall and Scaife, JGR, 2009
Difference of mean winter temperature between QBOphases in NCEP/NCAR reanalysis data.
ECMWF, 2 September 2015 – p. 6
gravity wave=
buoyancy wave6=
gravitational wave
ECMWF, 2 September 2015 – p. 7
Isolated Airparcel
temperature profileof backgroundatmosphere
air parcel
Γ
∆Τ
∆ρ
z
T
g
ECMWF, 2 September 2015 – p. 8
Plane Wave
cφ
warmco
lddownupward
ECMWF, 2 September 2015 – p. 9
Plane Wave
warmwarm
colddown
λx
λz
cφ
upward
upward
ECMWF, 2 September 2015 – p. 9
Plane Wave
s
dx
dz
φ
φ
warmco
ldwarm
λ x
λ z
upward
downward
upward
cφ air parcel
r
Ngr
avity
F
restorin
g force
F
ω2 =k2hN
2
k2h +m2(1)
ECMWF, 2 September 2015 – p. 9
Dispersion relation
The full dispersion relation in 3D, including exponentialdensity decrease and Earth rotation is:
ω2 =(k2 + l2)N2 + f2
(
m2 + 1
4H2
)
k2 + l2 +m2 + 1
4H2
≃ ω2 =k2hN
2
k2h +m2+ f2 (2)
This implies
N2 > ω2 > f2 (3)
⇒ phase velocity and group velocity
~cφ =ω
|~k|
(
k
|~k|,l
|~k|,m
|~k|
)
, ~cg =
(
∂ω
∂k,∂ω
∂l,∂ω
∂m
)
(4)
ECMWF, 2 September 2015 – p. 10
Plane Wave
warmwarmdown
λx
λz
cφ
upward
upward
cold
cg
Fpx = ρ(1− f2
ω2 )u′w′; X = −1
ρ∂∂zFpx
ECMWF, 2 September 2015 – p. 11
Wave clouds at Juelich
ECMWF, 2 September 2015 – p. 12
Wave clouds at Juelich
ECMWF, 2 September 2015 – p. 13
Saturation AmplitudeToo large amplitudes result in convectively instable layers.
Wave solution:
N2 > 0 ⇔ Γ = −10K
km<
∂(T + T sin(mz))
∂z(5)
⇒ Maximum stable amplitude:
Tmax =N2T
gm=
N2T
g2πλz (6)
Doppler shift
ω = ωgb − ~kh~U (7)
ω2 =k2hN
2
m2⇒ λz =
∣
∣
∣
∣
2πc− U||
N
∣
∣
∣
∣
(8)
ECMWF, 2 September 2015 – p. 14
The key quantitiesVery simplified:
✇ The momentum flux decides how much drag can beexerted
✇ The phase speed decides where the drag is exerted✇ The direction decides whether the drag accelerates or
decelerates the background flow
ECMWF, 2 September 2015 – p. 15
CRISTA-1 (1994)
Eckermann and Preusse, Science, 1999
F ∝ λz
λh(T ′)2
ECMWF, 2 September 2015 – p. 16
Absolute values of momentum fluxCRISTA-2, August 1997 F ∝ λz
λh(T ′)2
Absolute Values of Momentum Flux [mPa]
60N
40N
20N
Equ
20S
40S
60S
180W 150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180E
60N
40N
20N
Equ
20S
40S
60S
180W 150W 120W 90W 60W 30W 0 30E 60E 90E 120E 150E 180E
CRISTA Warner & McIntyre
Ern et al., JGR, 2004Orr et al., J. Clim., 2010 :GW scheme in ECMWF confined by CRISTA
ECMWF, 2 September 2015 – p. 17
HIDLS: Annual cycle for 2006
Ern et al., JGR, 2011
ECMWF, 2 September 2015 – p. 18
ECMWF global data and HIRDLS
abs.
GW
mom
entu
m fl
ux [m
Pa]100
10
1
0.1
0.01
Preusse et al., ACP, 2014
✇ Single day vs. whole month✇ General good agreement✇ Low latitudes: Satellites show hot spots over continents
ECMWF, 2 September 2015 – p. 19
Yonsei Convective GW Source
Choi et al., JGR, 2009ECMWF, 2 September 2015 – p. 20
Yonsei Convective GW Source
Song and Chun, JAS, 2005
Trinh et al., in preparation ECMWF, 2 September 2015 – p. 21
10 years of SABERla
titud
e [d
eg]
SABER GW momentum flux [ 10-3 Pa ]
50
-50
0
JAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJO2002 2003 2004 2005 2006 2007 2008 2009 2010
1.80
1.35
0.90
0.45
0.00
z = 30.0 km
λz= 5-25 km
Ern et al., JGR, 2011 ECMWF, 2 September 2015 – p. 22
SABER: poleward shiftla
titud
e [d
eg]
SABER GW momentum flux [ 10-3 Pa ]
z = 70.0 km
50
-50
0
JAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJOJAJO2002 2003 2004 2005 2006 2007 2008 2009 2010
0.10
0.08
0.06
0.04
0.020.00
λz= 5-25 km
Ern et al., JGR, 2011 ECMWF, 2 September 2015 – p. 23
QBO is wave-driven
wind u+c-c
Wave drag is exerted in the direction of phase speed c
= 2 c-u Nzλ π
cf. Lindzen and Holton, A theory of the Quasi-Biennial Oscillation, JAS,1968
ECMWF, 2 September 2015 – p. 24
QBO forcing
Ern et al., JGR, 2014
ECMWF, 2 September 2015 – p. 25
QBO forcing
Ern et al., JGR, 2014
ECMWF, 2 September 2015 – p. 26
Comparison of GCM and measurements
SPARC gravity wave initiative(reinstated in 2008; lead Joan Alexander).
A Comparison Between Gravity Wave Momentum Fluxes inObservations and Climate Models
Marvin A. Geller, M. Joan Alexander, Peter T. Love,Julio Bacmeister, Manfred Ern, Albert Hertzog, Elisa Manzini,Peter Preusse, Kaoru Sato, Adam A. Scaife, and Tiehan Zhou
J. Clim., 2013
ECMWF, 2 September 2015 – p. 27
Zonal mean climatologies
✇ general agreement of shape✇ quantitative agreement (better factor 2) in winter vortex✇ indicates problem at summer high latitudes
ECMWF, 2 September 2015 – p. 28
ECMWF, 2 September 2015 – p. 29
Findings and challenges
✇ General shape of global distribution bymodulation of tropospheric GWs by background windindividual sources such as orography, convection,spontaneous adjustmentlarge GWMF in southern winter: source still not fullyexplained
✇ sources need to be quantified✇ QBO driving seams clear, but direction could induce
artifacts✇ vertical gradient, propagation and dissipation need
good accuracy measurements
ECMWF, 2 September 2015 – p. 30
Infrared limb imaging from space
✇ 3D images of GWs✇ vertical resolution
about 1.0 km✇ temperature
precision0.5-1.0 K
✇ sources bybackwardray-tracing
ECMWF, 2 September 2015 – p. 31
Assessment
By simulated measurements (limb soundings) throughECMWF data fields:
✇ Infrared limb imaging will be able to measure all threekey quantities:
GW momentum fluxdirectionphase speed (inferred)
✇ zonal mean net GWMF accurate to ∼30 %✇ independent values at several altitudes throughout
entire stratosphere
ECMWF, 2 September 2015 – p. 32
Summary✇ Gravity waves are important for e.g. QBO and
Brewer-Dobson circulation and thus for climateprojection
✇ Gravity wave sources include: orography, convection,spontaneous adjustment, ?
✇ Oblique propagation is important → parametrisation vsresolved?
✇ GCM ↔ observations: General distributions betweenmodels and measurements are coarsly realistic
✇ Uncertainty ranges are large (factor 2 at best (e.g.30km, SH winter), order of magnitude at worst).
✇ 3D distributions from limb imager would be mostimportant break-through
ECMWF, 2 September 2015 – p. 33
Coupling by propagating waves
Global equation of motion (wind):
du
dt− fv +
1
ρ
∂p
∂x= X (9)
Separate into large scale u and GWs u′, u = u+ u′.Acceleration for u by GWs:
X = −1
ρ
∂
∂zFpx (10)
Pseudomentum flux:
Fpx = ρ(1−f2
ω2)u′w′ (11)
ECMWF, 2 September 2015 – p. 34