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1
The middle atmosphere and the parametrization of
non-orographic gravity wave drag
Peter Bechtold and Andrew Orr
2
Literature
• Holton, 2004: An introduction to Dynamic Meteorology, AP• Ern, M., P. Preusse, M.J. Alexander & C.D. Warner, 2004: Absolute values of
gravity wave momentum flux derived from satellite data. J. Geophys. Res., 109, D20103, doi:10.1029/2004JD004752
• Ern, M., P. Preusse & C.D. Warner, 2006: Some experimental constraints for spectral parameters used in the Warner and McIntyre gravity wave parameterization scheme. Atmos. Chem. Phys., 6, 4361-4381.
• Mc Landress, C. & J.F. Scinoccia, 2005: The GCM response to current parameterizations of nonorographic gravity wave drag. J. Atmos. Sci., 62, 2394-2413.
• Scinocca, J.F, 2003: An accurate spectral nonorographic gravity wave drag parameterization for general circulation models. J. Atmos. Sci., 60, 667-682.
• Warner, C.D & M.E. McIntyre, 1996: On the propagation and dissipation of gravity wave spectra through a realistic middle atmosphere. J. Atmos. Sci., 53, 3213-3235.
• Orr, Bechtold, Scinnoca, Ern, Janiskova, 2010 : JCL (see Lecture Note)• P. Bechtold et al. ECMWF Newsletter No 120, summer 2009
• Temperature decrease in the Troposphere is due to adiabatic decompression
• Midlatitude upper-tropospheric Jet form due to strong temperature gradient between Pole and Equator.
• The temperature in the Stratosphere increases due to the absorption of solar radiation by ozone
P (hP
a)
Typical Temperature and Zonal Wind profiles for July at 40S, together with the distribution of the 91-levels in the IFS. Tp denotes the Tropopause, Sp the Stratopause, the model top also corresponds to the Mesopause
Structure of the Atmosphere:
Troposphere & Stratosphere
CO2 and O3 zonal mean concentrations from GEMS as used in Cy35r3
SPARC
July climatology
• As heating due to ozone starts to decrease with height, so does the temperature
• Radiative heating/cooling of summer/winter hemispheres causes air to rise/sink at the summer/winter poles, inducing a summer to winter pole meridional circulation
• Coriolis torque induced by meridional circulation to produce easterly/westerly jets in the summer/winter hemispheres
• The large Coriolis torque implies the existence of some eddy forcing to balance the momentum budget. This is provided by the breaking/dissipation of vertically propagating planetary waves, and small-scale non-orographic gravity waves. GWs transport energy and momentum vertically
• Planetary waves add extra drag in the winter stratosphere, particularly northern winter (this is resolved by the model)
Structure of the Atmosphere: Stratosphere & Mesosphere
From Lindzen (1981)
Pronounced waviness in profiles due to gravity wavesVertical wavelength ~12km
Radiosonde observations of gravity waves
8OW 4OW 0O 4OE 8OE 12 OE 16 OE 20 OE 24 OE
60.5N1000
900800700600500
400
300
200
1009080706050
40
30
20
10
Cross section of - 19980317 1200 step 24 Expver F9I20.05 H-IFS 1279 Vertical velocity w
8OW 4OW 0O 4OE 8OE 12 OE 16 OE 20 OE 24 OE
60.5N1000
900800700600500
400
300
200
1009080706050
40
30
20
10
Cross section of - 19980317 1200 step 24 Expver F9I20.05 NH-IFS 3999 Vertical velocity w
(plotted at 1279 equivalent)
Jiang et al. 2005Seasonal climatology of gravity waves from UARS MLS
Latitudinal and seasonal dependence
Although no such thing as a gravity wave source climatology
Sources of (non-orographic) gravity waves: convection, fronts, jet-stream activity
Gravity wavesVertical wavelengths: 1-10s kmHorizontal wavelengths: 10s-1000s kmUnresolved or under-resolved by the IFS
Wave representation
( )
ˆ( , , ) ( ) cos( );
ˆ( , , ) ( )
; int
i k x mz t
g
t x z z k x mz t
t x z z e
frequency
k horizontal wavenumber
m vertical wavenumber
c horizontal phase speedk
c group velocityk
kU c c U rinsic frequency
and phas
e speed
What is a “ non-orographic” gravity wave?
Orographic gravity waves are supposed to be stationary (ω=0 => zero horizontal phase speed)
Non-orograpgic gravity waves are non-stationary, and therefore have non-zero phase speed. The parametrization problem is therefore 5-dimensional!
Depending on gridpoint j, height z, wavenumber k, frequency ω, and direction Φ
( , , , , )j z k
• Define a launch spectrum
• Define the relation between ω and κ. This is called the dispersion relation, and depends on the equation system (hydrostatic or non-hydrostatic shallow water) used to derive the waves (see Appendix in convection Lecture Note).
• Define in which physical space one wants to propagate the wave spectrum: either ω-κ coordinate frame or -m resp.
-m coordinate frame. For practical reasons one wants to have conservative
propagation from one level to the other as long as there is no dissipation.
• Define dissipation procedure, i.e. critical level filtering + some adhoc nonlinear dissipation mechanism to account for wave braking as amplitude increases with height due to decreasing density.
How to proceed for a simple parametrization
c
2 2 2 22
2 2 2
2 2
2 2
(1 / )
(1 / )
/ 0 ( )
/ 0 ( )
k N Nm
f
N hydrostatic wave dynamics
f no rotation
Dispersion relation for gravity waves
2
2
2
222
~~ c
NNkm
Wavelike solutions exist for
For simplicity we only consider hydrostatic, non-rotational waves which also allows to ignore the effect of back reflection of waves
Wavelike solutions exist for and critical level filtering occurs when the intrinsic phase speed approaches zero
2 0
2 2 2f N
Gravity wave source: launch globally constant isotropic spectrum of waves at each grid point as function of, for example, c. Assume constant input momentum flux
Consist of spectrum of waves via hydrostatic non-rotational dispersion relation
2
2
2
222
~~ c
NNkm
UcckUkmhz
~;~;2
;2
U: background wind; N: buoyancy
N
EW
S
Net input momentum flux = 0
Rely on realistic winds to filter the upward propagating (unrealistic) gravity wave source.
Physically based gravity wave scheme
Simplified hydrostatic non-rotational version of Warner and McIntyre (1996) scheme (Scinocca, 2003) - WMS
ts
ps
mm
N
m
mBmE
1
~),~,(ˆ
20
0
1. In any azimuth, φ, the launch spectrum is specified by the total wave energy per unit mass,
2. This is chosen to be the standard form of Fritts and VanZandt (1983), which in space is
3. It is assumed to be separable in terms of and .
4. The dependence on is
5. With
6. is a transitional wavenumber estimated to correspond to in the troposphere (Allen and Vincent, 1995)
7. The ‘tail spectrum’ is largely independent of time, season, and location (VanZandt, 1982)
0E
~m
** for ˆ and for ˆ mmmEmmmE ts
m ~
m
1, 3, 1 5 / 3s t p
km 32 z*m
large-m (small vertical scale) waves saturate at low altitudes
Increase in wave energy as the waves propagate vertically
m-3 slope
Larger scale wave saturate at progressively greater heights
3/5~
Observations: Fritts and VanZandt (1983) and VanZandt (1982)
Convective instabilities and dynamic (shear) instabilities (+ other not well understood processes) act to limit gravity wave amplitudes – gravity wave saturation
t=3
p=5/3
Theoretical considerations set 1≤p≤5/3 (Warner and McIntyre (1996))
unsaturated saturated
sm tm
m
Wav
e m
om
entu
m f
lux
• The characteristic vertical wavenumber m*, separating the saturated and unsaturated slopes is 2π/ 2km, with 2km the characteristic vertical wavelength.
• There is one free parameter in the scheme that allows to shift the saturation curve (dashed blue curve) to the right, with the result that non-linear dissipation is occuring at greater heights. As we will se, and as documented in the literature, this has important consequences for the simulation of the QBO
1) Specify in terms of momentum flux spectral density, using group velocity rule
2) Where
3) are not invariant to vertical changes in U(z) and N(z) (i.e. dependent variables). Hence spectral elements in space are also not invariant. Consequently are not conserved for conservative propagation, i.e. Eliassen-Palm theorem
4) Chose space to describe the vertical propagation of the wave field
5) Use dispersion relationship to remove out dependence on , and integrate out dependence on , which gives (Scinocca, 2003)
00F
dynamics chydrostatifor /~/~ mmcgz
~ and m
),~,(ˆ~),~,( 0000
mEk
cmF gz
),~,( and ˆ000 mFE~ and m
c
3
2
)(1
1),(
s
p
N
Ucmc
Uc
N
UcAcF
p
fNBmA
pp
2
2203
where the constant A is all terms which are independent of height
0 0 and U U U c c U
m ~
Non-hydrostatic limits (more physically realistic)
2( , , )( , , ) ( , , );
( , , )
m mF c J F m J
c N
Galilean Transform
At the launch level we have
Which sets an absolute lower bound for critical level absorption of .
If on the next vertical level z1(>z0) U increases such that
then waves with phase speeds in the range
encounter critical level absorption and the
momentum flux corresponding to these phase speeds is
removed from
How to do critical level absorption
0 1U c U
0 0U
0c
1 0U U
( , )F c N
S
EW
U0
Convective instabilities and dynamic (shear) instabilities (+ other not well understood processes) act to limit gravity wave amplitudes – gravity wave saturation
Results in the “universality” of the GW spectrum m-3 (Smith et al. 1987)
WMS scheme deals with non-linear dissipation in an empirical fashion by limiting the growth of the GW spectrum so as not to exceed saturated spectrum ~m-3.
1) Achieved by specifying a saturation upper bound on the value of the wave energy density at each level with the observed m-3 dependence at large-m
2) Which can be expressed as
3) Unlike the unsaturated spectrum, , the saturated spectrum is not conserved, ,and so decreases in amplitude with height as a result of diminishing density. This limits to a saturation condition
psat Nm
mCmE
~),~,(ˆ 2
3
p
sat
c
Uc
N
UcBCcF
2
),(
F satFF
),(),( cFcF sat
Parameter specification1) t=3 (fixed)2) p=1 (or 3/2 ; observed/theoretical )3) s=1 (or 0,1; s=1 most common, ie positive slope required)4) m* (= , see Ern et al (2006))5) C* (=1; raising this raises the height momentum is
deposited)6) φ =4 (number of azimuths, although can have 8, 16, …)
7) z0 (launch level; Ern et al. (2006) suggests either 450 hPa or 600 hPa)
8) input momentum flux into each azimuth is set to 3.75 x10-3 (Pa)
3/51 p
Ern, Preusse, and Warner, ‘Some experimental constraints for spectral parameters used in the Warner and McIntyre gravity wave parameterization scheme’, Atmos. Chem. Phys., 6, 4361-4381, 2006
00F
km2/2
Procedure1) Check for critical level absorption, i.e. if U increases such that
then waves with phase speeds in the range
2) Phase speeds which survive critical level absorption propagate conservatively to next level
3) Possible nonlinear dissipation is modelled by limiting the momentum flux
4) Implies for , that the momentum flux corresponding to these phase speeds is removed from and deposited to the flow in this layer, and that is set equal to .
5) Repeat procedure for subsequent layers and all azimuths
6) Results in momentum flux profiles used to derive the net eastward, , and northward, momentum flux
7) The wind tendency (i.e. gravity wave drag) in each of these directions is given by the vertical divergence of the momentum flux
Discretize speeds phase using cnc
min max0.25 m/s; 100 m/s; 20 50cc c n
)()( 01 zUzU
)()( 10 zUczU
),(),( cFcF sat
),~( cF
satF),~( cF
EFNF
p
FFg
t
VU NE
,,
Co-ordinate stretch applied: higher resolution at large-c (i.e. small-m)
Evaluation: Run ensemble of T159 (125 km) 1-year climate runs and compare mean circulation and temperature structure against SPARC dataset
• Cy35r2 (operational from 10 March 2009). Uses so called Rayleigh friction, a friction proportional to the zonal mean
wind speed, to avoid unrealistically high wind speeds (polar night jet) in middle atmosphere. The trace gas climatology (CO2, CH4 etc) consist of globally constant values, apart
from ozone
• Cy35r3 (became operational September 2009) includes a new trace gas climatology (GEMS reanalysis + D. Cariolle
fields), zonal mean fields for every month, and the described non-orographic gravity wave parametrisation
January
NH
July
SH
Polar winter vortex
ERA
Cy35r2
Operational since March 2009
Cy35r3
Operational in summer 2009 with GWD + GHG
SH wintertime vortex is quasi-symmetric, but not NH polar vortex, due to braking quasi-stationary Rossby waves emanating in the troposphere
Distinguishing between resolved and unresolved (parametrized) waves.
the Eliassen Palm flux vectors
• EP Flux vectors give the net wave propagation for stationary Rossby waves (derived from quasi-geostroph. eq.)
• Stationary Rossby waves are particularly prominent in the NH during winter. They propagate from the troposphere upward into the stratosphere
1* * * *cos , cos /
; ; .
*
& 1992, .388
EPVector R u v f R v p
f Coriolis latitude pot temperature
denote anomalies from zonal mean
Peixoto Oort p
ERA40
Resolved stationary Rossby waves: EP-Fluxes in Winter
Cy35r3-ERA40
Stationary Rossby waves are particularly prominent in the NH during winter. They propagate from the troposphere upward into the stratosphere
ERA40
Resolved stationary Rossby waves: EP-Fluxes in Summer
Cy35r3-ERA40
35r2
SPARC
July climatology
30
35r3
SPARC
July climatology
U Tendencies (m/s/day) July from non-oro GWD
32Comparison of observed and parametrized GW momentum flux for 8-14 August 1997 horizontal distributions of absolute values of momentum flux (mPa) Observed values are for CRISTA-2 (Ern et al. 2006). Observations measure temperature fluctuations with infrared spectrometer, momentum fluxes are derived via conversion formula.
Momentum fluxes from CRISTA campaign compared to parametr non-orogr GW momentum
fluxes
33
Total = resolved +parametrized (orograph+non-orograph.) wave momentum flux
Conclusions from comparison against SPARC & ERA-Interim reanalysis
• Polar vortex during SH winter quasi symmetric, but asymmetric NH winter polar vortex, due to vertically propagating quasi-stationary Rossby waves (linked to mountain ranges)
• In Cy35r2 (no GWD parameterization) SH polar vortex too strong, westerly polar night Jet is wrongly tilted with height, large T errors in mesosphere. Jet maximum in summer hemisphere easterly jet at wrong height (at stratopause instead of mesopause)
• In Cy35r3 improved tilt of the polar Jet with height towards the Tropics, allover improved winter hemisphere westerly and summer hemisphere easterly jets. The smaller warm bias around the stratopause is due to the improved greenhouse gas climatology
• Results qualitatively similar for January, invert NH and SH
35r2
SPARC
January climatology
35r3
SPARC
January climatology
U Tendencies (m/s/day) January from non-oro GWD
The QBO
Prominent oscillations in the tropical middle atmosphere are
• A quasi bi-annual oscillation in the stratosphere, and a
• Semi-annual oscillation in the upper stratosphere and mesosphere
These oscillations are wave induced. Whereas the waves are moving upward, these oscillations propagate downward. Why ? Waves deposit momentum at critical level, wind changes, and so does the critical level, etc
In the following 6-year integrations are carried out with Cy35r2, Cy35r3, and one sensitivity experiment with Cy35r3, but shifting the saturation spectrum to the right ->shifting wave braking to higher altitudes.
QBO : Hovmöller from free 6y integrations
=no nonoro GWD
Resolution dependent resolved gravity-wave activity as seen in Omega field (Pa/s) August 2006
Resolution dependence: Monthly mean parameterized (non-orographic & orographic ) & resolved momentum fluxes (mPa) August 2006
42
Resolution dependence Cy35r3 August