Coauthors: Kaoru Sato1, Shingo Watanabe2, Yoshio Kawatani2, Yoshihiro Tomikawa3, MasaakiTakahashi1

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Transport and mixing in the extratropical
tropopause region in a high vertical resolution
GCM
Kazuyuki Miyazaki
Japan Agency for Marine-Earth Science and
Technology
Co-authors Kaoru Sato1, Shingo Watanabe2,
Yoshio Kawatani2, Yoshihiro Tomikawa3,
MasaakiTakahashi1 1. Univ. of Tokyo, 2. JAMSTEC,
3. NIPR
2

• Just above the extratropical tropopause strong
  temperature gradient forms a TIL, and the
  chemical transition layer also lies at this
  level.
• The similar locations of the TIL and ExTL imply
  that the chemical and thermal structures interact
  with each other in the extratropical tropopause
  region (e.g., Randel et al., 2007).

• However, the relationship between the mechanisms
  of formation of the TIL and the ExTL is still
  unclear.
• In addition, relative importance of transport
  processes at different scales has not been fully
  understood.
• To clarify these processes, we used a vertically
  high resolved GCM.

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KANTO project A high vertical resolution GCM
T213L256 GCM vertical resolution 300 m,
top85km horizontal resolution 0.5625 No
gravity wave drag parameterization
Gravity wave propagations and the induced
circulation are explicitly simulated. allows
analyzing fine thermal/dynamical structure around
the tropopause. The importance of variously
scaled atmospheric processes can be examined.
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QBO variations
Gravity wave propagations
(Watanabe et al., 2008)
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Comparisons with a coarse-resolution GCM (?z?1km)
N2
MPV
tropopause based-coord.
Lat.
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Mean-meridional circulation January
Mean vertical wind W
Mean meridional wind V
4PVU
Compared to a coarse resolution model, the high
resolution model has stronger convergence of
the mean downward velocity in the lowermost
stratosphere stronger and sharper meridional
divergent flows in both the tropical Hadley
circulation and the extratropical direct
circulation
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The PV is a material invariant (i.e., passive
tracer) under inviscid (frictionless) and
adiabatic approximation (Hoskins, 1985).
Continuity equation of isentropic PV
Zonal mean equation based on the mass-weighted
isentropic zonal means (MIM)
Non-conservative term reflects changes in static
stability by diabatic processes (e.g., heating
at higher altitudes stabilizes the atmosphere and
produces the PV).
Analysis of maintain mechanisms of vertical PV
gradients
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PV budget analysis results
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Maintain mechanisms of PV gradients
EDYEDZsource
July
January
imply that the formation of large tracer
concentration gradients around the ExTL mainly
arises from mean downward transport in the lower
levels and isentropic (winter) and vertical
(summer) mixing in the upper levels.
MEY1
MEY2
MEY1
MEY2
MEZ1
MEZ2
MEZ2
MEZ1
EDZ
EDZ
EDY
EDY
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N2 50N-60N
TIL formation mechanisms
dN2/dt 50N-60N at TIL
In the isentropic formulation, eddy heat
transport terms are eliminated.
Thermodynamic analysis
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dN2/dt vertical profiles

• N2 increases during summer
• In the upper part TIL
• Radiation processes
• In the lower part TIL
• Downward advection of heat

Thermodynamic analysis
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July
dN2/dt by radiation
Log(H2O)

• Coarse-resolution model
• Excessive diffusion (adiabatic and diabatic
  eddies)? too small H2O gradient and N2 increase
  in the TIL

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Relative contributions of atmospheric motions
with different scales to mean-meridional
circulation and mixing
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Downward control calculation
PW n1-3
MW n4-20
GW n21-
Div. EP-F
Stream func.
January
?
V
Lat.
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Seasonal variation of horizontal and vertical
mixing
Horizontal diffusion coefficient
Vertical diffusion

• - Strong isentropic mixing between 20 K below and
  10 K above the tropopause
• Vertical eddy mixing are substantial in the
  tropopause region during summer, but are strongly
  suppressed just above the well-mixed region

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50N-60N January
Overworld stratosphere
400K
Middleworld stratosphere
Isentropic mixing
TIL
?
Zonal wave number (s)
Overworld stratosphere
400K
Middleworld stratosphere
TIL
?
Integrated flux
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50N-60N January
Overworld stratosphere
400K
Middleworld stratosphere
Vertical mixing
TIL
?
Zonal wave number (s)
Overworld stratosphere
400K
Middleworld stratosphere
TIL
?
Integrated flux
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PV for ngt21 at 360K ? Snap-shot picture
The small-scale disturbances occur over possible
sources of gravity waves (high mountains,
cyclones, fronts, and convection), suggesting
that the propagation and breaking of gravity
waves lead to active 3-D mixing in the TIL.
January
?Monthly mean amplitude
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Discussions GWs and small-scale mixing around
the TIL

• Following the linear wave theory, a large static
  stability allows the occurrence of
  large-amplitude GWs due to increase of the
  saturation amplitude.

• Even when GWs do not break, large-amplitude GWs
  can be dissipated due to radiative relaxation and
  can cause eddy vertical dispersions (under
  non-uniform diabatic heating rate fields).

- In the upper part of and just above the TIL,
GWs may easily reach their saturation limit and
breaking level because of a decrease in static
stability (and atmospheric density) with height.
- These situations associated with rapid changes
in static stability and the dissipation and
saturation of GWs seem to cause small-scale 3-D
mixing around the TIL.
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Summary

• The formation of large PV (or tracer) gradients
  around the ExTL mainly arises from mean downward
  transport in the lower levels and isentropic
  (during winter) and vertical (during summer)
  mixing in the upper levels.

• The locations of the TIL and the ExTL can be
  similar due to common dynamic processes (for
  constituent and heat) and interactions between
  constituent distributions and thermal structure.

• The small-scale dynamics associated with GWs play
  important roles in driving tracer transports by
  both the mean-meridional circulation and 3-D
  mixing. It may be important to include these
  small-scale dynamic effects in GCMs or CTMs to
  obtain better simulations of the TIL and ExTL.

Miyazaki et al., Transport and mixing in the
extratropical tropopause region in a high
resolution GCM - Part 1 Potential vorticity
and heat budget analysis, JAS (revised) -
Part 2 Relative importance of large-scale and
small-scale dynamics, JAS (submitted)
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Eddy PV transport fluxes U
Eddy transport plays an important role in
exchanging air between the stratosphere and
troposphere through processes such as
stratospheric intrusions, cutoff lows, and
gravity wave breakings, especially around the
subtropical jet stream.
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