1. Extratropical stratoshere-troposphere exchange in a 20-km-mesh AGCM
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Extratropical stratoshere-troposphere exchange in a 20-km-mesh AGCM
Ryo Mizuta
(Meteorological Research Institute / AESTO)
Hiromasa Yoshimura
(Meteorological Research Institute)

2. Introduction
Transport and mixing processes around UTLS region includes very fine filamental structures, but these cannot be simulated by conventional GCMs.
We had to restrict to regional models or two dimensional models in order to represent these processes.
Water vapor image (Meteosat)
Isentropically advected “controur” of PV isolines of 4 days before
Appenzeller et al. (1996)

3. AGCM with the grid size of 20km
Long-term simulations by a high-resolution AGCM improve the representation of regional-scale phenomena like tropical cyclones and that of local climate, due to better representation of topographical effects and physical processes.
OBS
Model
Surface temperature climatology
Precipitation climatology
OBS
Model

4. In addition to near-surface phenomena, the model can resolve
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In addition to near-surface phenomena, the model can resolve
sharp tropopause
filamental structures near the tropopause
PV 315K [PVU=10-6Km2s-1kg-1 ]
Using this high-resolution model, we have investigated
where and how the transport and mixing occur
what depends on the model resolutions PV
Water Vapor

5. Climate Simulation on the Earth Simulator
JMA/MRI AGCM -- used both for the operational numerical weather prediction and climate researches
TL959 (grid size of about 20km, 1920x960)
60 vertical layers with top at 0.1hPa (interval is ~900m at 250hPa)
Dynamics: Semi-Lagrangian Scheme (Yoshimura, 2004)
Cumulus parameterization:prognostic Arakawa-Schubert (Randall and Pan,1993)
Radiation: Shibata et al. (1999), Gravity wave drag: Iwasaki et al. (1989)
Time integrations over 20 years (as the “control” run against the global warming simulation) using climatological SST
Pick up one January and one July of a certain year because very huge data size is required
The horizontal resolution dependence is also examined using the coarse resolution (200km) model (TL95L40, almost the same settings as the TL959 model).
spin-up year integration
Jan Jul

6. Model Climatology zonal-mean U・T (DJF)
ERA40 Reanalysis ( ) TL959L60 (20years) TL959L60 – ERA40
＋ －
■ ■ 95% significant difference
The model's ability of simulating the present-day climate has been confirmed from global scale through small scale in the sense of seasonal mean (Mizuta et al. 2006).

7. Model Climatology Jet stream Storm tracks U300 [m/s] NH DJF
stddev of Z days bandpass-filtered (DJF)
ERA40( ) TL959L60 (20years)

8. Quantification of Transport by Passive Tracer Advection
1. Tracer initialized to 1 only above the 2PVU tropopause
2. Tracer advection for 24 hours without source/sink
3. Compare with the tropopause at final time
Gray (2006)
χ= 1 at PV > 2 (PVU)
χ= 0 at PV < 2 (PVU)
χ at PV < 2 : ST
1 – χ at PV > 2 : TS
Semi-Lagrangian advection scheme, same as the model dynamical core
3D online calculation
initialized to 0 or 1 at 00UTC every day
Averaged over 30-day calculations
Day

9. Vertical distribution (20N-90N, Jan)
Strat.  Trop Trop.  Strat Net
TL959
TL95
TL959
TL95
TL95
TL959
Less transport in each direction above 400hPa in the high-resolution model, but more exchange below 500hPa.
Net transport does not much depend on model resolution. Vertically integrated amount is consistent with the residual mean stratospheric circulation (1-2 x 1010 kg/s S  T)

10. Exchange in the lower levels
Exchange in the lower levels is not well simulated in the low-resolution model, because tropopause folding is simulated only in the high-resolution model.
 less exchange in the lower level of the low-resolution model.
TL959
TL95
---- PV=2PVU
TL959
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11. Horizontal Distribution (Jan)
StratosphereTroposphere TroposphereStratosphere
Horizontal Distribution (Jan)
hPa
TS transport around the subtropical jet over Eurasia
ST transport in northern winter around the storm tracks at lower altitude
hPa

12. July
TL95
TL95
TL95
TL959
Vertical distribution is similar to January, with upward shift of the peak because of higher tropopause
Transport occurs mainly around the Pacific and the Atlantic at 200hPa, due to Rossby wave breaking (Postel and Hitchman 1999)
weaker in the lower altitudes due to weak storm activity
TL959
TL959
Strat.  Trop. Trop.  Strat. Net
hPa
hPa

13. Contributions of the PV nonconservative terms
A nonconservative process has to work for transport across PV surface
Gravity-wave drag
Convective momentum transport
(Diffusion)
Shortwave Radiation
Longwave Radiation
Heat release by Large-scale condensation
Heat release by Convection
(Diffusion)
will move to the stratosphere in Δt
--- stored as monthly-averaged data (except for diffusion)
will move to the troposphere in Δt

14. Contribution by Longwave (300hPa, Jan)

15. Contributions of the PV nonconservative terms
Jan
Jul
Estimated transport by the effect of longwave can explain over half of TST
The other contributions are too small to explain the transport

16. Summary
Amount of exchange estimated by passive tracer has resolution dependence. In the high-resolution model,
less exchange at higher levels --- due to better representation of sharp tropopause
more exchange at lower altitudes --- due to better representation of small-scale structures
net transport have small resolution dependence
Net stratosphere to troposphere transport below 400hPa
large over the Pacific and Atlantic storm track in January
Net troposphere to stratosphere transport above 300hPa
near the subtropical jet over Eurasia in January
around the Pacific and the Atlantic in July
Large part of this transport estimated from PV change by vertical difference of longwave radiation.
Please check Mizuta and Yoshimura (2009, JGR) for more detail !