1. Double tropopauses during idealized baroclinic life cycles
Shuguang Wang
Lorenzo M. Polvani
Columbia University

2. Classic picture of the tropopause (Holton et al. 1995)
Breaks in the thermal tropopause near the subtropical jet.
The low latitude tropopauses extend to higher latitudes above the jet, while the lower extratropical tropopauses extend below the jet to middle latitudes.
In the overlying region, static stability increases to high stratospheric values above the first tropopause and decreases to low tropospheric values below the second tropopause. 2

3. Classic picture of the tropopause (Holton et al. 1995)
Breaks in the thermal tropopause near the subtropical jet.
The low latitude tropopauses extend to higher latitudes above the jet, while the lower extratropical tropopauses extend below the jet to middle latitudes.
In the overlying region, static stability increases to high stratospheric values above the first tropopause and decreases to low tropospheric values below the second tropopause. 3

4. Double tropopauses from new observations
Double tropopauses are ubiquitous in the middle latitudes from different measurements and dataset. (Schmidt et al., 2006; Randel et al., 2007; Pan et. al., 2009). e.g., radiosondes, GPS sounding, Limb sounder, Reanalysis data etc.
DT strongly correlate to the intense cyclonic flows (Randel et al. 2007)
Ozone sounding and tracer analysis suggest that DT associated with the air masses originated from low latitudes
Randel et al. 2007 4

5. Double tropopauses Goals of this research
Investigate double tropopauses using baroclinic life cycles
Study the role of synoptic eddies in shaping double tropopauses
First tries
Initialized with a few well known jets: Rotunno’s jet, Polvani’s jet etc.
Turning parameters in these jets
NO luck. No double tropopauses in the baroclinic life cycles 5

6. Initialization of baroclinic life cycles
WRF 3.0, f plane channel model, periodic in x, symmetric in y; 50x90x65; dx = 100 km, dz ~ 450 m; positive advective scheme for tracer
Width
TIL
Cold point Y Z
JET
Specify temperature lapse rate pairs at low and high latitudes:
then piece together temperature using the lapse rate pairs
Build in Tropopause inversion layer (Birner 2002, 2006):
temperature increases above the local cold point tropopause
Balance wind from temperature to baroclinic jets
Vary the strength of cold point to ensure jet is closed 6

7. Initial lapse rate profiles
Low latitudes
High latitudes
Try four different initial lapse profiles with increasingly stronger TIL
Without TIL Weak TIL Medium TIL Stong TIL; 7

8. Initial baroclinic jet
Blue: zonal wind (5m/s); red: potential temperature (10k), green: -dT/dz = 2K/km
Try four different initial lapse profiles with increasingly stronger TIL
Without TIL Weak TIL Medium TIL Stong TIL; 8

9. Baroclinic waves initialized without initial TIL
Shading: depth between the 1st and 2nd tropopause; Gray: PV at 335K (ci=0.5) 9

10. Baroclinic waves initialized without weak TIL
Shading: depth between the 1st and 2nd tropopause; Gray: PV at 335K (ci=0.5) 10

11. Baroclinic waves initialized without medium TIL
Shading: depth between the 1st and 2nd tropopause; Gray: PV at 335K (ci=0.5) 11

12. Baroclinic waves initialized with strong initial TIL
Shading: depth between the 1st and 2nd tropopause; Gray: PV at 335K (ci=0.5) 12

13. Double tropopauses in four different baroclinic life cycles
More DT due to initial TIL
Double tropopauses increases with time
DT in the region of cyclonic shear and anticyclonic shear at 200 hPa
DT in cyclonic shear >> DT in the anticyclonic regions 13

14. Double tropopauses during different baroclinic life cycles: LC1 vs. LC2
Blue: zonal wind (5m/s); red: potential temperature (10k), green: -dT/dz = 2K/km
Double tropopauses do not increases as that in cyclonic type of baroclinic life cycles, despite of the presence of initial TIL 14

15. Baroclinic waves initialized with strong TIL, but in LC1
Shading: depth between the 1st and 2nd tropopause; Gray: PV at 335K (ci=0.5) 15

16. Double tropopauses during anticyclonic baroclinic life cycles (LC1)
Double tropopauses do not increases as that in cyclonic type of baroclinic life cycles, despite of the presence of initial TIL 16

17. Baroclinic waves initialized with strong initial TIL
Shading: depth between the 1st and 2nd tropopause; Gray: PV at 335K (ci=0.5) 17

18. Air mass mixing form a passive tracer
Shading: tracer; dotted: thermal tropopause; gray: dynamic tropopause; dash: wind
Indicate the air masses mixing between low and high latitudes
Tracer has initial value of the corresponding y coordinate at each grid point.
Red regions shows that air masses originate from high latitudes
Not the same as the observations 18

19. Double tropopauses in observations Pan et al. 2009
Shading: dθ/dz
From limited number of events: Ozone sounding and tracer analysis suggest that the air masses originated from low latitudes are responsible for double tropopause. 19

20. Conclusions
Double tropopauses may increase significantly during baroclinic life cycles at the its late stage
Stronger TIL, more DT; Weaker TIL, less DT
Strong vorticity dependence consistent with observations; the amount of double tropopause increases in the region of cyclonic shear but not in the region of anticyclonic shear.
DT NOT increase during the life cycles of anticyclonic type baroclinic waves
A passive tracer demonstrates that the air masses responsible for the simulated the double tropopauses are originated from high latitudes: opposite to the observations.
It is open question that double tropopauses are consistently formed and maintained throughout all the season in the middle latitudes. We are open to suggestions.
Caveat. TIL and DT have different seasonal cycles. DT maximize in winter but TIL in summer. Related: does the intrusion also maximize in winter? 20

21. Questions?

22. Baroclinic waves initialized with strong initial TIL
Shading: depth between the 1st and 2nd tropopause; Gray: PV at 335K (ci=0.5) 22