1. Research Center for Environmental Changes, Academia Sinica
InterTropical Convergence Zone (ITCZ) Breakdown and Reformation in the Moist Atmosphere
Chia-Chi Wang
王嘉琪
Research Center for Environmental Changes, Academia Sinica
中央研究院 環境變遷研究中心

2. Outline
Introduction of the ITCZ on the synoptic timescale (mainly ITCZ breakdown in the eastern Pacific)
Model (Quasi-equilibrium Tropical Circulation Model, QTCM 1) and experiment designs
Results and Discussion
Concluding remarks

3. Climatological ITCZ
Waliser and Gautier Averaged from 17 years of monthly Highly reflective cloud (HRC) data. The number of days per month the given grid point was covered by a large-scale deep convective system, subjectively determined.

4. ITCZ on the Synoptic Timescale
HURSAT-Basin, VS, Sept, 2000
Timescale ~ 1—3 weeks

5. GOES west 20—29 Aug, 2002, VS

6. QuikSCAT surface wind anomaly and relative vorticity
QuikSCAT surface wind anomaly and relative vorticity. (mean: all available JJA wind)
2000/08/09
2000/08/07
2000/08/11
2000/08/13
QuikSCAT 0.5x0.5
1E-5 1/s

7. The produced disturbances
VS images
Aug. 22, 2000
Aug. 27, 2000
The produced disturbances
QuikSCAT 0.25 x 0.25

8. Previous studies: modeling
Hack et al. 1989, Schubert et al. 1991: used 2-D (lat—height) model to simulate the ITCZ. Heating produced PV gradient reversal is not unique about the formation of African waves (easterly waves). The ITCZ can induce its own breakdown (through barotropic instability).
Guinn and Schubert 1993: (a f-plane one layer model) Hurricane spiral bands may be produced from ITCZ breakdown.
Nieto Ferreira and Schubert 1997: Simulate barotropic aspects of ITCZ breakdown in lower troposphere in shallow water model.
Wang and Magnusdottir 2005: simulate ITCZ breakdown in dry primitive equation model with different background flows.

9. Previous studies: data analysis
Agee 1972: formation of tropical storm Anna from ITCZ wave disturbances.
Wang and Magnusdottir 2006: survey 3 independent datasets (QuikSCAT, NCEP analysis, and GOES cloud images) over1999—2003 to count the occurrence of ITCZ breakdown (subjectively). ITCZ breakdown does happen frequently during summer and fall over the eastern Pacific.
Magnusdottir and Wang 2008:spectral analysis on ERA year 850hPa relative vorticity. Conclude that the ITCZ over the eastern north Pacific has strong wave-like signal on the synoptic timescale.

10. Importance
An efficient way to pool vorticity in the tropics which represents the early stages of tropical cyclogenesis
Understand the basic dynamics of the ITCZ on the synoptic timescale.

11. Quasi-Equilibrium Tropical Circulation Model 1 (QTCM 1)
Two vertical modes: barotropical mode and the first baroclinic mode
Good in deep convective areas
2 layer model outside convective areas

12. QTCM 1—cont. A form of Betts-Miller convective scheme
Simple radiation scheme
Bulk formulae in planetary boundary layer

13. Experiment design QTCM settings:
domain: 360x157, 78.5S-78.5N (1x1 degree)
Aqua-planet
Prescribed uniform SST 300K
Dry case (no moisture, radiation, sfc fluxes)
Moist-on case
All-physics-on case
Budget analysis

14. Dry case
Prescribed heating: 6 K/day, 5 days. Red dash line. 850 hPa

15. PE model result Day=5 Day=9 Day=7 Day=11 5 K /day peak on 600 hPa
Wang and Magnusdottir(2005)

16. Moisture-on
Relative vorticity 850 hPa

17. All-physics-on

18. QuikSCAT surface anomaly wind and relative vorticity
QuikSCAT surface anomaly wind and relative vorticity. (mean: all available JJA wind)
2000/08/09
2000/08/07
2000/08/11
2000/08/13
QuikSCAT 0.5x0.5
1E-5 1/s

19. Evap (color), sfc wind speed (contour) and wind vectorK K
Contour interval 5 m/s

20. A disturbance can induce surface convergent flow on its southwest.
Surface wind induces surface evaporation (energy source).
wind-evaporation feedback
The tail can be seen as a new ITCZ.
The intensity is weak.
Model resolution (in vertical modes and horizontal resolution. Strong numerical damping)
Lifetime is controlled by large-scale wave propagation.

21. Budget analysis
All-physics-on

22. Moist static energy equation
Vertical moisture convergence
Precipitation
Moisture equation:
Horizontal moisture convergence
Temperature equation:
Convective heating
Horizontal energy convergence
Vertical energy convergence
M = Ms - Mq

23. Budget analysis:experiment design

24. horizontal
Vertical
(EXP 1 – control)
vertical
horizontal

25. horizontal
Adiabatic cooling
Adiabatic cooling (plotted with minus sign)
horizontal

26. Moist static energy budget
FTs
Horizontal energy convergence
Vertical energy convergence
Evap
Horizontal q
Vertical energy

27. Budget analysis:experiment design

28. Prescribed horizontal moisture convergence (EXP 2)
(EXP 2 – control)

29. Prescribed evaporation (EXP 3)
(EXP 3 – control)

30. Budget analysis:experiment design
Convection failed to develop
Model over heated

31. Warmer SST (305K) (EXP 6)
(EXP 6 – control)
Hori_T
Hori_q

32. Budget analysis:experiment design
Stronger tail
Domainant processes
Contribution of horizontal moisture convergence may change, but this process is passive

33. Concluding remarks The breakdown is dominant by dry dynamic processes.
In a moist atmosphere, a long tail appears on the southwest of a vortex. The development of the tail suggests a mechanism (positive wind-evaporation feedback) for quick reformation of an ITCZ (within a couple of days).
Two major processes that maintain the tail is surface evaporation and vertical motion. Horizontal motion is a passive process.
Energy source: Surface evaporation
Energy sink: Vertical convergence

34. Concluding remarks—cont.
The role of the tropical disturbance:
disturbs the ITCZ and breaks it
Induce a tail (a new convergence zone) on its southwest side
The passage of large-scale waves can suppress the convection of the tail by modifying surface wind pattern.

35. Effect of Kelvin wave Thickened: Kelvin wave is filtered Solid lines
Dashed lines

36. Future work The role of the ocean?
Coupled with mixed layer ocean: Life time of the tail is shortened for a few days
TMI SST (30-day high pass filtering): warmer SST before breakdown, cooler SST during/after breakdown.
Coupled with simple ocean dynamics (ex. parameterized Ekman pumping)
Different meridional SST gradients
Coupled with daily SST (i.e., larger SST variation)

37. Warmer SST (305K) prescribed advq1