InterTropical Convergence Zone (ITCZ) Breakdown and Reformati

on in the Moist Atmosphere

Chia-Chi Wang

王嘉琪Research Center for Environmental Changes, A

cademia Sinica

中央研究院 環境變遷研究中心

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

Climatological ITCZ

Waliser and Gautier 1993. 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.

GOES west 20—29 Aug, 2002, VS

QuikSCAT surface wind anomaly and relative vorticity. (mean: all available JJA wind)

2000/08/09

2000/08/07 2000/08/11

2000/08/13

1E-5 1/s

Aug. 22, 2000

Aug. 27, 2000

VS images

The produced disturbances

Previous studies: modeling• Hack et al. 1989, Schubert et al. 1991: used 2-D (lat—he

ight) 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.

Previous studies: data analysis• Agee 1972: formation of tropical storm Anna from I

TCZ wave disturbances.• Wang and Magnusdottir 2006: survey 3 independ

ent 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-40 23-year 850hPa relative vorticity. Conclude that the ITCZ over the eastern north Pacific has strong wave-like signal on the synoptic timescale.

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.

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

QTCM 1—cont.

• A form of Betts-Miller convective scheme

• Simple radiation scheme

• Bulk formulae in planetary boundary layer

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

Dry case

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

PE model result

Day=5

Day=7

Day=9

Day=11

5 K /day peak on 600 hPa

Wang and Magnusdottir(2005)

Relative vorticity 850 hPaMoisture-on

All-physics-on

QuikSCAT surface anomaly wind and relative vorticity. (mean: all available JJA wind)

2000/08/09

2000/08/07 2000/08/11

2000/08/13

1E-5 1/s

Evap (color), sfc wind speed (contour) and wind vector

Contour interval 5 m/s

K

K

• 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.

All-physics-on

Budget analysis

Moist static energy equation

EvapQvMqvqb qqt 111~~

Tsradcst FFQvMTvTa 111~~

Tsradtt FFEvapvMqTvqbTa 11111~)(~

Moisture equation:

Temperature equation:

Precipitation

Convective heating

Horizontal moisture convergence

Vertical moisture convergence

p

q

1

Horizontal energy convergence

Vertical energy convergence M = Ms - Mq

Budget analysis:experiment design

EvapQvMqvqb qqt 111~~

horizontal Vertical

(EXP 1 – control)

vertical

horizontal

Tsradcst FFQvMTvTa 111~~

horizontal Adiabatic cooling

Adiabatic cooling (plotted with minus sign)

horizontal

FTs

Moist static energy budget

Tsradtt FFEvapvMqTvqbTa 11111~)(~

Horizontal energy convergence

Vertical energy convergence

Horizontal q

Evap

Vertical energy

Budget analysis:experiment design

(EXP 2 – control)

Prescribed horizontal moisture convergence (EXP 2)

Prescribed evaporation (EXP 3)

(EXP 3 – control)

Budget analysis:experiment design

Convection failed to develop

Model over heated

Warmer SST (305K) (EXP 6)

(EXP 6 – control)

Hori_q

Hori_T

Budget analysis:experiment design

Stronger tail

Contribution of horizontal moisture convergence may change, but this process is passive

Domainant processes

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

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.

Effect of Kelvin wave

Solid lines Dashed

lines

Thickened: Kelvin wave is filtered

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)