Explicit Simulations of the Intertropical Convergence Zone

1
Explicit Simulations of the Intertropical
Convergence Zone

• Changhai Liu and Mitchell W. Moncrieff
• Present by Xiaoyu Liu

2
Outline

• Motivation
• Model description
• Assumptions
• Initial state
• Simulation
• Off-equator maximum convection stage
• Equatorial maximum convection stage
• Formation of the equatorial easterlies
• Conclusion and future work
• Acknowledgment

3
Motivation

• ITCZ is one of the most important components of
  the global circulation
• What is the physical mechanism regulating the
  formation and latitude preference of ITCZ?
• SST?
• Ekman pumping and moisture availability?
• Low level convergence?
• Cross-equatorial pressure gradients and
  radiative-convective instability?

4
Model

• Two dimensional Eulerian version of the
  nonhydrostatic Eulerian/Semi-Lagrangian anelastic
  model
• Features
• Domain 16000Km 24Km
• Grid horizontal 5Km vertical 0.3Km
• Boundary conditions Rigid bottom free-slip top

Z
16000 km
24 km
S.P
EQ
N.P
Y
5
Assumptions

• theta and r equal to average value anytime
• Absorbing layer at boundaries
• Constant SST 302.5K
• Surface moisture and sensible heat flux are given
  by TOGA COARE surface flux algorithm
• Time-independent and horizontal uniform radiative
  cooling
• -1.5K/day below 12km decrease linearly to zero at
  the top

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Initial State

• A resting atmosphere
• T and q given by average condition of Dec 19-26
  1992 in TOGA COARE
• Start with small random perturbations of theta
  and r

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Simulation

• Period 100 days
• Goal Statistical quasi-equilibrium
• Convective pattern
• Off-equator maximum convection stage
• Equatorial maximum convection stage

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FIG. 1. Spacetime distributions of surface
precipitation rate during (a) days 125, (b) days
2650, (c) days 5175, and (d) days 76100. The
light and dark shading correspond to rainfall
intensity greater than 1 and 10 mm h-1,
respectively. The equator is located at the
center of the domain.
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• FIG. 2. Spatial distributions of surface
  precipitation rate averaged over (a) the early
  40-day integration and (b) the late 60-day
  integration. The field is smoothed with a 500-km
  running mean filter.

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FIG. 3. Spatial distributions of cloud amount
averaged over (a) the early 40-day integration
and (b) the late 60-day integration. The light,
moderate dark, and heavy dark shading correspond
to cloud fraction greater than 5, 15, and 25,
respectively.
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Off-equator maximum convection stage

• FIG. 4a. Physical fields averaged from days 16
  to 25. Meridional wind (1 m s-1 contour
  interval), The white and dark shadings correspond
  to vertical velocity less than -2.5 10-3 m s-1
  and greater than 2.5 10-3 m s-1, respectively

12
Off-equator maximum convection stage

• FIG. 4b. Physical fields averaged from days
  16 to 25. zonal wind (2 m s-1 contour interval)
  The white and dark shadings correspond to
  vertical velocity less than -2.5 10-3 m s-1 and
  greater than 2.5 10-3 m s-1, respectively

13
Off-equator maximum convection stage

• FIG. 4c. Physical fields averaged from days
  16 to 25. temperature perturbation (1-K contour
  interval)

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Off-equator maximum convection stage

• FIG. 4d. Physical fields averaged from days
  16 to 25. water vapor mixing ratio perturbation
  (0.5 g kg-1 contour interval)

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Summary of off-equator maximum convection stage

• Vigorous convection is off-equator, rarely occurs
  near equator
• Convection is asymmetric
• Equatorward flow at upper levels and poleward
  flow at lower levels
• Deep equatorial easterly wind
• Westerly jets

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Equatorial maximum convection stage

• FIG. 5. Evolution of the space-averaged
  precipitation rate (solid line) and CAPE (dashed
  line) over a 1500-km-wide area centered at the
  equator during (a) days 5075 and (b) days
  75100.

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Equatorial maximum convection stage

• FIG. 6a. Physical fields averaged from days 81 to
  90. Meridional wind (1 m s-1 contour interval),
  The white and dark shadings correspond to
  vertical velocity less than -2.5 10-3 m s-1 and
  greater than 2.5 10-3 m s-1, respectively

18
Equatorial maximum convection stage

• FIG. 6b. Physical fields averaged from days 81
  to 90. zonal wind (2 m s-1 contour interval) The
  white and dark shadings correspond to vertical
  velocity less than -2.5 10-3 m s-1 and greater
  than 2.5 10-3 m s-1, respectively

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Equatorial maximum convection stage

• FIG. 6c. Physical fields averaged from days 81
  to 90. temperature perturbation (1-K contour
  interval)

20
Equatorial maximum convection stage

• FIG. 6d. Physical fields averaged from days 81
  to 90. water vapor mixing ratio perturbation (0.5
  g kg-1 contour interval)

21
Summary of equatorial maximum convection
stage

• Single equatorial ITCZ-like morphology
• Convection concentrated in a narrow area around
  the equator and not continuous
• Wavelike vertical structure and opposite in sign
  in two hemispheres
• Two-cell vertical structure
• Easterly uniformly distributed in vertical
• No jet like structure

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Formation of the equatorial easterlies

• FIG. 7a. Evolution of the equatorial easterly
  wind averaged over a 1000-km-wide area centered
  at equator for control simulation

23
Formation of the equatorial easterlies

• Zonal momentum equation

Coriolis torque
Zonal wind tendency
Horizontal Mom. Flux convergence
Vertical Mom. Flux convergence
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Formation of the equatorial easterlies

• FIG. 8. Evolution of the equatorial zonal
  wind tendency by (a) horizontal momentum flux
  convergences, (b) vertical momentum flux
  convergences, and (c) Coriolis torques averaged
  over a 1000-km-wide area centered at the equator
  during days 1060. Contour interval is 1 m s-1
  day-1

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Conclusion and future work

• Two distinct convective patterns in the Tropics
  are obtained during the 100-day integration
• Off-equator ITCZs
• Single ITCZ at the equator
• Highly idealized experimental setup and 2-D
  assumption exclude some features
• Future work
• 3-D explicit studies

26
Acknowledgment

• Genuine authors Changhai Liu and Mitchell W.
  Moncrieff
• Dr. David Nolan