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ICMCS-V_061101. On the mechanism of eastward-propagation of super cloud clusters (SCCs) over the equator – Impact of precipitation activities on climate of East Asia –. Masanori YOSHIZAKI and Tomoe NASUNO (IORGC/JAMSTEC). Topics 1. Simple model: linear, 4-layer model - PowerPoint PPT Presentation
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On the mechanism of eastward-propagation ofsuper cloud clusters (SCCs) over the equator
– Impact of precipitation activitieson climate of East Asia –
ICMCS-V_061101
Masanori YOSHIZAKI and Tomoe NASUNO(IORGC/JAMSTEC)
Topics1. Simple model: linear, 4-layer model with constant N and no basic wind2. Extension of simple model using a NICAM output (Diabatic heating: positive-only wave CISK)
Thanks to Drs. T. Nasuno and M. Sato for providing NICAM data
History and motivations
・ Hayashi ・ Sumi (1986) found an eastward- propagating mode around the equator in the aqua-planet numerical experiment.・ Eastward-propagating super cloud clusters ( SCCs) were obtained by satellite data, too. (e.g., Nakazawa , Murakami , Takayabu et al.)
Many theories to explain the mechanisms of eastward-propagating modes:
1) Atmospheric instability ・ Moisture convergence・ Surface evaporation2) Atmospheric response to independent
forcing・ Tropical intraseasonal stationary forcing・ Tropical stochastic forcing・ Lateral forcing Zhang (2003)
day
East
Westward propagating
Eastward propagating
Nakazawa
Which mechanisms are working?
Atmospheric instability, or atmospheric response to independent forcing?
Intraseasonal variation >>> MJO (Madden-Julian Oscillation)
・ Atmosphere with constant N and no basic wind,・ Equatorial-beta plane system (βE),・ 4 layers in the vertical,・ Linear system,・ Heating: positive-only wave-CISK,・ Large second-order horizontal diffusion.
Yoshizaki (1991a,1991b): a simple model of S CC s
0 :w B < 0 Q= w B ・ f(z) :w B > 0
.0
,
),,(0
,
,
2
2
z
w
y
v
x
u
bQNwt
b
zd
dgNgbb
z
vuyyt
v
uvyxt
u
HH
HH
HH
Model:* Horizontal direction: Grid* Vertical direction: Mode expansion
Height
QNw
t
b 2
w B
bottom
Model top
Q /N2 10
Total QEach modes
of Q
* Vertical mode expansion
Height Combination of two modesOnly 1st mode
w B
* Two heating profiles were considered:
* Top-heavy heating profile can be expressed as a combination of positive 1st mode and negative 2nd mode
η1 = 1.5, η2=0.0
η1=1.5, η2=-1.5
Height
w B
η1 = 1.5, η2=0.0
η1=1.5, η2=-1.5
Time
along the equator
Convective mode moves
westward in βE .
Eastward-propagating mode grows faster than westward-
propagating mode in βE .
→ Changes of vertical heating profiles induce different characteristic features of propagation!
* In this model, it is assumed that diabatic heating is greater than adibatic cooling due to upward motion in some layers.
However, is the ‘>’ case right, observationally or numerically?; Disturbance driven by convection for the ‘>’ case, or neutral wave in the stable stratification for the ‘<‘ case.
Further study could not be pursued in 1990’s, however, because there was no step to check above-mentioned features.
Recently, numerical outputs using a global NH model (NICAM)were available.
22 NwQQNwt
b
>>> Which mechanisms are working? Atmospheric instability , or atmospheric response to independent forcing?
Snapshot of ‘NICAM’ precipitation- Aqua planet -
NICAM: Nonhydrostatic ICosahedral Atmospheric Model = Global cloud-resolving nonhydrostatic model
40000 km / 30 days~ 15.4 m / s
SCC
7 km resolution
2S – 2N average
x - t distribution of diabatic heating
)10( 14
sK
tC
qLQ
p
v )(K
)1.0( 1sm
w
)( 1sm
u
Qzd
dw
(1) Comparison of Q (diabatic heating) and adiabatic cooling due to upward motion
(2) Comparison of Q (diabatic heating) and adiabatic cooling due to upward motion
)10( 14 sKzd
dwQD
Disturbances driven by convection ( or atmospheric instability )
D is positive in some layers in the vertical direction.
.0
,')(
,1
0
,'1
)(
,'1
)(
312
312
3122
z
w
y
v
x
u
rQz
wNLx
ut
gz
p
vrvuyy
pvNL
x
vu
t
v
uruvyx
puNL
z
uw
x
uu
t
u
HH
HH
HH
Governing equations
* 54 vertical grid model is used.* Parameter ε: 0 or 1 ε1 : Linear or nonlinear (NL) ε2 : With or without basic eastward wind ε3 : Including or excluding Rayleigh damping (function of z)
Positive only wave-CISK
X - time section of vertical motions at the height of 3.7 km along the equator
Blue : upward motion
Red : downward motion
Full model: ε1=ε2=ε3 = 1 η=60
Tim
e (d
ay)
X (10,000 km)
Vertical section
Horizontal section
16 days~ 29 m / s
Yoshizaki (1991a,1991b)* Linear * No zonal wind* Constant N* 4 layers in the vertical * Mode expansion in the vertical * Combination of 1st and 2nd modes
Present calculation* Nonlinear* Zonal wind* Variable N* 54 layers in the vertical* Grid in the vertical * Diabatic heating simulated by NICAM
θ
Heating
Full model : Basic wind u + N + positive-only wave-CISK + Rayleigh damping
Rayleigh damping is working well.
Z
X
Vertical pattern of simulated SCC s
Conclusions1) Diabatic heating is larger than adiabatic cooling due to upward motion in some vertical layers: SCCs appeared in NICAM is disturbances driven by convection. Then, SCCs are excited due to atmospheric instability.2) The simple model is extended using the NICAM output.3) When positive-only wave-CISK is applied as diabatic heating, eastward-propagating disturbances appear as a dominant mode.4) MJO (or SCC) is responsible for the formation of tropical cyclones affecting East Asia. Thus, this study is important.
Further studies1) This vertical grid model should extend to a vertical mode model, to confirm results obtained by a simple vertical mode.2) Rayleigh damping is important to eliminate the reflection of vertically propagating gravity waves. The differences between inclusion/exclusion of Rayleigh damping should be studied.3) Multi-scale horizontal feature is not simulated due to selection rule of convection.
Horizontal pattern of simulated SCC s
Height
Case of two vertical modes
Case of one vertical mode
w B
Two heating profiles were considered:
* Top-heavy heating profile can be expressed as a combination of positive 1st mode and negative 2nd mode
η1 = 1.5, η2=0.0 η1=1.5, η2=-1.5
Time
along the equator
→ Changes of vertical profiles of heating induce different characteristic features of propagation!
Why does the difference of heating profiles produce different features?
Case of one vertical mode
(η1>1)
Only convective mode excited
η1 = 1.5, η2=0.0Similarly to an usual convection, disturbances with no propagation are excited
Convective mode growswithout propagation in no βE.
Convective mode moves westward in βE .
Why does the difference of heating profiles produce different features?
Case of two vertical modes
(η1>1, η2<0)
Convective and oscillation modes excited simultaneously
Oscillation mode can be separatedinto EP and WP modes.
EP mode grows faster than WP mode in βE .
Growth rate
Horizontal wavenumberSmall Large
Horizontal diffusion = 0
Growth rate
Horizontal wavenumberSmall Large
Small horizontal diffusion
Growth rate
Horizontal wavenumberSmall Large
Large horizontal diffusion
Selection rule of convection
In the linear atmosphere system, there are two independent modes;(1) neutral wave modes and (2) exponentially growing modes.
(1) Gravity wave, Kelvin wave, Rossby wave and so on: When forced, a selection rule does not work: All waves stimulated by forcing are evenly excited and appear following a dispersion relation.
(2) Baroclinic waves, Benard convection, shear instability and so on: Modes with maximum growth rate grow fastest and a selection rule works.
In this model, a positive-only wave-CISK works like usual convectionand disturbances with maximum growth rate are infinitesimally small without horizontal diffusion (and viscosity).>>>> A large horizontal diffusion is included to get modes with horizontal scales of 1000 km. >>>> No multi-scale horizontal structure!
)0u(
02
X - time section of vertical motions at the height of 3.7 km along the equator
Blue : upward motion
Red : downward motion
Full model: ε1=ε2=ε3 = 1 η=60
Tim
e (d
ay)
X (10,000 km)
.0
,')(
,1
0
,'1
)(
,'1
)(
312
312
3122
z
w
y
v
x
u
rQz
wNLx
ut
gz
p
vrvuyy
pvNL
x
vu
t
v
uruvyx
puNL
z
uw
x
uu
t
u
HH
HH
HH
Governing equations
Parameter ε: 0 or 1*ε1 : Linear or nonlinear (NL)*ε2 : With or without basic eastward wind *ε3 : Including or excluding Rayleigh damping (function of z)
Height
Case of two vertical modes
Case of one vertical mode
w B
η1 = 1.5, η2=0.0
η1=1.5, η2=-1.5
Time
along the equator
→ Changes of vertical profiles of heating induce different characteristic features of propagation!
Convective mode moves
westward in βE .
Eastward-propagating mode grows faster than westward-
propagating mode in βE .
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