A Numerical Study of the Mesoscale Convective

A Numerical Study of the Mesoscale
Convective System Observed Okinawa
Islands in June 1987
(MONTHLY WEATHER REVIEW 1991)
CHUAN-YONG CHANG
AND
MASANORI YOSHIZAKI
2004年9月3日野村真奈美
Introduction
A mesoscale convective system (MCS) that
developed early in the morning of 6 June 1987
over Okinawa Island .
Observation analysis →Akaeda et al.(1991)
Leeside convection
Banta and Schaaf (1987)
This study
Privailing wind
at low levels
→south easterly
Akaeda et al.(1991)
What is the
mechanism?
The leeside convergence zone (Banta（1984）)
The evolution of the system
The MCS has two stages of evolution.
Stationary
Orographical effect?
Propagate rapidly
eastward
Objective
In order to solve the following two questions, using
a two-dimensional cloud model, a series of simulations
were performed regarding the formation and evolution
of the MCS.
1. Why did the system form in the leeside of the
mountain? Did it form accidentally or was it
orographically forced to form in spite of the
relatively low mountain height ?
2. Did the transition from the stationary stage to
propagating stage has a anything to do with
orographically?
Model and environmental fields
A two dimensional compressible model developed
by Yoshizaki and Ogura (1988).
Terrain following coordinates
The variables
wind velocity (u,v,w), pressure p
potential temperature θ
mixing water ratios of water substances
(water vapor, cloud water, rain)
Warm rain process
The mountain shape is smooth with a single peak.
(along the X1X2 line )
The elevation of the terrain surface
zS = HMAX exp [-(x/xH)2]
Domain the x direction 360km
Vertical direction Grid interval 63m
The model top 21km
Model and environmental fields
Model and environmental fields
Results of the CNTL simulation
1. A time-X section
・The system moves
right ward very slowly.
・Individual cells move
leftward.
・The system has two
stages of evolution.
Stationary stage
Propagating stage
New cells form at the
leading edge.
Speed 4.5 m/s
leeside
upwind
2. Vertical structures
mountain
Cloud water
(solid) and
rain (dashed)
Deviation of
potential
temperature
H
H
H
L
Comparison with the observation result
Sensitivity of the MCS to the mountain height
mountain height
0m
400m
800m
・The high-mountain simulation
The model MCSs stay near the mountain without propagation.
・The low-mountain simulation
The model MCSs have stationary and propagating stages
in their evolution.
Vertical structures in the H4 simulation time
Stationary stage
The mountain
・forced lifting to
the MCS in the
upwind side
・block the outflow
spreading in the
leeside.
Propagating stage
The density current
overtakes the
orpgraphic effects.
Sensitivity of the MCS to the environmental field
S1
S2
The moist region near 780 hPa is
crucial for the system to form in the
lee side, while the inversion near the
surface is not.
Sensitivity of the MCS to the environmental field
Examine the moist distribution
by using the dry model
Hmax= 400m, 200m
Features are similar to Hmax=800m.
Region A is more humid.
Hmax = 800m
X
Deep convection can
develop in the leeside by
forcing associated with
mountain waves.
Conclusion
Using a two-dimensional cloud model, a series of
simulations were performed to answer two questions
regarding the formation and evolution of the MCSs
that occurred in the early morning of 6 June 1987
over Okinawa Island.
1. Why did the system form in the leeside of the mountain?
Ascending motion associated with mountain-induced
waves made the low-to midlevel air saturated in the
leeside and the model convection formed there.
( The model was able to produce deep convection
only when the observed sounding was modified to make
the lower atmosphere more moist and unstable. )
Conclusion
2. Why did the system undergo two stages of evolution ( the
stationary and propagating stage ) ?
・The convective system stayed leeside until the cold pool
beneath the cloud base deepened sufficiently and overtook
the orographic effects.
( Forced lifting , damming of the cold pool )
・The cold air overflowed the crest of the mountain and
system started propagating.
The formation of the cold pool in the leeside
of the mountain and orographic effects make
the stationary stage longer and remarkable.

Download Report