Cheng-Zhong Zhang and Hiroshi Uyeda

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Possible Mechanism for Prevalence of
Midlevel Convections to the South of the Meiyu
Front
Cheng-Zhong Zhang and Hiroshi Uyeda Hydroshperic
Atmospheric Research Center, Nagoya University
1 November 2006 in Boulder, Colorado
2
Possible Mechanism for Prevalence of
Convection of Medium Depth
to the South of the Meiyu Front
Cheng-Zhong Zhang and Hiroshi Uyeda Hydroshperic
Atmospheric Research Center, Nagoya University
1 November 2006 in Boulder, Colorado
3
Target Area
4
Statistical Analyses of Vertical Structure on
Precipitation Systems
3 years data (Jun/Jul, 2001-2003)
HeFei Doppler radar
0 ?
Convection of Medium Depth
Zhang et al. 2006 (JMSJ)
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Analyses of 3D Doppler radar data
Classification of precipitation type Used
Bright Band Fraction (BBF) Method for 3km x 3km
column. CMD is extracted from convective
portion. lt Deep Convection, CMD, Transition,
Stratiform gt
Classification of meso-beta scale convective
systems Decide the location of the
Meiyu front by using Doppler velocity gradient.
Consider the relative location and the
motion speed of the precipitation system
ltSlow-moving and South-of-Front (SSF),
Slow-moving and Along-Front (SAF), gt
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Convection of Medium Depth (CMD)
Criteria 1) The altitude of reflectivity core
is lower than 4km. 2) The height of 15 dBZ
echo-top is equal to or less than 8 km.
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BBF method

• Resonfeld (1995) Cheng et al. (2001)
• BBF is defined as the fraction of grid point
  with intensive reflectivity in 2 km thickness air
  layer including 0 C level to that in the whole
  air column.
• B BF a/N
• a-- the number of point with reflectivity
  larger than threshold within 2 km thickness air
  layer including melting level .
• The threshold is determined firstly by place
  the reflectivity in the whole column to
  descend order from Zmax to Zmin, then set the Nth
  intense reflectivity within whole air column as
  threshold.
• N-- the number of grid point within 2 km
  thickness air layer around the melting level.

BBFlt0.4 convective 0.4ltBBFlt0.6
transition BBFgt0.6 stratiform
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Distribution of different properties of
precipitation
H 2 km
A
A'
B'
B
CMD
2150UTC 09JUL2003
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Analyses of 3D Doppler radar data
Classification of precipitation type Used
Bright Band Fraction (BBF) Method for 3km x 3km
column. CMD is extracted from convective
portion. lt Deep Convection, CMD, Transition,
Stratiform gt
Classification of meso-beta scale convective
systems Decide the location of the
Meiyu front by using Doppler velocity gradient.
Consider the relative location and the
motion speed of the meso-beta scale
convective system ltSlow-moving and
South-of-Front (SSF), Slow-moving and
Along-Front (SAF), gt
10
Schematic illustration of CMD and DC over the
Meiyu front
Zhang et al. 2006 (JMSJ)
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Ratio of estimated precipitation amount for four
types
SAF TYPE (6 cases)
SNF TYPE (1 case)
FAF TYPE (4 cases)
SSF TYPE (12 cases)
Z300 R 1.37 for convective (CMD)
Z205 R 1.48 for stratiform and transition
(Fujiwara 1965)
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5 km
1 km
CReSS
300 ? 300 grid points, with the spacing of 5 and
1 km (outer and inner domains) 70 levels
vertically, with stretching from 100 m to 400 m
with increasing to 20 km. Initialized at 00 UTC
22 July 2002 by using RANAL data. (for 12 hours)
The simulation for the inner domain
started 3 hours after that for the outer domain.
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Radar Echo
Radar reflectivity at 3 km at 0628 and 0700 UTC
22 July 2002. The solid line indicates the Meiyu
front, and five cells (CMD) are respectively
labeled as C1, C2, C3, C4, and C5. The radar site
is indicated by the symbol
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Simulation
Model-simulated mixing ratio of rainwater (g
kg-1) and wind (vector) at 3 km at 0730 and 0800
UTC. The convective cells CMD1?5 (CMD2 dissipated
at 0730 UTC) are convection of medium depth. DPC1
indicates deep convection. The thick solid line
represents the Meiyu front. S1 and S2 at 08 UTC
represent the areas immediately ahead and south
of the front, respectively.
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CMD3
East-west cross section of CMD3 along the lines
AA?, BB? and CC? in the previous figure. The
black contour line indicates divergence, the red
contour line indicates convergence.
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CMD3 2kmx2km Column
(a) Time variation of the maximum mixing ratio
of precipitation hydrometeors and corresponding
vertical velocity for CMD3. The solid and
short-dashed lines represent the vertical
velocity the color area is representative of the
mixing ratio of rainwater and the red dashed
line represents the mixing ratio of snow and
graupel. (b)Time variation of simulated hourly
rainfall intensity within 2 km? 2 km rectangle
region under the peak.
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DPC1 2kmx2km Column
Same as in the previous figure, but for deep
convection DPC1.
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RELATIVE HUMIDITY
LEVEL OF NEUTRAL BUOYANCY
Low LNB
MEIYU FRONT
S
N
(a) Distribution of LNB and (b) vertical cross
section for relative humidity along AA' in (a).
The solid line in (a) and the arrow in (b)
indicate the position of the Meiyu front. The
blank areas in (a) indicate atmosphere is
absolutely stable.
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Summary

• In order to understand the characteristics of
  precipitation systems to south of the Meiyu/Baiu
  front, a concept of Convection of Medium Depth
  (CMD) is introduced.
• Data analyzed is Hefei Doppler radar for June and
  July from 2001 to 2003 over the downstream region
  of Yangtze River.
• We found that CMD (echo top height ranges from 3
  km to 8 km during its duration) predominates in
  the convective systems categorized as slow-moving
  (lt 3 m/s) and south-of-front (SSF) types.
• We employed cloud-resolving numerical model to
  replicate the evolution of CMD.
• The structure of CMD from the simulation well
  agreed with the observation through the life.
• It is concluded that because of existence of
  abundant moist air to south of the Meiyu front,
  convection is easily formed in low atmosphere.
• The moderate updraft within the convective cell
  couldnt uplift large amount of precipitation
  particles penetrating the melting level,
  resulting in formation of CMD to south of the
  Meiyu front.
• Further studies on CMD would be necessary to
  understand the formation mechanism of heavy
  rainfalls along the Meiyu/Baiu front.