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Meng, Z., F. Zhang, P. Markoswki, D. Wu, and K. Zhao, 2012: A modeling study on the development of a
bowing structure and associated rear infl ow within a squall line over south China. J. Atmos. Sci. , 69, 1182–1207.
A Modeling Study on the Development of a Bowing Structure and Associated
Rear Inflow within a Squall Line over South China
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Line end vortices develop on either side of the system, and the vortices help enhance the mid-level rear inflow.
Cyclonic on the northern end of the system and anticyclonic on the southern end, for an environment of westerly vertical wind shear
The Coriolis force acts to intensify the cyclonic vortex and weaken the anticyclonic vortex.
Bookend vortex:
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This study examines the formation and evolution of the bowing process for a squall line that occurred in spring 2007 in South China.
Introduction
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Observation shaded : dBZ
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Environmental features of the squall line
contour : geopotential height (m)shaded : precipitation (kgm-2)
thick contours : θ (K)
thin contours : θe (K)
(wind at 850 hPa)
(kgm-2)
(kgm-2)
(kgm-2)
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Radiosondes (Wuxhou, Qingyuan) QingyuanWuzho
u
CAPE : 1067 Jkg-1
CAPE : 747 Jkg-
1
solid : normal
dash : parallel
Wind shear : 15 ms-1
Wind shear : 30 ms-1
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Numerical model and configurationsWRF V3.2Starting at 1200 UTC 23 April 2007.Initial and boundary conditions provided by
6-hourly FNL NCEP analysis of 1 。 x1 。
Do.1 Do.2 Do.3 Do.4Grid points 150*120 205*172 316*205 481*451Horizontal intervals
40.5 km 13.5 km 4.5 km 1.5 km
Vertical layers 35 layersMicrophysics WSM6
Planetary boundary processes
Yonsei State University
Cumulus parameterization
Grell-Devenyi No
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Result from the numerical simulation
c ≈ 17 ms-1
c ≈ 16 ms-1
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Radar reflectivity and rear inflow
shaded : dBZ
blue : 1.5 km RI (> 4 ms-1)
red : 2.5 km RI (> 4 ms-1)
line-relative wind vector at 1.5 km height
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The formation mechanism of the large
bowing structure
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Θ’v = Θc - Θv
Θ’v(H) -1 K
Speed of cold poolB Buoyancy
Θ’vVirtual potential temperature
perturbations
ΘvEnvironmental Virtual potential
temperature
qcMixing ratio of condensate
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south north
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Environmental vertical shear
shaded : vertical wind shear (0.25~3 km)
red : 50 & 60 dBZ
black : 4 & 8 ms-1 RI (2.5 km)
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environmental flow
pressure gradient
bookend vortices
Formation mechanism of rear inflow
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Environmental flow shaded : dBZ
line-relative wind vector at 1.5 km height
blue : 1.5 km RI
red : 2.5 km RI
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23/2130 Z
Pressure gradientcontours : θ ,
shaded : dBZ
shaded : RIcontours : p ,
(dBZ)
(dBZ)
(ms-1)
(ms-1)
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23/2230 Z
23/2330 Z
(ms-1)
(ms-1)
shaded : wind speed
contours : p ,
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bookend vortices
vector : line-relative streamlinecontour : rear inflow
(dBZ)
(dBZ)(dBZ)
(dBZ)
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southern bow segmentshaded : dBZred contour: RI at 2.5 kmblue contour: RI at 1.5 km
shaded : 1.5 km vertical vorticitycontours : 2.5 km vertical vorticity
(dBZ)
(dBZ)
line-relative wind vector at 2.5 km height
(s-1)
(s-1)
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23/2130 zshaded : dBZblack contours : RI
shaded : 2.5 km vertical vorticityblack contours : RI
shaded : wind speed(ms-1)
line-relative wind vector at 2.5 km height
(dBZ)(dBZ)
(ms-1)
(ms-1)
(s-1)
(s-1)
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23/2230 zshaded : dBZblack contours : RI
shaded : 2.5 km vertical vorticityblack contours : RI
shaded : wind speed(ms-1)
(dBZ)(dBZ)
(ms-1)
(ms-1)
(s-1)
(s-1)
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Vorticity budget analysis
shaded : vertical vorticitycontours : tilting terms ()
contours : stretching terms ( )
(s-1)
(s-1)
(s-1)
(s-1)
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23/2130 zshaded : 1.5 km vertical vorticitycontours : verticial velocity
shaded : vertical velocitycontours : horizontal vorticity
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Vortex line (southern bow segment)
shaded : 1.5 km vertical vorticity
(s-1) (s-1)
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northern bow segmentshaded : dBZred contour: RI at 2.5 kmblue contour: RI at 1.5 km
shaded : 1.5 km vertical vorticitycontours : 2.5 km vertical vorticity
line-relative wind vector at 2.5 km height
(s-1)
(s-1)
(dBZ)
(dBZ)
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23/2300zshaded : dBZblack contours : RI
shaded : 2.5 km vertical vorticityblack contours : RI
shaded : wind speed (ms-1)
(s-1)
(s-1)
(dBZ)(dBZ)
(ms-1)
(ms-1)
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23/2400zshaded : dBZblack contours : RI
shaded : 2.5 km vertical vorticityblack contours : RI
shaded : wind speed(ms-1)
(s-1)
(s-1)
(dBZ)(dBZ)
(ms-1)
(ms-1)
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Vorticity budget analysis
shaded : vertical vorticitycontours : tilting terms ()
(s-1)
(s-1) (s-1)
(s-1)
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shaded : 1.5km vertical vorticitycontours : verticial velocity
shaded : vertical velocitycontours : horizontal vorticity
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Vortex line (northern bow segment)
shaded : 1.5 km vertical vorticity
(s-1) (s-1)
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The collapse of the big bowing structure
shaded : dBZstream line : 2.5 kmblack contour : RI
(dBZ) (dBZ)
(dBZ) (dBZ)
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shaded : wind shear
thick contours : θ (K)thin contours : θe (K)
contour : geopotential height (m)
(wind at 850 hPa)
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The formation of the bowing structure is found to be closely associated with the rear inflow.
Midtropospheric rear inflow is forced by bookend vortices, environmental flow and horizontal pressure gradient are not primarily responsible for the formation of the rear inflow.
Bookend vortices form mainly through tilting of horizontal vorticity: southern part:Vortices are formed through the upward tilting of lower-
level baroclinic horizontal vorticity.
northern part: vortices are formed through the downward tilting of
upper-level baroclinic horizontal vorticity.
Conclusion
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THE END
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冷池與環境風切的平衡
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2330 UTC
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