Shu-Hua Chen 12 , Yuh-Lang Lin 3 , Zhan Zhao 2 , and Heather Reeves 3
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Transcript of Shu-Hua Chen 12 , Yuh-Lang Lin 3 , Zhan Zhao 2 , and Heather Reeves 3
Parameters Controlling Precipitation Associated with a Conditionally Unsaturated, Unstable Flow over a Two-
Dimensional Mesoscale Mountain
Shu-Hua Chen12, Yuh-Lang Lin3,
Zhan Zhao2, and Heather Reeves3
1 National Central University2 University of California, Davis3 North Carolina State University
Outline
o Introduction
o Numerical Simulations
and
Results
o Summary
Upstream Propagating Convective System
Introduction
Jou, 1997Jou, 1997
Quasi-stationary Convective System
Introduction
Introduction
Downstream Propagating Convective System
Objective
Perform idealized simulations
for a conditionally unstable flow
over a 2D mountain ridge to
investigate the propagation and
types of cloud precipitation
systems controlled by three
control parameters.
Introduction
Control Parameters
► Moist Froude number, Fw
U Basic flow speed (m/s)
h Mountain height (m)
Nw Moist Brunt-Vaisala
frequency
► CAPE
► Orographic aspect ratio
(h/a)
a half mountain width
hNU
Fw
w
zg
N v
vw
2
• Weather Research and
Forecasting (WRF) model
• 2D simulations
• Domain = 1000 km x 20 km,
∆x = 1km, and vertical grid
interval stretched
• Purdue-Lin microphysics
scheme
• Bell-shaped mountain with a
half width, a, and a mountain
ridge, h.
• Integration time = 10 hours
Model Configuration
21 a/)xx(
hh
osfc
Experiment Design (CAPE)
Long-dashed lines from right to left are soundings for CP0, CP1, CP2, CP3, CP3,CP4, and CP5, respectively. The CAPE values for them are 487, 1372, 1895, 2438, 3000, and 3578 J/kg, respectively.
Nw = .942 ~ 1.01 x 10-2 s-1
LFC
U = 2.5, 5, 10, 15, 20, 30
m/s for
F1 – F6, respectively
h = 2 km
a = 30 km
hNU
Fw
w
Fw
CAPE(J/kg)
0.131 0.262 0.524 0.786 1.048 1.572
487
1372
1895
2438
3000
3578
CP0F1
CP1F1
CP2F1
CP3F1
CP4F1
CP5F1
CP0F2
CP1F2
CP2F2
CP3F2
CP4F2
CP5F2
CP0F3
CP1F3
CP2F3
CP3F3
CP4F3
CP5F3
CP0F4
CP1F4
CP2F4
CP3F4
CP4F4
CP5F4
CP0F5
CP1F5
CP2F5
CP3F5
CP4F5
CP5F5
CP0F6
CP1F6
CP2F6
CP3F6
CP4F6
CP5F6
Fw
CAPE(J/kg)
0.131 0.262 0.524 0.786 1.048 1.572
487
1372
1895
2438
3000
3578
CP0F1
CP1F1
CP2F1
CP3F1
CP4F1
CP5F1
CP0F2
CP1F2
CP2F2
CP3F2
CP4F2
CP5F2
CP0F3
CP1F3
CP2F3
CP3F3
CP4F3
CP5F3
CP0F4
CP1F4
CP2F4
CP3F4
CP4F4
CP5F4
CP0F5
CP1F5
CP2F5
CP3F5
CP4F5
CP5F5
CP0F6
CP1F6
CP2F6
CP3F6
CP4F6
CP5F6
Experiment Design (U and CAPE)
Shading – rainfall Contours – w at 3.6km
Shading – w Contours – θ
7h
CP4F1Fw=0.131
(2.5m/s)
CP4F3Fw=0.524
(10 m/s)
I
II
Flow Regimes(CAPE = 3000 J/kg)
(CAPE = 3000 J/kg)
CP4F4Fw=0.786
(15 m/s)
CP4F6Fw=1.572
(30 m/s)
Mixed convective and stratiform clouds
Stratiform clouds
III
IV
Flow Regimes
What makes a straitform cloud
develop over a mountain ? (Large CAPE)
3 time scales are relevant:
1) Advection time: Tadv ~ a/U
2) Cloud growth time: Tc
(controlled by microphys processes)
3) Orographic perturbation time:
Assume Tc=20 min, a = 30 km:
For U = 15 m/s, Tadv = 20 min ~ Tc
=> convective cloud may develop
For U = 30 m/s: Tadv = 10 min > Tc
=> not enough time for a convective cloud to develop
Toro ~ 4 – 5 min
CAPE
ZZ~Toro LFCEL
2
Total Accumulated Rainfall (10h)
Small Fw
Large Fw
When Fw increases,
the flow shifts to a higher number
Flow regime.
II
IIIII
IVIV
Small CAPE
Large CAPE
IIII
III
I
When CAPE increases,
the flow shifts to a lower number
Flow regime.
Total Accumulated Rainfall (10h)
Orographic Rainfall
Small Fw
Large Fw
II
Orographic Rainfall(Large Fw, Strong Wind)
Small CAPE
Large CAPE
2D Flow Regime Diagram
(Fw and CAPE)
Bifurcation?
II IIIIII
IVIVIIII
Flow Regime Table(Fw and h/a)
hNU
Fw
w
A1(2/7.5)(7.5km
)
A2(1/7.5
)(15km
)
A3(1/7.5)(22.5k
m)
A4(1/15)(30km
)
A5(2/45)(45km)
F1 (0.131)(2.5m/s)
I I I I I
F2 (0.262)(5m/s)
I I I I I
F3 (0.393)(7.5m/s)
I I I I I
F4 (0.524)(10m/s)
II I I I I
F2 (0.786)(15m/s)
III III II II II
F2 (1.048)(20m/s)
III III III III III
F2 (1.572)(30m/s)
IV III III III III
Fw
h/a
h = 2 km, CAPE=3000 J/kg,
Accumulated Rainfall(vary mountain width)
U = 7.5 m/s
0
4
8
12
16
tota
l rai
n (x
1000
mm
)
7.5km 15km 22.5km30km 45km
Time (h)
2 4 6 8 10
U = 30 m/s
0
4
8
12
16
20
24
28
tota
l rai
n (x
1000
mm
)
7.5km 15km 22.5km30km 45km
Time (h)
2 4 6 8 10
a = 7.5
km
a = 22.5
km
a =45 km
IV III III
Accumulated Rainfall(vary mountain width)
Accumulated Total Rainfall
a = 22.5 km
0
4
8
12
16
20
24
tota
l rai
n (x
1000
mm
)
2.5 m/s 5 m/s 7.5 m/s10 m/s 15 m/s 20 m/s30 m/s
Time (h)2 4 6 8 10
a = 7.5 km
0
4
8
12
16
20
tota
l rai
n (x
1000
mm
)
2.5 m/s 5 m/s 7.5 m/s10 m/s 15 m/s 20 m/s30 m/s
Time (h)2 4 6 8 10
IV
Summary
FW FD
Regime I
C
FW FD
Regime II
C
Regime I: Flow with an upstream propagating convective system
Regime II: Flow with a long-lasting orographic convective system over the mountain peak, upslope or downslope
Regime III: Flow with a long-lasting orographic convective or mixed convective and stratiform precipitation system over the mountain peak and a downstream propagating convective system ; and
Regime IV: Flow with a long-lasting orographic stratiform precipitation system over the mountain and possibly a downstream propagating cloud system.
FW FD
Regime III
S/C C/S
FW FD
Regime IV
SC/S/N
Summary
When the Fw (or basic wind speed) increases, the flow tends to shift to a higher number flow regime.
When the CAPE increases, the flow shifts to a lower number regime.
When h/a increases, the flow shifts to a higher number flow regime.
When the CAPE is large, the orographic rainfall amount is not very sensitive to the Fw.
When the CAPE is small, the orographic rainfall amount is strongly dependent on the Fw.
The domain integrated rainfall amount is sensitive to Fw, but not to the aspect ratio, in particular for flow Regimes I and II.
Local orographic rainfall amount from straitiform precipitation systems can be as heavy as that from convective or mixed type precipitation systems.
Summary