Transcript From noon to dusk on April 6 2001, Ningxia Hui Autonomous

Mechanism, Kinematics and
Topographic Effect Analysis on
Sandstorm Satellite data in Ningxia:
A Case Study
Hu Wendong1,2 Gao Xiaoqing3
(1Key Laboratory of Meteorological Disaster Preventing and
Reducing in Ningxia, Yinchuan, China 750002; 2Ningxia
Meteorological Observatory, Yinchuan, China 750002; 3Cold and
Arid Regions Environment and Engineering Research Institute,
Chinese Academy of Sciences, Lanzhou, China 73000)
1．Summary
From noon to dusk on April 6 2001, Ningxia
Hui Autonomous Region was attacked by a
gust of gale which attained 8-9 degree at
maximum and companied with sand
invoked by the quick movement of cold
mass from Mongolia. Eight meteorological
stations in Ningxia observed sandstorm
such as Yinchuan, Pingluo. Here below is
the meteorological record.
Table 1 instantaneous peak gust (unit: m/s), sandstorm and their
appearing time of the stations in Ningxia on April 6, 2001.
Yanchi
Mahuangxhan
Tongxin
Weizhou
Xingren
Haiyuan
Liupanshan
Jingyuan
53
916
18
22
17
19
19
19
22
18
12：
39
14：
52
13：
44
14：
13
12：
38
13：
46
18:3
2
17：
02
53
614
53
612
53
617
53
705
20
17
18
18
17
14：
59
14：
42
14：
44
14：
31
13：
20
Huinong
Zhongning
53
910
Qingtongxia
53
806
Wzhong
53
707
Yinchuann
53
881
Taole
53
810
Pingluo
53
727
53
615
53
519
Maxwi
nd
time
Station
53
723
53
611
ID
Duststorm
√
√
√
√
√
√
√
√
Durati
on of
duststo
rm
13：
2817：
56
15：
1016：
08
15：
0815：
40
15：
1716：
45
13：
4116：
51
16：
4518：
39
14：
2318：
09
12：
2412：
29
2．Synoptic analysis
2.1 Prophase features
• At 08:00 BST on April 1st 2001, at 500hP, east Asia
trough lay in east Japan, Area expanded from Xinjiang to
Siberia was controlled by a high ridge, the strong cold
mass followed the trough.
• Because of the ridge, the temperature of Ningxia from
April 1st to 5 was higher than that of historical mean. The
anomaly was 5.3℃ over 24 stations, among them the
lowest was 3.3℃ at Xiji, and the highest was 6.6℃ at
Dawukuo. The precipitation anomaly was negative
except the 4 stations in the southern part of Ningxia. The
shortage of precipitation, higher temperature made the
topsoil much drier; it was favourable for the sandstorm.
2.2 Upper air circulations
Fig 1a, 20:00 BST April 5 2001, geopotential height at 500hPa (10gpm)
2.3 Surface situation
Fig1b, 08:00 BST, April 6 2001, surface analysis
Thin line: sea-level pressure, thick line: allobar in 24h, (hPa).
In this case, the sondages in Yinchuan and Minqin (fig 2,
Yinchuan) showed stable stratification, and it was not
favourable for vertical movement.
The strong northwest jet at low and medium level of
atmosphere and the cold front in surface were the major
reasons to the sandstorm.
3. Development and evolvement of system
• At 08:00 BST April 6, the wind at 500hPa increased.
Meanwhile the surface chart indicated the Ningxia was
clear with pretty high visibility, it was easy to absorb solar
radiation here, and provided energy for the development
of weather system. At noon, the cold air coming from
Hexi and Alaxar, and invaded Ningxia, cold front made
the visibility, temperature and humidity dropped greatly,
and the wind speed increased. The flying dust and
sandstorm were observed after the cold front passed.
Take Yinchuan as an example (fig 3), the cold front was
passing around 14:00, instantaneous velocity of wind
reached 18.1m/s, the sand with gale made the visibility
fell, and during 15:08 to 15:40 attained the standard of
sandstorm.
Fig 3, April 6 2001, temporal variations of pressure (a), temperature (b),
relative humidity(c), visibility (d), and wind speed (e) in Yinchuan
4. Satellite image features
4.1 Identify and characteristics of satellite image
• The humidity at low level in this case was very low.
• High humidity areas were high clouds in satellite images.
• The positions of high clouds could be identified from vapour
image, and the other high grey level regions in infrared images
were sands according to experience.
• In the image at 09:00 BST, the cold front clouds could be found
stretching from east part of Alaxar to Hexi, the bright
temperature was low and the low clouds covered east and
south part of Gansu province.
• Stable stratification
• The sand movement was confined to the vicinity of surface, the
even bright temperature in the image is the major feature and
the gradient of bright temperature was pretty low.
4.2 Movement of sandstorm
• The sand area in Hexi corridor was with high integrity at 09:00
BST and the integrality of that in Alaxar was much worse.
• Considering the surface and 700hPa wind, it is easy to find that
the wind field in Hexi was stronger and with better integrality.
The southeast movement of sandstorm in Hexi advanced
rapidly and that in Alaxar was much slower at 10:00 BST, it
comes from the difference of wind at low level atmosphere. The
bright temperature of sandstorm in Hexi got lower and much
integrated.
• At 12:00 BST Lanzhou and Baiyin were controlled by the sands
and the sandstorm approached to west border of Ningxia. The
sands intruded Ningxia from south and north passes of Mt.
Helan at 13:00 BST, and the speed of the sandstorm dropped a
little. Most of the north and middle part of Ningxia completely
fell into sands at 14:00 BST, and south city Guyuan was
attacked by sands at 15:00.
4.3 Change of sandstorm movement
4.3.1 Typical particles and calculation
• 4 particles were selected in sandstorm and
the particles passed different regions, a:
Shizuishan
in
north
Ningxia,
B:
Qingtonhxia at south pass of Mt. Helan, C:
Zhongning-Zhongwei in central Ningxia,
and D: Guyuan-Xiji in south Ningxia. The
particles above represented the movement
of different sections of sandstorm (fig 4).
Fig 4, leading edge hourly movement of sandstorm
Fig 5, speed temporal variations of particles A, B,
C and D, which lie from south to north.
速度（Km/hr）
质点A速度
120
100
质点C速度
80
60
40
20
0
120
速度（Km/hr）
质点B速度
10
11
12
13
14
15
16
17
18
19
16
17
18
19
北京时
100
80
60
40
20
0
10
11
12
13
14
15
4.3.2 Analysis
• The speeds of 3 particles in north part shared the
consistency (fig 5a), low for the initial several hours,
and reached their peaks around 14:00 BST, the
northern the later of its maximum. The feature of
particle C was similar to that of particle D, attained
peak velocity earlier.
• It is important to note that the particle B did not go
along the line in the figure. The fact is it came from
south pass of Mt. Helan laterally.
• The speed of particle D in south most was higher
initially (fig 5b), and reduced rapidly afterward. At
18:00 BST its speed regained suddenly because of
the geographic move-around. It means the particle
went laterally in previous hours and returned the
path at 19:00 BST. It is incorrect to think the speed
increased abruptly.
• The
movement
was
related
with
topography. The north particles passed the
plain area in the morning and it is much
easier to push ahead, but the south particle
had to trek in mountain area, and it is much
difficult.
• Even though the pursuing and orientating
work was carried out by hand work, and
there must be errors, the movement
tendency of 4 particles were very clear,
4.4 Structure of sandstorm
• Distinct
texture
characteristics
of sandstorm
movement in the images because of strong wind.
• Automatic equalization process.
• The motion direction, sand distribution inside the
sandstorm can be seen in fig 6. The severe centers of
sandstorm had high continuity at 12:00 BST or 04:00
GMT (fig 6a) with an excellent ability of moving and
developing.
• Trend of decline at 13:00 BST or 05:00 GMT, the
severe centers were discrete and there were 2 centers
which were shrinking. It can be conducted the
sandstorm in Hexi was weakening by analyzing
continuity, density and covering area of severe center.
Fig 6, equalized images of sand-dust area in Hexi at 12:00 BST (a) and
13:00 BST (b) on April 6, Here the time signed in the images is in GMT
5．Discussion and explanation
5.1 Weather background
• It is a typical sandstorm, strong jet at low
and medium level in atmosphere, quickly
moving cold front on surface, high allobar
and pressure gradient caused gale. Since
the air was dry, it was difficult to form
precipitation. Land surface with sand in
Hexi and Alaxar was very dry in spring, it
is very easy to blow the sand into air and
cause sandstorm.
5.2 Dynamic-thermo mechanism discussion
• It is reasonable to consider the vapour
condensation heat had little effect. The energy
came from atmosphere kinetic and the solar
radiation also.
• Kinetic dominating period: In the morning, the
solar radiation was weak, the kinetic effect was
much more important than that of radiation. The
sands activity was violent wherever the wind
was strong, so the sand in Hexi where was
controlled b northwest gale was intense and the
moving speed was higher, and the strip texture
and intensity were obvious. Meanwhile the
sands activity in Alaxar was relatively weak.
• Thermo enhancing period:
• At noon, Ningxia surface had accumulated energy because of
solar radiation. The sandstorm came from Alaxar regained and
the velocity increased after passed the Mt. Helan. The sands
came from Hexi slowed down as the kinetic dropped.
• Comprehensive waning out period:
• The sands covered most part of Ningxia the afternoon. The
visible channel image shows the albedo was high, the radiation
energy was reflected by sands, and the thermo mechanism was
destroyed. The main body of cold air mass occupied Ningxia,
pressure gradient was very low and wind bated, dynamic was
losing, there were no mechanisms to maintain sandstorm and it
could not go ahead any more.
• At 23:00 BST sands in some sections were blown low and fell
down soon after the wind became calm, and these parts of
sandstorm retrogressed. The large area swept by sandstorm
went to die out because of the energy dissipation.
5.3 Topographic impact on sandstorm
• Blocking of Qinghai-Tibetan plateau: The satellite image at
08:00 BST shows the front expanded to Qinghai and the wind
was favourable for the sands to move to Qinghai. But there
were no sandstorm record except Mang'ai and Lenghu. The low
moving sands could not fly over the high plateau, and the
plateau prevented Qinghai province from being attacked by
sandstorm.
• Hindering impact of Mt. Helan: Mt. Helan is orthogonal to the
motion of sandstorm. Its height is not high enough to hold back
the sands. Satellite images show, at 11:00 BST, the sands
approached to Mt. Helan, and sands accumulated in front of the
mountain in stead of went over. At 13:00 the east side of
mountain was covered by sands, but little of sands activity in
Yinchuan. The satellite images indicate the sands went around
the north and south ends of mountain, and the 2 airflows met in
Yinchuan. The Mt. Helan hindered the movement of sandstorm
about 1-2 hour.
• Guiding impact of Mts. Xihua-Yueliang-Liupan:
Mts. Xihua-Yueliang-Liupan is along the
direction of sandstorm, and the mountains
guided the sandstorm into the valley area. It
could be found the sandstorm approached to Mt.
Xihua at 14:00 BST but could not over pass it
and the sandstorm was conducted into valley of
river Qingshui. The sandstorm advanced along
the valley, and passed Guyuan closed to
Jingyuan, but it could not cover the mountains at
end.
5.4 Height of sands
• If the clouds or sands are high enough,
their shadows can be found in visible
channel usually in afternoon. In this case,
there was no shadow of sandstorm, so the
height of sands was not high. Calculated
by infrared data, combing with sondage
and comparing with topography of Ningxia,
we found the motion of sandstorm was
along the contour of 2200m, so the height
of sandstorm was about 2200m.
6．Conclusion
• 1 Strong northwest jet at low and medium level of
atmosphere and rapidly moving cold front with great
gradient of pressure and allobar were the basic
systems for the sandstorm. The sands were
confined to the low layer of atmosphere; the height of
sands was about 2200m according to satellite data.
• 2 Surface layer kinetic and solar radiation were the
energy sources of sandstorm. In the morning, kinetic
was the major factor and the sands in Hexi advanced
quickly. In the afternoon, solar radiation played a
great role and the sands came from Alaxar developed
dramatically. The 2 mechanisms vanished at dusk,
and the sandstorm died away. It is very important to
understand the inner rule of sandstorm to forecast
sandstorm correct in detail.
• 3 Satellite images have a perfect capability to monitor
the leading edge of sandstorm; the particles in
different positions were consistent with the mechanism,
the intensity of sands was distinct on the images, and
it is crucial for sandstorm warning.
• 4 The interior information could be revealed by image
processing, and with this information the motion;
evolvement could be shown which is very difficult to
get with other methods.
• 5 Landscape impacts on sandstorm. Landform with
different directions, scales affected the sandstorm in
different ways. Large-scale terrain blocked the sands,
smaller one attacked directly hindered the motion of
sands, and the mountains share the same direction
with the motion of sandstorm guided it into valley area.
Topography in different area made different process
and intensity of sandstorm.
Thanks
谢谢
Merci
• HU Wendong 胡文东
• Email: [email protected]