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Advances in Sciences and Engineering
6:1 (2014)
Analysis on Radar Echo of Active Cold Front
Xianda Bai
. Guilin meteorological Bureau in Guilin 541001, China
E-mail:[email protected]
Abstract: Active cold front often occurred in Guilin area among April to May, it often accompany with
thunderstorm winds and hail and heavy rain process. Study on this disasters weather, it can provide scientific
gist on disaster reduction and prevention. Use the radar observation data to analyze the weather process, we can
find that the frontal radar echo are ribbon pattern, and it arrangement form northeast to southwest, there are
some scattered block echo in the warmer area ahead of the cold front, the echo band move toward south quickly,
Echo band constituted with many small and medium scale of convective echo block. It often caused local heavy
rain and strong wind or hail. Analysis on radar velocity diagram we can find that wind reversal with height in
Guilin and cold advection is obvious, so the cooling is larger after the frontal.
Keywords: Active cold front; radar observation; echo analysis.
1. Introduction
Guilin is located in the south of Nanling mountain, it often impacted by cool or warm air in spring. When th
e strong cold air came from north cross the Nanling, it move can accelerate and form as active cold front. It ofte
n brine short-term thunderstorm winds, hail and strong convective weather where impacted by active front. Guili
n CINRAD/SB business application was started in 2004. It obtained the complete observation data of all active c
old front weather in 2004-2013. It can provide the basic data for the analysis.
Spring is a transition season from winter to summer. The south warm moist air flows began to active and co
ntinuously to affect northward, in the same time, north cold air still have some activated, strong cold air will be
pressure to the south. As a result of the action of Nanling mountain, cold air will be resist in the process when th
e cold air move southward, frontal moving slowly and slope is larger, cold air accumulation in the back of the fr
ontal. The temperature gradient is larger between the frontal. When a cold front across the mountains,action b
y the cold air gravity and sinking, the front moving will be speed up, and form as active cold front. That is the se
cond type cold front of meteorology[1], Due to the frontal zone move faster, slope large, and the atmosphere are
convection instability, convection movement booming, the weather changes is dramatic. Liu Zhaomin[2] used th
e density current to parse the nature of the active cold front. The density current forward movement speed formu
la is:
C 2  ( K 2 gh)
T2  T1
T2 and T1 in formula (1) are virtual temperature of cool or warm air mass, g are acceleration of gravity, h are
thickness of the cold mass, K2 is empirical coefficient that value is 1.4.
From formula (1), we can see that the value of T1- T2 is larger, the temperature difference before and after
cold front are greater,the frontal moving speed of the C are faster. If the cold thickness (h) is larger, the cold
air forces are stronger,the frontal move faster.
This kind of weather forecast is difficult only according to the weather analysis and numerical forecast prod
ucts analytical applications. The cold air can be better forecast, but it is difficult to forecast the specific influenc
e place and time. Because of data limitation and interval cannot satisfy with the needs of the change faster weath
er system monitoring, it is difficult to accurately forecast on active front impact by conventional meteorological
satellite cloud image analysis. The application of a new generation of weather radar can implements the continu
ous uninterrupted monitoring weather system. The abundant radar products can provides the conditions for anal
ysis on sharp change of weather systems. Radar products are widely applied to the weather analysis [3-4]. Yang Y
inming etc studied on the structure and characteristics of convective echo system of the tropical depression, cons
idered of the tropical depression rain storm was mutual influenced mainly by multi-scale system, and the mesosc
ale systems include with convective echo, thunderstorm group [5]. Ma Zhongyuan etc. [6] using the Doppler weat
her radar information to studied on the gale of mesoscale systems. Si Lei etc. [7] made an analysis on the strong c
6:1 (2014)
Advances in Sciences and Engineering
onvective weather caused by cold vortex that observation by radar. There are many research for the strong echo
characteristics of hail monomer[8-9], local strong convective weather caused by mesoscale system has become th
e consensus of scholars; Jia Xianfeng etc. analysis on Guangxi rainstorm radar echo characteristics[10], notice the
precipitation echo stability is the important cause of heavy rain. But the study of active front is less. The discuss
ed on the active front radar echo characteristics affecting of Guilin using new generation of weather radar observ
ations data are given in this paper.
2. The influence of active clod front
There are 7 of more typical active cold front weather process in Guilin region from 2004 to 2013, (20040503,
20050505, 20060410, 20070417, 20100401, 20120425, 20130515). It has appeared a thunderstorm, heavy rain,
wind, hail and strong convective weather successively when the active cold front influence in Guilin area. Syste
m impact time is short. The temperature decreases significantly when the cold front impact. The hail and thunde
rstorm winds affect in Guilin from north to south east on April 17, 2007. South Guilin city also appeared a short
hail on April 25, 2012, this frontal weather made large losses. Using formula (1) to calculate the moving speed o
f active clod front influence of Guilin, the cold front moving average speed are 15-17 m/s, and the fastest is to 2
6 m/s.
3. The weather situation of active cold front
We can found that they all have common weather circulation characteristics of these 7 active clod front weath
er situation. There is an upper trough on the 500hPa, a shear line on 850hPa and a clod front on the ground, this
are configured excellently. These systems are slightly forward increasing with altitude. It meets the fundamental
circulation situation of the cold front [11]. For example on 20040503, 500hPa west wind trough is located in long
itude 107 °at 08:00 of May 3, (figure 1), it is already very close to Guilin. From Guangxi coastal, Guilin, go to
Hunan all are controlled by southwest airflow before the 500hPa trough; 850hPa shear line is located in the cent
ral of Hunan to southern of Guizhou, Guilin are controlled by southwest air flow in the south area of the shear li
ne. Ground front located in the junction of Hunan and Guangxi provinces. These weather systems location slight
ly forward as height lifting. Consistent southwest wind has observed in 08:00 in Guilin counties, the southwest
wind speed are 8m/s in Guilin station. A wet tongue can be found from south China coastal to Guilin and to the
central of Hunan by analysis on the dew point temperature of each high layer. The active cold front move south
ward faster when it is over the Nanling mountain at 11:00, it affected Guilin area at about 18:00. It changed as n
ortheaster wind all of Guilin area surface at 20:00. The active cold front moved 400km in 7 hours. This move sp
eed accordance with the calculation results of formula (1). When a active cold front arriving to Guilin, the north
ward wind and cooling is obviously, the temperature has dropped a 8-10℃, the wind speed has reached the 12m
/s, and short-term wind speed reached 18m/s. Due to active cold front moving fast and the rainfall intensity is ve
ry big, so the precipitation is uneven, local rainfall reached 44 mm in some place in 1 hour.
Figure 1, Weather system place sketch map. Red five stars location Guilin, Blue arrow for water vapor transmission channel.
Advances in Sciences and Engineering
6:1 (2014)
4. Analyzed on Radar echo characteristic of active cold front
By radar observed on 7 times of the process of active cold front respectively, we can fond that the radar echo
is narrow band for active cold front. Several mesoscale of echo is inlaid with echo band. Velocity diagram confi
guration is better and response to cold advection is obvious.
4.1. Banding echo characteristics
4.1.1. Analyzed on precipitation reflectivity factor of radar
Active cold front of radar echo characteristics mainly is a narrow echo band on the reflectivity graph of rada
r. Figure 2 is evolution feature of active cold front radar echo when it impact of Guilin on May 3, 2004(Elevatio
n=0.5°), the center are in Guilin radar station in the figure. A banded precipitation echo is in Hunan and Guangx
i junction at 9:00 (fig.2a), a bulk echo in Guizhou moving to band echo and merger with it, it make echo to stren
gthen in the merging process. There is some scattered precipitation in the southwest warm moist flows area whic
h is in the southeast of Guilin, the radar echo generation or dissipation is soon. Echo band begin to enhancement
at 11:00 (fig.2d) and influence to Guilin. Behind the main echo belt, there is another weak echo belt moving sou
thward. Two echo belt move southward and the back of the weak echo with reduced at 13:00 (fig.2f), both echo
belt move pass Guilin centre at 14:00 and move out of Guilin area at 15:00 (fig.2h).
Figure 2, Reflectivity products of Guilin CINRAD/SB On May 3, 2004, the elevation angle is 0.5 °, The time (utc) in the
graph are: a(01:05)、b(02:31)、c(03:02)、d(03:51)、e(04:04)、f(05:29)、g(06:24)、h
Echo center strength reach to 45dBz when it begin to enters Guangxi region, and the center strength developmen
t to 55dBz when the echo move closer to Guilin, next, the center intensity has been less change with stable. Narr
owly radar echo band are very small, it is about 50km width, the strong echo center range are even smaller, it wi
dth are only 5km or so. Echo thickness is uneven and changed a lot over time. Echo top height is slightly elevate
d with echo belt move southward, the echo top height is between 6-8km when it entering Guangxi border. The e
cho top height increase to 8-11km when the echo belt moved near to Guilin. Before radar echo remove out Guili
n range, the echo top height is 8-12km. It shows that the active cold front echo belt continuously development in
the process of echo belt moving southward. The scattered precipitation echo of warmer area ahead of active col
d front combined with the cold front echo continuously. Merger caused echo intensity increase and echo top hig
h increase. Echo belt top average height is about 5-6km. The top high of the strong echo center are about 9-12k
m. (echo top height figure been omit).
4.1.2. The bulk echo characteristics
There are some mesoscale convective monomer always inlay the active cold front echo band. The most comm
only characterized is that there is a strong narrow-band echo with the cover of the echo band. The medium and s
mall scale systems often appear more heavy rain center or hail [12] corresponding to the ground. Scattered patche
6:1 (2014)
Advances in Sciences and Engineering
s echo in warmer area of ahead of front may be merger together and cause the strong convective weather in this
area. As follows figure 3 are analysis on typical process on April 17, 2007, for an example.
Figure 3: Guilin CINRAD/SB reflectivity factor (production No.19) on April 17,2007. elevation angle is 2.4 °, Detection
time (Beijing) are: a(07:44)、b(09:35)、c(10:32)、d(11:28)、e(12:23)、f(13:25)、g(14:
On April 17, 2007, in the morning, Guilin is located in warm area, the southern margin of the cold front, the tem
perature is about 21℃. At 6 a.m., the echo monomer generated in the south of Maoer mountain of Nanling was
development in the process when the radar echo is moving eastward. At 7:30-7:50, there have been hail weather
in the Quanzhou county, the northeast of Guilin. (fig.3A). Echo is in isolated patches, and the echo intensity reac
hed to 65dBz, the echo top up to 18 km; As the echo belt move southward from the south of Hunan, the bulk ech
o intensity in Ziyuan county development to 70dBz, (fig.3C), the echo top up to 17 km, it cause the hail weather
in the south of Ziyuan county; At 2 p.m., (fig.3C), the hail weather were caused by frontal echo with strong cent
er in the Gongcheng and Lipu county, south of Guilin. The whole city of Guilin from north to south has appeare
d the thunderstorm winds and short-time strong rainfall. With the echo belt southward moving, the temperature i
n Guilin is declined by 8-10℃.
Analysis on profile of caused hail strong echo block, we can see the echo dome, nail three body scattering etc.
characteristics of typical strong convective monomer.
4.2. The average radial velocity product characteristics
Figure 4 is on May 3, 2004, Guilin radar average radial velocity diagram. (Elevation Angle is 2.4°), the cool
color graph is the towards radar radial velocity, (negative velocity), warm color is to leave the radar radial veloci
ty, (positive velocity).
According to the configuration of positive and negative velocity and radial velocity analysis principle [4,13], it
can be found that there is unanimous northwest wind for all of the active cold front process. It is southwest wind
before the cold front, after the cold front it is big northwest wind in all layers, the wind speed is up to 10~20m/s,
it rapidly promote cold front southward pressure. According to the average radar radial velocity diagram config
uration relations, from figure 4a, 4b, 4c between the positive and negative speed, a cold front can be analyzed. A
t 10a.m. on May 3, the active cold front move southward from the border of Hunan to Guangxi, it affect to Guili
n at 11a.m. (Fig. 4d).
Advances in Sciences and Engineering
6:1 (2014)
Figuer4 Guilin CINRAD/SB average radial velocity of products in May 3,2004, in the figure radar levation Angle is 2.4 °.
Detection time (Beijing), respectively are: a(10:31)、b(11:26)、c(11:32)、d(12:34)、e(13:23)、f
Guilin was controlled by north wind at 11a.m., It can be seen from the figure 4e that Guilin near the ground has t
urned to northeast wind; In fig. 4f, north wind first appear in close to the ground, at same time, weak southwest
wind appear in high altitude. Guilin was control by north wind after 3p.m.(fig.4g). The north wind is left radar i
n central to southern part of Guilin in Figure 4h, when storm come out of Guilin, the weather in north part of Gu
ilin are sunny, it is no radar echo in the north area.
Due to the active cold front moving quickly, the center of the positive and negative speed is greater than 20m/
s can be analyzed on the velocity diagram, 200km away from the center of radar station has a speed of fuzzy are
a, this is the instruction significance for forecasting active cold front.
4.3. Analysis on VAD wind profile
To analyze in the evolution of wind profile characteristics of the active cold front process, the wind profile dat
a are put together, on May 3,2004, 8a.m.~6p.m. (universal time in figure, it differ 8 hour from Beijing time). Th
e wind direction evolution of the active cold front can be found (see figure 5).
On May 3, 11:26, Guilin is northwest wind from surface to 4 km height, wind direction is very mess, after 12:
20, there are north wind from surface to the high altitude, at 12:52, it is north wind in high altitude again, it begi
n to turn north wind at 13:51 at ground floor. The wind changes showed that the upper trough has transit out Gui
lin at 1p.m. The high altitude is north wind first, and then the north wind change down, finally all layers are nort
h wind. The trough place forward significantly. According to the principle of synoptic meteorology, the weather
caused by forward trough system is vehemence, the strong wind and hail convectional weather is likely to happe
By analysis on the wind speed evolution in figure 5, it can be found that before active cold front transit pass G
uilin, wind direction with height increase clockwise rotation of Guilin from ground to 5km altitude, nearly groun
d are warm advection. 12:58, it is north wind at 10km altitude, 13:41, active cold front has passed over Guilin, it
is north wind on the ground, at the same time in 2~3km and 5~8km altitude, the wind with height increase rever
se, cold advection obviously. After the cold front parsed, Guilin temperature drop around 8~10℃. But due to lac
k supplement of cold air, the temperature is quickly recovered. According to the configured of temperature adve
ction, Guilin's atmosphere stratification is very unsteadiness, easy to produce convection weather.
6:1 (2014)
Advances in Sciences and Engineering
Figure 5. On May 3, Guilin CINRAD/SB radar vertical wind profile product, (For universal time).
5. Analysis on rainfall data
From radar’s vertical liquid water content, 1 hour and cumulative rainfall, 3 hours accumulated rainfall, and o
ther products analysis found that, all these active cold front process affected to Guilin has a narrow belt of rainfa
ll and it move quickly from north to south. There is some small and scattered rainfall in the south side of the ban
d echo. The north side of the band echo is clear sky area, explains it has for high pressure control and sunny wea
ther after cold front.
Use the Guilin sounding data to analysis on this several times of active cold front process, from logarithmic
pressure chart can analyze the unstable stratification of atmospheric layer existed and show that Guilin atmosphe
re is convective instability, The favorable environment field configuration, easy to cause convection we
ather [14].
Using encryption automatic rain station data for analyzed on the active cold front process, 1 hour rai
nfall of more than 30mm site distribution is very asymmetry. It shows that the rainfall affect by cold fr
ontal passage are obviously uniformity.
6. Brief Summary
(1) Analysis of active cold front of Guilin, we can found the active cold front is mainly develops in the spring, a
nd have a similar same weather situation. Frontal slightly forward, 500hPa upper trough is not deep, 850hPa coo
led type wind shear is obvious. When the cold front system moved across the Nanling, it will move faster effecti
ng by go downhill action.
Due to lack of cold air supplement, cold air modified fast, temperature recovery fast. This is a familiar feat
ure of the active cold front. As the weather system is not strong enough, from the weather map analysis is diffic
ult to forecast a cold front that will impact on Guilin with quick south moving.
(2) The new generation of weather radar has a strong ability for monitoring of fast moving weather process. Bef
ore the frontal system entering to Guilin area, 1~2 hours in advance we can see a narrow echo pressuring southw
ard quickly, the rain echo in 2~3km height appeared first, after two individual sweep, echo transit just can be ob
served near surface. This forward configuration of weather system easy to generated the short-term thunderstor
m winds, hail and heavy rain.
(3) Before active cold front impact, Guilin local belongs to warmer area, echo is mainly scattered patches. This s
cattered echo area in favorable weather situation will be developed as mesoscale echo enhancement area (MEE
R), the mesoscale convective will generated a strong convective weather. When active cold front impact, the ech
o characteristic of ribbon is obvious, a number of small and medium scale echo system has inlaid with echo belt.
This strong monomer system often caused thunderstorm winds and strong convective weather at ground.
Advances in Sciences and Engineering
6:1 (2014)
(4) Analyzed on radar profile product of active cold wind process, it can be find that the wind anticlockwise cha
nge with height, as the cold advection, corresponding surface cooling is obvious.
(5) Zonal echo top high is average of 7~12km, echo block strong center top high of medium and small scale can
reach 18km to 20km. The small and medium scale patches echo intensity commonly are 60dBz or above, and ec
ho top high are 15km or so in Guilin area of hail weather.
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