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Vol.23 No.3
September
JOURNAL OF TROPICAL METEOROLOGY
2017
Article ID: 1006-8775(2017) 03-online-first paper
Structure Features and Composite Analysis of Convective Cells in a Warm Sector
Heavy Rainfall Event over Southern China
QIAN Lei (钱
磊)
1, 2, 3
, DING Zhi-ying (丁治英)
1, 3
, ZHAO Xiang-jun (赵向军)
1, 3
, XIA Fan (夏
蘩)
1, 3
(1. Key Laboratory of Meteorological Disaster, Nanjing University of Information Science and Technology,
Nanjing, 210044/ State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences,
Beijing, 10081; 2. Anhui Meteorological Observatory, Hefei, 230061; 3. College of Atmospheric Sciences,
Nanjing University of Information Science and Technology, Nanjing, 210044)
Abstract: This paper uses the ARW-WRF model to carry out a numerical simulation of a warm sector
heavy rainfall event over southern China on the 22-23 May, 2014. A composite analysis method was used to
analyze the evolution process and structural features of the convective cells on the convection line during
this rainfall event. This analysis identified three stages: (1) Stage of activation - the equivalent potential
temperature surfaces as lower layers start to bulge and form warm cells and weak vertical convective cloud
towers which are subject to the impact of low-level warm moist updrafts in the rainfall sector; (2) Stage of
development - the warm cells continue to bulge and form warm air columns and the convective cloud towers
develop upwards becoming stronger as they rise; (3) Stage of maturity - the warm air columns start to
connect with the stable layer in the upper air; the convective cloud tower will bend and tilt westward with
each increase in height, and the convection cell is characterized by a “crescent-shaped echo” above the
700hPa plane. During this stage the internal temperature of the cell is higher than the ambient temperature
and the dynamic structural field is manifested as intensive vertical upward movement. The large-value
centers of the northerly and westerly winds in the middle layer correspond to the warm moist center in the
cells and the relatively cold center south of the warm air column. Further analysis shows that the formation
of the “crescent-shaped” convective cell is associated with horizontal vorticity. Horizontal vorticity in the
center and west of the warm cell experiences stronger cyclonic and anticyclonic shear transformation over
time; this not only causes the original suborbicular cell echo form shape to develop into a crescent-like
shape, it also makes a convection line consisting of cells to develop to the northwest.
Key words: Convective cells; structural features; horizontal vorticity; composite analysis
doi: 10.16555/j.1006-8775.2016.03.paper serial number to be decided
1
INTRODUCTION
As one of the systems that affect heavy rain processes, the structural features of the small scale convection
cells have been closely studied by meteorologists. The internal structure and development processes of
thunderstorms were first investigated by Byers and Brahan by using field observations, after which they began to
establish the life cycle model of common cell thunderstorms[1]. Based on the improvement on the cell life cycle
theory of Byers and Brahan, Doswell proposed that the development of cell thunderstorms would experience three
stages: tower cumulus clouds; maturity; and dissipation[2]. The differences among the stages centered on aspects
such as cloud internal vertical airflow, temperature and water particle morphology (Doswell[2]). In recent years,
with the rapid popularization and application of radar data, a new understanding of the convective cell structure
has been developed. Based on the analysis of the meso-γ scale 3D dynamic structure of a Mei-Yu front rainfall
system, Zhou and Guo proposed that the meso-γ convective cell characterized by strong convection was the main
contributor of heavy rainfall, and that the cell dynamic structure with central and lateral airflows rising and
sinking, respectively, is advantageous for maintaining this heavy rainfall system[3]. In a dual Doppler radar
inversion experiment on local heavy rainfall in southern China, Zhou also found a similar cell dynamic structure
and proposed that strong echo areas of the convection cells resulted from a convergent rise of aqueous vapor in
the intermediate and lower layers[4]. The convective cell can affect heavy rainfall and it can further affect the
Received 2016-01-05; Revised 2017-03-09; Accepted 2017-03-14
Foundation item: National Basic Research Program of China (Project 973: No.2013CB430103); National
Natural Science Foundation of China (NO. 41530427); Chinese Academy of Meteorological Sciences
(2015LASW-A07); State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences
Biography: QIAN Lei, primarily undertaking research on numerical simulation and meso-scale meteorology.
Corresponding author: DING Zhi-ying, e-mail: [email protected]
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rainfall process by cell-to-cell interactions. Fu and Guo proposed that, during the cell merging process,
enhancements of internal rising and sinking airflows within the cell are beneficial to both convection
development and aqueous vapor transformation[5]. A large volume of super-cooled cloud water and ice phase
particles will therefore be generated which will result in heavy rain (Fu and Guo[5]). Sun and Wang also identified
that convective cells can not only produce strong rainfall, they can further strengthen their influence on rainfall by
forming a β mesoscale linear convective system through the organization process across several cells[6]. It can be
seen that research on the structural features of convective cells in the process of heavy rainfall events mainly
focus on the impact of the dynamic structure of the rainfall, and rarely on the thermal structure.
Apart from requiring certain thermodynamic conditions, the development of small scale convective cells need
stronger vertical wind shear in the lower layers to provide dynamic lifting conditions. Jones[7] and Xu[8] identified
that horizontal vorticity is associated with the vertical shear of the environmental wind field, and when
convection is enhanced in the convection system, the horizontal vortex line will become tilted due to the action of
the vertical movement and then converted to the vertical vorticity. Further investigations suggested that the
transformation of the horizontal vorticity to the vertical vorticity will not only cause rainfall, it will also have an
impact on the deformation of the convective system, division and other processes (Houze and Hobbs[9]; Davis and
Weisman[10]; Ding et al.[11]). Meteorologists have also identified a close connection between horizontal vorticity
and vertical motion. Fujita, in studying the micro downburst structure, found that when the downdraft of the
downburst reached the ground, it would not only cause a strong horizontal divergence wind, it would also form an
upward winding air flow by touching the ground and creating a horizontal vortex ring around the downdraft[12].
This phenomenon was validated by Straka et al.[13] and Markowski et al.[14] by using a numerical model. Ding et
al., using a continuous equation under the isobaric surface to analyze the relationship between the vertical
velocity and the horizontal vorticity, identified that the horizontal vorticity vector demonstrated a cyclone
(anticyclone) shear around the rising (sinking) sectors[15]. These investigations have raised the following
questions which require further analysis: would such structural variations resulting from the development of the
convective cell bring about vertical wind shear variations in the surrounding environmental field? And what
mechanism will the vertical wind shear and horizontal vorticity variations follow to further influence the
development of the convective cells?
In this paper, based on the diagnostic analysis and numerical simulation of the β mesoscale structure in a
warm sector heavy rainfall event over southern China, results from the model closely reflect observed results.
Relevant data have been used to further analyze the relationship between the convective cell structural features
and the horizontal vorticity in a rainfall system. The content of our research includes: (1) the evolution process
and characteristics of the cell structure during the occurrence and development of convection; and (2) the
relationship between the thermal structural variations of convective cells and the horizontal vorticity in the
environmental dynamic field.
2
OBSERVATIONS ON THE SYNOPTIC PROCESS AND RELEVANT DATA
2.1 Data
The data used in this paper includes the NCEP (National Center for environmental prediction) 1°×1°
reanalysis data (every 6 hours), as well as the hour-by-hour rainfall and surface meteorological elements obtained
at conventional and automated weather stations. In addition, this paper also focuses on the radar data (every 6
minutes) of two monostatic stations (Guangzhou and Yangjiang) from 00:00 on May 22 to 23:30 on May 23, with
a horizontal resolution of 1°×4km.
2.2 Observed rainfall analysis
An extensive and continuous heavy rainfall event occurred across southern China in May, 2014: from the
22-23 May the central region of Guangdong Province experienced torrential rainfall, this event being influenced
by the southern trough and the low-level jet. Figure 1a shows the cumulative rainfall distribution observed over
22 hours from 18:00 (universal time, similarly referred to hereafter) on May 22 to 16:00 on May 23. The rainfall
region was dominated by a northwest-southeast distribution, in which the center of the strongest rainfall was
located around 23.6° N, 113.9° E. Cumulative rainfall was up to 420 mm. Analysis of the weather situation
showed that at 00:00 on May 23 an east-west ground frontline (at around 900hPa) was present in the northern
Guangdong Province, and that the rainfall sector was under the control of warm air to the south of the frontline
(Fig. 2a). At around 500hPa there was a two-trough-one-ridge situation in the intermediate and high-latitude areas.
The low-latitude southern trough was located in the central part of Guangxi Province and the area of heavy
rainfall was located in front of the southern trough (Fig. 2b); the 850hPa and 200hPa areas of heavy rainfall were
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3 (subject to change)
under the control of the northeast-southwest orientated low-level jet and the divergence airflow in the northeast of
the South Asia High area. These conditions were not only conducive to convective occurrence and development,
they also provided a water vapor channel for the heavy rain (Fig. 2c-d). In summary, the major systems which
affected this heavy rainfall event included the 200hPa South Asia High, the 500hPa southern trough and the
850hPa low-level jet.
Fig. 1. 18:00 on May 22 to 16:00 on May 23: (a) 22 h observed cumulative rainfall distribution (shadings, unit:
mm); (b) simulated 22 h cumulative rainfall distribution (shadings, unit: mm); (c) regional average hourly rainfall
variations (unit: mm) at the center of the heavy rainfall (113.2-114.2° E, 23.5- 24° N) from 15:00 on May 22 to
18:00 on May 23, in which the straight square column indicates the observed results while the broken line
indicates the simulation results. The black crosses indicate the positions of the two radar stations (Guangzhou and
Yangjiang), while the red box indicates the central area of heavy rainfall indicated in (c).
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Fig. 2. 00:00 on May 23: (a) 900hPa equivalent potential temperature (isoline, unit: K); (b) 500hPa height field
(solid line, unit: dagpm) and wind field (wind vector, unit: m/s); (c) 200hPa flow field (streamline, unit: m/s) and
divergence field (shaded areas, unit: 10-5s-1); (d) 850hPa low-level jet (arrow, unit: m/s), full wind speed (shaded
area, unit: m/s). The capital letters L and N indicate 900hPa cold and warm sectors, respectively.
3
NUMERICAL SIMULATION SCHEME AND ANALYSIS RESULTS
3.1 Simulation scheme details
In this paper we adopted the mesoscale model WRFV3.2.1 (Skamarock and co-authors[16]) which was jointly
developed by US scientific research institutions such as the National Centers for Environmental Prediction
(NCEP) and the National Center for Atmospheric Research. The NCEP 1°×1° reanalysis data was recorded every
6 hours and used as the initial field and boundary conditions on the model to simulate the warm sector heavy
rainfall process occurring in the central Guangdong Province from 12:00 on May 22, to 18:00 on May 23, 2014.
The model used a triple nested scheme with a total of 40 layers located in a vertical direction. The center of the
simulated region was located at 24°N, 113.5°E. The model integration lasted for a total of 30 hours with the
innermost grid output being produced every 10 minutes. Table 1 shows the specific parameterization scheme for
this simulation:
Table 1.
WRFV3.2.1
Horizontal resolution
Horizontal grid points
Micro physical process scheme
Long-wave radiation scheme
Shortwave radiation scheme
Cumulus convection scheme
Surface-layer physics scheme
Planetary boundary layer scheme
Land surface physics scheme
4
Parameterization scheme for the simulation
D01
D02
13.5km
4.5km
226×145
328×271
Lin
Lin
RRTM
RRTM
Dudhia
Dudhia
K-F
——
Monin-Obukhov
Monin-Obukhov
YSU
YSU
Noah
Noah
D03
1.5km
478×481
Lin
RRTM
Dudhia
——
Monin-Obukhov
YSU
Noah
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QIAN Lei (钱
磊), DING Zhi-ying (丁治英), et al.
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3.2 Rainfall distribution contrast
A comparison of the simulated cumulative rainfall distribution (Fig. 1b) with the observed conditions (Fig. 1a)
show close similarity. Both figures show a northwest-southeast rainfall zone, and that the cumulative rainfall in
the central area was up to 330 mm throughout the 22 hour period. However, compared to the observed rainfall
zone, the simulated rainfall zone is relatively narrow and its position moves slightly towards the southeast. Figure
1c shows that observed and simulated average rainfall in the central area was almost similar in its hour-by-hour
variation; both graphs present a unimodal distribution and the strongest rainfall time only occurred at one-hour
intervals. However, compared with the observed conditions, simulated rainfall declined sharply after 04:00 on the
23rd, thus making the rainfall in the central area far lower than the observed conditions. This difference arises
because the simulated rainfall zone moves southwest after 04:00. In general, the simulation results for rainfall
coverage, strength and trend are consistent with the observed rainfall event.
3.3 Contrast of radar echo distribution
The horizontal distribution of the observed radar reflectivity around 3km above the area of heavy rainfall
(17:30 to 20:30 on May 22) is shown in Figure 3a-c. From Figure 3a, we can deduce that there were three
northeast-southwest linear convection structures to the north, middle and south of the area of heavy rainfall at
17:30 on the 22nd. At this time there were also new-born convective cells continually appearing to the northwest
and southwest of these convection lines. The new-born cells to the southwest constantly moved towards the
northeast to maintain the convection lines while the new-born cells to the northwest developed convection lines in
the northwestern region. This meant that the southeastern side gradually became weaker and evolved into
stratiform clouds (Fig. 3b). After 20:30, the three convection lines and the accompanying stratiform clouds
gradually merged (Fig. 3c) to form a northwest-southeast mesoscale convective system (MCS).
When we compared the simulated radar echo with the observed results it was evident that the simulated
convection line and convective cells appeared 1-2 hours later than the actual results; however the evolution
process was still relatively close to the observed results. The simulated radar echo was therefore set 1 hour later
than the observed results for our analysis (Fig. 3d-f). When the simulated radar echo results (Fig. 3d-f) are
compared with the observed results, it can be seen that both sets of results have similar evolution characteristics.
Figure 3d shows a major northeast-southwest convection line (with intensity more than 50dBZ) in the area of
heavy rainfall at 18:30, with several new convective cells scattered to the northwest. At 19:30 (Fig. 3e) another
convection line appeared to the south of the main convection line, with convection currents still developing to the
north of the main convection line. Several convection lines then formed to the southeast which merged to form
the larger-sized MCS (Fig. 3f) at 21:30.
Compared with the observed results, although three wide-range convection lines were not clear in the area of
heavy rainfall at the initial stage of the model, the observed and simulated results both showed the evolution
process of multiple convection lines (northeast-southwest) that developed into the MCS during the period of
heavy rainfall. In general, therefore, the simulated convection lines were similar to the observed conditions in
terms of position and evolution, thus this model was suitable for mesoscale and small scale structure analysis.
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Fig. 3. (a)-(c) indicate the observed radar echo (shaded areas, unit: dBZ) at 17:30, 18:30 and 20:30 on May 22,
respectively; (d)-(f) indicate the simulated radar echo (shaded areas, unit: dBZ) at 18:30, 19:30 and 21:30 on May
22, respectively. The black ovals show the positions of the convection line at each corresponding moment.
4
CONVECTIVE CELL EVOLUTION AND STRUCTURAL ANALYSIS DURING THE PERIOD OF
HEAVY RAINFALL
4.1 Evolution of the convective cell above the convection line
From the analysis in section 3.3 it was highlighted that the observed and simulated radar echoes are both
characterized by several northeast-southwest convention lines which combine to form the MCS. Further analysis
of the evolution processes in the simulation results show that the banded echoes of the main convection line
before the formation of the MCS are characterized by multiple weak convective cells which gradually combined
into both linear and complex linear forms. After 17:20 on the 22nd (Fig. 4a-e), several meso-γ scale suborbicular
convective cells (A-E) appeared in succession in the rainfall sector. These cells were more than 35dBZ in strength
and they continuously developed (increasing in strength) and gradually moved towards the northeastern region.
At the same time an individual cell (A) started to extend towards the north and south, followed by a
center-oriented cupped echo-free sector; the cell gradually evolved from the original suborbicular echo to a
crescent-like shape. At 18:00, multiple convective cells were characterized by a northeast-southwest linear
arrangement and they formed the main convection line (Fig. 4e). New convective cells started to appear in the
southwestern region and several convective cells developed to the northwest. By 18:10 several crescent cells
started to appear and assemble above the convection line (Fig. 4f). In general, all the cells above the convection
line were initially characterized by suborbicular echoes and the nearby 500hPa equivalent potential temperature
consisted of warm cells. When the cell echoes gradually changed into a crescent shape, the 500hPa warm cells
continuously increased in strength and moved to the northeast. Soon after, each convection cell center gradually
developed on its north side and the echoes to the south gradually weakened so as to form stratiform clouds; the
main convection line was characterized by a northwesterly developmental trend. Comparison of the evolution
process of the convection line radar echo in the center of the area of heavy rainfall in the observed results showed
similar characteristics to the simulation results. The convection line consisted of several convective cells created
through their development and northeasterly movement, and the majority of the cells in the later period of
development were all characterized by obvious crescent echoes (Fig. 5).
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Fig. 4. 500hPa equivalent potential temperature(isoline, unit: K) and simulated 700hPa radar echo (shaded areas,
unit: dBZ), the black solid lines represent the locations of the inclined sections in Figure 6a-d and the letters A-E
indicate multiple cells forming the convection line. (a), (b), (c), (d), (e) and (f) indicate the time periods of 17:20,
17:30, 17:40, 17:50, 18:00 and 18:10, respectively.
Fig. 5. Observed radar echo (shaded areas, unit: dBZ). The black circles and numbers 1-4 represent the cells
which form the convection line. The black solid line shows the position of the inclined sections in Figure 6e-h.
(a)-(d) indicate the time periods of : 16:18, 16:30,17:06 and 17:48, respectively.
A tilted vertical section of the equivalent potential temperature and radar echo along the main convection line
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(black solid line, northeast-southwest) was made in Figure 4 (Fig. 6 shows the tilted vertical sections of the
equivalent potential temperature, the simulated radar echo and vertical circulation along the black solid line of Fig.
4). After 17:10, an equivalent potential temperature low-value zone existed around 500hPa of the convection line
and a weaker upward movement appeared below 800hPa of the lower layer (Fig. 6a). Warm moist air therefore
flowed upward and gradually started to release latent heat caused by condensation; the low-level equivalent
potential temperature surface rose and was characterized by an obvious disturbance state at 17:20 (Fig. 6b). With
the continued strengthening of the low-rise updraft, the equivalent potential temperature surface continuously
protruded upwards and it was characterized by three local warm air columns extending upwards to around 500hPa
at 17:40 (Fig. 6c). This caused the middle-layered equivalent potential temperature low-value zone to break and
form relatively cold zones between the air columns. At the same time, all of the convective cloud towers were
also rising and quickly extending upwards to 450hPa under the influence of the unstable energy release; these all
showed an intensity of more than 45dBZ. At 18:00, the three warm air columns further extended upwards to
around 350hPa and they started to connect with the high-altitude stable layer. The low-level equivalent potential
temperature surface of the air column broke away from its high-altitude structure and dropped back to 750hPa so
that a local warm moist center appeared at around 500hPa. This had an equivalent potential temperature of more
than 350K. The convective cloud tower also further developed, moving up to 350hPa, and an obvious
center-toward cupped echo-free sector came into existence to the rear around 600-750hPa. This created a cloud
tower echo bending and tilting to the west with variations in height (Fig. 6d). If the echo of the vertical
convective cloud towers of cells 2-3 (Fig. 5) above the convection line are compared, we can see that it shares
similar evolution characteristics to the simulated cloud towers, though they developed faster (Fig. 6e-f). In
addition, the echo of the observed cloud towers was also characterized by a westward bending and tilting motion
with variations in height during the later period of cell development (Fig. 6g-h). However, cloud tower bending is
significantly weaker in the observed results than in the simulation results; this may be a result of lower resolution
radar data.
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QIAN Lei (钱
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Fig. 6. (a)-(d) indicate the tilted vertical sections of the equivalent potential temperature (isoline, unit: K),
simulated radar echo (shaded areas, unit: dBZ) and vertical circulation (arrow, showing vertical speed multiplied
by 10, unit: m/s) along the black solid line in Figure 4. (a), (b), (c) and (d) indicate the time periods of: 17:10,
17:20, 17:40 and 18:00, respectively. (e) - (f) indicate the tilted vertical sections of the observed radar echo
(shaded areas, unit: dBZ) along the black solid line in Figure 5. Numbers 2-3 represent cells 2-3 in Figure 5. (e),
(f), (g) and (h) indicate the time periods of: 16:18, 16:30, 17:06 and 17:48, respectively.
In summary, the evolution process of the convective cells above the convection line in the area of heavy
rainfall can be roughly summarized as undergoing the following three stages: (1) Stage of activation - the
low-level warm moist air in the area of heavy rainfall moved upward and released latent heat (from condensation)
under the influence of the weak updraft, making the low-level equivalent potential temperature surface start to
bulge and form warm cells with a vertical stretching height below 600hPa. At the same time, vertical convective
cloud towers started to appear around these warm cells; they had a limited upward extent and an intensity of no
more than 35dBZ. (2) Stage of development - the low-level equivalent potential temperature surface continued to
bulge upward under the influence of warm moist updrafts to form warm air columns vertically stretching upwards
to 500hPa. Once at a height of 500hPa they then formed warm cells and the convective cloud towers continued to
extend up to 450hPa. The echo intensity of these towers was more than 45dBZ. (3) Stage of maturity - the warm
air columns continued to extend to 350hPa, and they started to connect with the stable layer in the upper air. A
local warm moist center was formed around 500hPa with the equivalent potential temperature being more than
350K; the convective cloud tower, with an intensity of more than 50dBZ, vertically extended up to 350hPa and an
obvious center-toward cupped echo-free sector in the middle and lower layers at the rear of the cloud tower came
into existence. This resulted in the cloud tower echo to bend and tilt westward with each variation in height.
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4.2 Evolution of the convective cell above the convection line
The thermal and dynamic structural characteristics of the convective cells (A-E in Fig. 4) were analyzed using
the composite analysis method: initially, the strongest hourly radar echo of each individual convective cell
(generally the reflection rate will be above 45dBZ) was identified to establish a 0.3°×0.3° square area (as shown
in the black box in Fig. 4a), this formed the research area for a single convective cell. Then, using previous
analysis of the evolution processes of cells and warm air columns, the moment when closed warm cells first
appeared at 500hPa near the cell (set as t) was recorded as an indication of a warm air column being created in a
cell. Finally, the composite research areas of the five convective cells were separated according to their respective
moment of t.
Figure 7 shows the meridional section of the composite cell at the moment of t + 2. It can be seen at the
moment of t that the warm air column was preliminarily formed in the cell with the vertical extension height
reaching 450hPa. It can also be seen that a vertical convective cloud tower had already emerged near the air
column, this having an intensity of more than 45dBZ (figure omitted). Subsequently, subject to the continuous
impact of the low-level warm moist updrafts, the warm air column further stretched up to 350hPa (strongest) at
the moment of t + 2 and formed a local warm moist center above 352K near 450hPa. In the air column there was
an intensive vertical upward movement, from 850hPa up to 350hPa, and the center of the maximum rising speed
was located in the middle layer of the troposphere, this had a velocity of more than 6 m/s (Fig. 7a). The deviation
between the temperature of the warm air column and the nearby ambient temperature was calculated, and it was
found that the internal temperature was higher than the ambient temperature, with the center of high temperature
disturbance rising to 2K near 500hPa (Fig. 7b). The vertical convective cloud tower became stronger when the
strength exceeded 50dBZ. Figure 7b shows the north-south section along the cell center, this can be seen to differ
from the northeast-southwest section in Figure 6a-d. The convective cloud tower can be seen to tilt southward
rather than bending or tilting westward with each variation in height.
The observed thermal structural characteristics of the convective cells in the heavy rain episode were very
similar to the typhoon convective hot tower structure suggested by Montgomery et al.[17] and Zhang et al.[18-19].
Aspects such as the tower-shaped equivalent potential temperature surface distribution, the strong vertical upward
movement in the air columns, and the middle and high-level warm moist centers were very similar. However, the
air columns in the cells in our observed rainfall event extended lower than in the typhoon convective hot tower
(Zhang et al.[18]), and both the strength of the high temperature disturbance center and of the center of strongest
upward movement is relatively weak. These differences may arise because water vapor and the strength of
convection in the typhoon were stronger than the heavy rain processes in the warm sector.
Analysis of the composite cell dynamic structure showed that at the moment of t, the westerly large-value
sector around the cells was located near 750-550hPa, and it was distributed on both sides of the air column;
400-300hPa was a weak westerly sector (figure omitted). At the moment of t + 2, with the development of the
warm air column, the westerly large-value sector on both sides of the column strengthened; its large-value center
corresponded to the cold center south of the column (Fig. 7c). The distribution of the meridional wind speed was
notably different from the zonal wind speed. This was demonstrated by the southerly wind prevailing below
600hPa while the northerly wind dominated above 500hPa; the warm, moist center of the cell corresponded to the
large-value center of the northerly winds (Fig. 7d).
10
No.3
change)
QIAN Lei (钱
磊), DING Zhi-ying (丁治英), et al.
11 (subject to
Fig. 7. Composite cell at the moment of t + 2: (a) equivalent potential temperature (black isoline, unit: K) and
vertical velocity (red dotted line, unit: m/s); (b) simulated radar echo (shaded areas, unit: dBZ) and deviation
between the warm air column temperature and the nearby ambient temperature (isoline, unit: K); (c) equivalent
potential temperature (isoline, unit: K) and zonal wind speed (shaded areas, unit: m/s); (d) vertical section
equivalent potential temperature (isoline, unit: K) and meridional wind (shaded areas, unit: m/s).
5
RELATIONSHIP BETWEEN THE HORIZONTAL VORTICITY, AND THE DEVELOPMENT OF
CONVECTIVE CELLS AND CONVECTION LINE
5.1 Relationship between convective cell development and horizontal vorticity
From the research of Ding et al. on the relationship between the horizontal vorticity and the vertical
movement, it can be seen that the meso-β and γ-scale atmospheric movement was in a state of quasi-static
equilibrium[15]. The expression of the horizontal vorticity component can therefore be changed from the z
coordinate system to the p coordinate system and simplified using a means of scale analysis:
∂w ∂v
∂w
∂v
∂v ∂v
= ( ) p − (− ρg ) ≈ ρg
∝
−
∂y
∂p
∂p ∂p
∂y ∂z
∂u ∂w
∂u ∂w
∂u
∂u
ζy =
= − ρg
− ( ) p ≈ − ρg
∝−
−
∂p
∂p
∂y
∂p
∂z ∂x
ζx =
where u, v and w indicate the speed of the x, y and z directions, respectively; and ζx and ζy indicate the horizontal
vorticity of the x and y directions, respectively. Further analysis of the relationship of the horizontal vorticity and
the vertical motion follows the continuous equation of the p coordinate system to introduce a Poisson equation,
this is expressed as (Ding et al.[15]):
∂ζ y ∂ζ x
∂ 2ω
∂ ∂u ∂v
∂
∂u
∂ ∂v
=
−
+
=
−
−
=
−
(
)
(
)
(
)
∝ −ω ∝ w
∂p 2
∂p ∂x ∂y
∂x ∂p ∂y ∂p
∂x
∂y
From this equation, when the horizontal vorticity vector is characterized by a cyclonic shear, it will be conducive
to the development of an upward movement. When an anticyclonic shear appears, it will be conducive to the
development of a sinking movement.
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From the analysis result in the fourth chapter of the paper, it was seen that with the development of a
convective cell the upward extension of its internal warm air column led to a horizontal vorticity vector in the
center and west of the warm cells, making the cyclonic and anticyclonic shear stronger over time. And variation
of the horizontal vorticity vector will further influence the development of the convective cell and its internal
warm air column.
Figure 8 shows the evolution conditions of horizontal vorticity over time at 500hPa around the composite
convective cell. In order to clearly express the variations of the horizontal vorticity before and after the warm
cells appear in the intermediate layer, the local variation rate of the horizontal vorticity over time was calculated.
This was done by subtracting the moment-by-moment horizontal vorticity component from the one at the previous
moment and dividing by the time interval. The results show that at the moment of t - 1, the convective cell was
relatively weak in intensity and there was no warm cell forming around the height of 500hPa. This indicated that
the warm air column at this time had not yet spread to the intermediate layer and that the horizontal vorticity
showed no significant variation (Fig. 8a) compared with that at the previous moment. At the moment of t, the
warm air column extended upward to the intermediate layer of the atmosphere under the influence of updrafts
resulting in the occurrence of warm cells at the height of 500hPa. At the same time, the local variation rate of the
horizontal vorticity component around the warm cells showed an obvious cyclonic shear, this indicated that the
cyclonic shear tends to get stronger compared with the horizontal vorticity vector at the previous moment (Fig.
8b). If we compare the local variation rate of the average vertical speed in the whole layer from 700-500hPa at the
same moment, it can be seen that the center of strong positive values appear around the center of warm cells. This
indicates that the cyclonic shear of the horizontal vorticity at this point strengthens which is conducive to
enhancing the updrafts (Fig. 8f). After moment t, the local variation of the horizontal vorticity around the center
displayed a more obvious cyclonic shear as the warm cells developed further; this was conducive to the
sustainable development of convection at this point. At the same time, the horizontal vorticity vector on the west
side of the warm cells started to show an anticyclonic shear tendency which strengthened over time (Fig. 8c-d),
while the local variation of the vertical velocity also showed a center of a strong negative value (Fig. 8g-h). This
was conducive to the enhancement of the downdraft at this point which suppressed the upward transportation of
the low-level warm moist airflow and the development of convection; the rear cell echo started to reveal a
center-toward cupped echo-free sector, thus producing a cell form which gradually evolved from the original
suborbicular shape into a crescent shape. While the anticyclonic shearing area for the local variations of the
horizontal vorticity was gradually developing to the southwest of the cell, the radar echo in its southern region
gradually weakened. This was also the main reason for the cell echo center to continuously develop to the north.
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QIAN Lei (钱
磊), DING Zhi-ying (丁治英), et al.
13 (subject to
Fig. 8. Simulated radar echo of the composite cell near 700hPa (shaded areas, unit: dBZ) :(a) - (d) equivalent
potential temperature at 500hPa (isoline, unit: K) and local variation rate of the horizontal vorticity component
(arrow, 10-6m·Pa-1s-2); (e) - (h) local variation rate of the whole-layer average vertical speed at 700-500hPa
(isoline, unit: 10-3m·s-2). (a) (e), (b) (f), (c) (g) and (d) (h) indicate the moments of t - 1, t, t + 1 and t + 2,
respectively. The x and y-axis scale values indicate the number of grids in the direction of x and y.
This analysis shows that the positive and negative centers of the local variations of the vertical speed have a
strong relation to the cyclonic and anticyclonic shearing sectors of the local variations of the horizontal vorticity.
The development of the warm air column in the convective cell will create an increase in the horizontal
temperature gradient around the middle-layer warm cells, and thus create a vertical wind shear which is subject to
significant variations. This will cause the horizontal vorticity vector in the center and to the west of the warm
cells to tend towards the cyclonic and anticyclonic shear which becomes stronger and stronger over time. This is
advantageous to the strengthening of the rising and sinking motion at each corresponding position, ultimately
creating a cell echo pattern which changes significantly over time.
5.2 Relationship between convection line development and the horizontal vorticity
From the analysis in section 5.1, it can be seen that the upward extension of the warm air column will create a
horizontal vorticity vector in the center and west of the warm cells at 500hPa which tends to strengthen the
cyclonic and anticyclonic shear over time. The variations in horizontal vorticity will further promote the
development of the warm air column and the convective cell. The relationship between the development of the
convection line which is composed of multiple cells and the horizontal vorticity is examined in this section.
The distribution of the local variation rate of the horizontal vorticity directly calculated at 500hPa near the
main convection line is shown on Figure 9. This figure shows that the local variations of the horizontal vorticity
are characterized by cyclonic (Fig. 9a-b) and anticyclonic (Fig. 9c-d) shearing stages; this occurs in the center and
to the southwest of the majority of cells at the stages of development and maturity. These results are consistent
with the analysis results of the composite cell in Figure 8. Further observations of the echo variations of each cell
above the convection line demonstrate that the local variations of the horizontal vorticity are characterized by an
anticyclonic shear at the back of each cell. As this is not conducive to the development of convection, a
center-toward cupped echo-free sector starts to appear. However, due to the impact of the cyclonic shear on the
local variations of the horizontal vorticity in the center and north of the cell, powerful convection and updrafts
developed. These caused the cell echo center to gradually move north, while in the south it was reduced to
stratiform clouds matching the anticyclonic shear sector of the horizontal vorticity local variations, thus creating a
convection line which tended to gradually develop northwest over time (Fig. 9c-d).
If a tilted vertical section of the local variation rate for the vertical speed is made near the convection line
along the purple solid line, it can be seen that multiple cells consisting of the convection line develop (Fig. 9a).
The cell center and its southwest region correspond to the positive and negative sectors for local variations of the
vertical velocity, but the negative value sector is relatively weak in intensity. There are no negative value sectors
at the back of the northeastern cells which may be associated with the relatively weak intensity of various cells
(Fig. 9e). This kind of dynamic configuration features weaken restraints in the low-level warm moist updrafts and
convection at the back of each cell, so the convective cloud tower has not yet bent or tilted westward. Figure 9
shows that before 18:00 all cells above the convection line reached the stage of maturity, while the positive and
negative centers of local variations in vertical speed were notably strengthening in intensity in the cell center to
the southwest. With each further development of the cloud tower the positive and negative sectors tended to
extend towards the middle and high layers of the atmosphere. This also corresponded to the cyclonic and
anticyclonic shearing sectors for local variations of the horizontal vorticity in the cell center and its southwestern
region, as shown in Figure 9c-d (and Fig. 9f). This kind of dynamic configuration restrains and strengthens the
upward transportation and convection development of the low-level warm moist air flows at the back of the cell
and around the center. The echo at the back of each cell convective cloud tower therefore gradually weakens
whilst the front echo gradually strengthens. This then causes the cloud tower to tilt and bend westward.
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Fig. 9. (a) - (d) Simulated 700hPa radar echo (shaded areas, unit: dBZ), local variation rate of the horizontal
vorticity component at 500hPa (arrow, unit: 10-6m·Pa-1s-2), with the purple solid lines indicating the inclined
section positions in (e) - (f). (a) - (d) indicate the time periods of 17:40, 17:50, 18:00 and 18:10 on the 22nd,
respectively. (e) - (f) indicate the tilted vertical sections of the simulated radar echo (shaded areas, unit: dBZ) and
the local variation rate of the vertical speed (isoline, unit: 10-3m·s-2) along the purple solid lines. (e) - (f) indicate
the time period of 17:40 and 18:00 on 22nd, respectively.
6
CONCLUSIONS
The ARW-WRF model was used in this investigation to undertake numerical simulations of a warm sector
heavy rainfall event over southern China in the central region of Guangdong Province, May 22-23, 2014. A
composite analysis method was adopted to analyze the structural features of the convective cell above the
convection line in the heavy rainfall event and to further investigate the impact of the horizontal vorticity on the
structure and developmental processes of the convective cell and convection line. It was found that:
(1) The internal temperature of the convective cells above the convection line in a heavy rainfall event is
higher than the ambient temperature; in the middle layer there is a center of disturbance subject to a high
temperature, up to 2K. The dynamic structural field is manifested as intensive vertical upward movement, and the
large-value centers of the northerly and westerly winds in the middle layer correspond to the warm moist center in
the cells and the relatively cold center south of the warm air column.
(2) The development of the warm air column in the convective cell will create a gradient as the horizontal
temperature increases around the middle-layer warm cells, which cause a horizontal vorticity vector in the center
and west of the warm cells thus making the cyclonic and anticyclonic shear stronger over time. This is
advantageous to the strengthening of the rising and sinking motion at all corresponding positions which
ultimately creates a cell echo pattern which changes significantly over time.
(3) Analysis of the impact of the horizontal vorticity on the development process of the convection line show
that each cell center above the convection line is influenced by the cyclonic shear of the local variations in
horizontal vorticity. This results in the strong echo center to gradually move north, while in the south it is reduced
to stratiform clouds matching the anticyclonic shear sector. The convection line therefore gradually develops to
the northwest over time and the cloud tower, as a whole, tilts and bends westward.
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QIAN Lei (钱 磊), DING Zhi-ying (丁治英), et al.
15 (subject to
change)
Acknowledgement: We hereby express our thanks for such observation data provided in this paper by Southern China Monsoon
Rainfall Experiment (SCMREX) laboratory sponsored by Chinese Academy of Meteorological Sciences and the China
Meteorological Administration. The data can be available via relevant applications on http://scmrex.cma.gov.cn. Thanks to State
Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences as the first (joint) signature unit to finance the
open topic of this paper. Here we shall still give our thanks to such support from National Basic Research Program of China (Project
973: No.2013CB430103), National Natural Science Foundation of China (NO. 41530427) and Chinese Academy of Meteorological
Sciences (2015LASW-A07).
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Citation: QIAN Lei, DING Zhi-ying, ZHAO Xiang-jun et al. Structure Features and Composite Analysis of Convective Cells in a
Warm Sector Heavy Rainfall Event over Southern China [J]. J Trop Meteorol, 2017, 22(3): starting and ending pages to be decided.
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