MAINTENACE MECHANISM OF CONVECTIVE CELLS WITHINMESOSCALE CONVECTIVE SYSTEM IN HUMID SUBTROPICAL REGION*Shingo Shimizu1, Hiroshi Uyeda1, Taro Shinoda1, Kazuhisa Tsuboki1
H. Yamada2, B. Geng 2, and T. Kato3(1: HyARC, Nagoya University, 2: Frontier Observational Research System for Global Change,
3: Meteorological Research Institute, Japan Meteorological Agency)
HyARC, Nagoya University, Japan, 464-8601, Furo-cho, Chikusa Nagoya, Japan.
e-mail: shimizu@rain.hyarc.nagoya-u.ac.jp
Abstract
In oder to reveal relation between maintenance mechanism and storm environment, two back-building rainband
observed near Shanghai during Meiyu season in 2001 which had different lifetime of convective cell were
investigated using variational dual-Doppler radar analysis and regional objective analysis data. The lifetime of
convective cells within MCS in a moist mid-level environment (90 min) was longer than that in a dry mid-level
environment (42 min). The moist environment caused evaporative cooling of rain to be inactive and caused a weak
downdraft. The weak downdraft did not cut off a moist air supply to the long-lived convective cell.
Keyword: MCS, Lifetime of convective cell, mid-level humidity, dual-Doppler radar analysis, CReSS

1. Introduction
The relation between environment and types of
Mesoscale Convective System (MCS) has not been
documented well in a moist environment such as that ofEast Asia. On the contrary, in a dry environment such asthat of the Great Plains in U.S., it is well known that
Convective Available Potential Energy (CAPE) andvertical wind shear are important environmental
parameters for determining formation types of MCS(Bluestein and Jain, 1985). In a moist environment, ahumidity distribution, as well as CAPE and vertical wind
shear, is now focused as an important parameter for
maintenance of convective cell within MCS.
In order to reveal maintenance mechanism of convective
cells within linear MCS in a moist environment, weselected two back-building rainbands with distinct
characteristics of convective cells observed during a field
experiment in the downstream region of Yangtze riverduring Baiu/Meiyu season in 2001 (Yamada et al., 2004).
We investigated the lifetime and downdraft strength ofconvective cells within the two back-building rainbands bydual-Doppler analysis, and estimated three-dimensional
humidity distribution by using Regional objective
ANALysis (RANAL), Regional Spectral Model (RSM),
which are produced by the Japan Meteorology Agency. To
confirm the relation between mid-level humidity anddowndraft strength, we performed a numerical simulation
with Cloud Resolving Storm Simulator (CReSS) (Tsuboki
and Sakakibara,2002).

2. Data
Two X-band Doppler radars (at Wuxian and Zhouzhuang,
covering the downstream of Yangtze river with 64 km in
radius) obtained sets of volume scan data of reflectivity
and Doppler velocity every six minutes.
We analyzed lifetime and three dimensional structure ofconvective cells within two back-building rainbands by

Fig. 1: The observation field around two X-band
Doppler radars at Wuxian and Zhouzhuang (Fig. 1a).
Two circles in Fig. 1a show the radar observation area
(64 km in radius). Five of 2o x 2o regions(NW,NE,SW,NE,and CT) in Fig. 1a indicate analysis
regions for estimating environmental parameters using
RANAL. The RSM calculated the parameters within the
10o x 10o region shown in Fig. 1b. Domains for
1km-CReSS and 5km-CReSS are shown in Fig. 1b.
using dual-Doppler radar analysis with variational method(Gao et al.,1999) within the overlapped radar observation
area of the two radars (Fig. 1a).

In order to evaluate the environment around theobservation area, humidity distribution is investigated in
the five regions (2o×2o regions: NE, NW, SE, SW, and CT(covering the radar observation area)) using RANAL data
(Fig. 1a). The time resolution of RANAL is 6 hours (02, 08,
14, 20 LST (LST = UTC + 8 hours)). The horizontal
esolution of RANAL is 20 km. In the vertical direction,
RANAL contains 20 levels. The mean value ofenvironmental parameters in the each region is calculated:vertical integrated specific humidity deficit from 850 hPa
to 500 hPa (HUDE), and vertical integrated vapor flux
convergence from surface to 850 hPa (VFC), vertical
integrated specific humidity from surface to 850 hPa(VISH), CAPE and CIN. HUDE is an index for dryness of
midlevel layer. In order to investigate the time variation of
humidity distribution, we used the forecast of RSM within
10o × 10o region (Fig. 1b). The time resolution of RSM is 1
hour. The horizontal resolution is 20 km. The RSM was run
for 24 hours from 20 LST 18 Jun and 20 LST 23 Jun.
The numerical simulations were conducted with a
1km-resolusion CReSS (1km-CReSS). The 1km-CReSS
was one-way nested within the outputs of a 5km-CReSS(Fig.1b). The 5km-CReSS was also nested within the
outputs of RSM.

3. Result
3.1 Dual-Doppler analysis
We investigated lifetime and the maximum downdraft
velocity of convective cells within two back-building
rainbands (CASE1: 0348 LST 24 June, CASE2: 0118 LST
19 June). We defined lifetime of convective cell as theperiod when the maximum reflectivity at 4 km above sea
level (ASL) was over 30 dBZe. The longest lifetime of
convective cell was 90 minutes in CASE1 (0318 LST 0448 LST), and 42 minutes in CASE2 (0106 LST - 0148
LST). The maximum reflectivity during the lifetime was 39
dBZe at a height of 4 km ASL in CASE1, and 44 dBZe at aheight of 2 km ASL in CASE2 (not shown). We named the
back-building rainband with long-lived convective cells as
BBL, and the back-building rainband with short-lived
convective cells as BBS. The BBL consists of steadyconvective cells. On the contrary, the BBS was maintainedby repeated replacement of the convective cells.
We observed a remarkable difference in the maximum
downdraft velocity. Reflectivity at a height of 1 km ASL of
CASE1 and CASE2 is shown in Fig. 2. Vertical crosssections along the A-A' and B-B' lines in Fig. 2a and Fig.
2b are shown in Fig. 2c and Fig. 2d, respectively. The
maximum downdraft velocity at 2 km ASL was 2.0 m s-1(CASE1) and 5.0 m s-1 (CASE2), respectively. The locationof downdraft core differed from the location of thereflectivity core in two cases (not shown). The cause of thedowndraft seems to be mainly due to evaporative coolingin two cases because the downdraft core located at theconfluence of the wind at a height of 4 km ASL and theedge of strong echo in two cases (not shown).
It is known that a strong downdraft of a convective cellFig. 2: The radar reflectivity overlaid withstorm-relative wind vectors at a height of 1 km ASL of
CASE1(a),CASE2 (b). Vertical cross sections along
A-A’ (Fig. 2c) and B-B’ (Fig. 2d) lines are indicated.
Thick contours show downdraft every 1 m s-1. Openarrows show system movement speed.
causes a strong outflow cutting warm and moist air supply
to an updraft of the convective cell (Weisman and Klemp,
1982). In present cases, actually short-lived convective
cells were observed corresponding to the strong downdraftin CASE2, and long-lived cells were obserbed
corresponding to the weak downdraft in CASE1.

Fig. 3: The distribution of HUDE on 19 Jun (a) and on 24 Jun(b) calculated by RANAL data, and those of 19 Jun (c) and 24
Jun (d) estimated by RSM. In Fig. 3a and Fig. 3b, the locationsof two rainbands are shown by open area within radar range(over 30 dBZe). Open arrows indicate storm-relative wind at 4
km ASL estimated by dual-Doppler analysis. The mean valueof HUDE is calculated in SW (a) and NW (b) region. Contours-2in Fig. 3c and Fig. 3d show HUDE every 3 kg m estimated by
RSM.

3.2 Environments of two rainbands
We investigated the environment around the two
rainbands, in order to find environmental parameters todetermine lifetime and downdraft velocity of convective
cell, and to understand the maintenance mechanism of
convective cells within a rainband in a moist environment.
We compared the two cases, and calculated the mean valueof HUDE in the region where the storm-relative wind at 4
km ASL blew from. The other parameters were calculatedin CT region. In Fig. 3, HUDE distributions in two cases
are indicated. In CASE1, low CAPE (0 J kg-1) in CT region
and large CIN (104 J kg-1) over the ocean restrained
vertical transport of vapor, thus abundant vapor in lower
layer (VISH in SE region was 24.1 kg m-2) was
horizontally transported from ocean to the observation area.
The transported abundant vapor was accumulated in lower
layer because the VFC had high value (6.95 × 104 kg m-1
s-1). In the middle layer, a moist air (HUDE was 3.14 kgm-2) existed over the windward region of storm-relative
wind at the mid-level (Fig. 3a). In CASE2, CAPE ( 777 Jkg-1) and HUDE (9.99 kg m-2 in Fig. 3b) were higher, and
VFC (3.66 ×104 kg m-1 s-1) was lower than that of CASE1.
A mid-level layer in CASE2 was drier than that of CASE1(Fig. 3b).
CAPE was important parameter to transport vapor fromlower layer to middle layer in CASE2. In fact, maximum

Fig. 4: The simulated mixing ratio of rain and storm-relative

wind at a height of 970 m (a), and vertical cross section of
mixing ratio of rain (b) and relative humidity(c) along A-A’ line
at 0500 LST on 24 JUN 2001. In Fig.4b, the black contour
indicates 0 m/s vertical velocity. Red (blue) contour shows
updraft (downdraft) evrery 1 m/s (0.2 m/s). In Fig. 4c, blue
contour shows relative humidity evrery 2 %, and black contour
shows mixing ratio of rain every 1 g/Kg.

updraft in CASE2 was more that 7 m s-1, and maximum
reflectivity was more than 42 dBZe (Fig. 2d)corresponding to high CAPE. On the contrary, in CASE1,
VFC, instead of CAPE, was important parameter totransport vapor vertically. The maximum updraft was lessthan 2 m s-1, and maximum reflectivity was less than 40
dBZe (Fig. 2c) corresponding to low CAPE. The weak
updraft was quasi-steady during 90 minutes. It caused alarge amount of rainfall corresponding to high VFC.
Distributions of HUDE in Fig. 1b are shown in Fig. 3c
and Fig. 3d, respectively. The belt of HUDE less than 6 kg
m-2 from east to west (Fig. 3a and Fig. 3b) approximately
corresponded to the Baiu/Meiyu frontal zone. In both northand south sides of the Baiu/Meiyu front, HUDE was morethan 20 kg m-2. From 23 LST 23 Jun to 05 LST 24 Jun(three hours before and after Fig. 3c), a dry air (HUDE wasmore than 5 kg m-2) advected from northwest to the NW
region in Fig. 3b. On the contrary, from 23 LST 18 Jun to 5
LST 19 Jun (three hours before and after Fig. 3d), a moist
air (HUDE was less than 5 kg m-2) advected fromsouthwest to SW region (Fig. 3a.).

3.3 Numerical simulations
We conducted numerical simulations with the
1km-CReSS in order to confirm the relation between thedowndraft strength and the mid-level humidity (we show
only CASE1 here). Figure 4a shows the horizontal
distribution of mixing ratio of rain (QR) corresponding toreflectivity distribution observed at 0348 LST (Fig.2a). The
orientation of rainband and the wind field at a height of 1
km around rainband were well simulated. A newconvective cell generated to the southwest of the oldconvective cell as well as observation. The lifetime of
convective cell was long-lived (about 1 hour).
The pattern of vertical wind was well simulated (Fig. 4b).
The storm-relative inflow from southeast turned to weak
updraft (2-3 m/s) below the melting layer. Little snow andgraupel were formed above the melting layer (about 1g/Kg). Therefore, the height of QR more than 0.5 g/kg wellcorresponds to the echo top height (Fig. 2). The maximum
downdraft was very weak (about 0.2 m/s). The weak
downdraft core was located in the moist region more than
90 % (Fig. 4c). The moist environment caused evaporative
cooling to be inactive, and caused the weak downdraft.

4. DISCUSSION
We summarized our results in Fig. 5. In a dry mid-level
environment (high HUDE environment (Fig. 5b)), thevapor was evaporated well and caused strong downdraft in
CASE2. The strong downdraft cut off warm and moist air
supply to the updraft of convective cells in the lower layer,
and caused the short-lived convective cells in CASE2. The
vertical vapor transportation in CASE2 is explained byhigh CAPE. On the contrary, in a moist mid-level
environment (low HUDE environment (Fig. 5a)), theevaporative cooling is inactive, therefore, long-lived
convective cells with weak downdraft velocity wereobserved in CASE1. The vertical vapor transportation in
CASE1 is explained by high VFC in spite of low CAPE.
We propose that HUDE is a parameter to determine thelifetime of convective cells within MCS and the strength ofdowndraft caused by evaporative cooling of rain. CAPE
and VFC would be important environmental parameters todetermine the amount of vertical vapor transportation.
The research on environmental parameters focused on
mid-level humidity (HUDE and VFC), such as these twocase studies, gives us strong possibility to understand themaintenance mechanism of MCS in a moist environment.
5. CONCLUSIONS
We investigated the humidity distribution for determining
the lifetime of convective cell within two rainbands
observed near Shanghai during Baiu/Meiyu period in 2001

Fig. 5: Characteristics (lifetime and downdraft velocity) andschematic model of two rainbands developed indifferent distribution of mid-level humidity on 24 Jun
24 (CASE1: Fig. 4a) and 19 Jun (CASE2: Fig. 4b),2001. Black dot indicates the longest-lived convective
cell (90 min in CASE1, 42 min in CASE2). In a moist
mid-level environment (CASE1), evaporative cooling
was inactive and a weak downdraft could not cut off
warm and moist air supply to the convective cell.

by using dual-Doppler radar analysis with variational
method, RANAL, and RSM. We propose that mid-level
dryness between 850hPa and 500 hPa (HUDE) behind a
rainband determines the maximum downdraft velocity andlifetime of convective cell within MCS. Moreobservational and numerical study on the relation between
this HUDE and MCS would lead to a clear understandingof maintenance mechanism of MCS in a moist
environment.

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