Journal of the Meteorological Society of Japan, Vol. 89A, pp. 299--305, 2011.

299

DOI:10.2151/jmsj.2011-A20

NOTES AND CORRESPONDENCE
Diurnal Variation of Radar Echo Area in the Middle of Indochina
Takehiko SATOMURA, Keiko YAMAMOTOy
Graduate School of Science, Kyoto University, Kyoto, Japan

Bounteum SYSOUPHANTHAVONG, and Souvanny PHONEVILAY
Department of Meteorology and Hydrology, Vientiane, Lao PDR(Manuscript received 13 May 2010, in ﬁnal form 12 October 2010)

Abstract
First results of diurnal variation of radar echo area observed at Vientiane, Lao PDR, in the middle of Indochina are reported. Radar echo intensity data from April to October 2008 is analyzed.
The monthly averaged diurnal cycles of radar echo area have a peak in the late afternoon in April andmidnight in July. The echo area per one echo at the time of the maximum total echo area in the diurnal cycle in
April (@600 km 2 ) is smaller than that in July (@1,200 km 2 ). The phases of diurnal cycle of echo area in 6 banded
regions oriented from southwest to northeast are compared to examine spatial di¤erences of diurnal cycles. It isfound that the phase of each band delays from southwestern to northeastern banded regions. The propagation
speed of phase of echo area diurnal cycle is estimated as about 14 m s1 .

1. Introduction
The tropics occupies more than one third of
the Earth surface and latent heat release producing
precipitation in tropics plays one of the essential
roles in the energy transfer processes of the Earth.
Among disturbances in wide range of time scales
observed in tropics, the diurnal variation is one of
the most signiﬁcant variations of precipitation intropics (e.g., Nitta and Sekine 1994; Ohsawa et al.
2001). It is also known, however, that global atmospheric models fail to reproduce diurnal variationsof precipitation over land, especially over tropicalland and warm-season extra-tropical land (e.g.,
Dai 2006). Because the diurnal phase of cloudinessand precipitation notably a¤ects the energy balance
Corresponding author: Takehiko Satomura, Graduate
School of Science, Kyoto University, Kitashirakawa
Oiwakecho, Sakyo, Kyoto 606-8502, Japan.
E-mail: satomura@kugi.kyoto-u.ac.jp
yPresent a‰liation: Japan Meteorological Agency.
6 2011, Meteorological Society of Japanof the earth system, it is considered that the phaseerror of diurnal variation should be decreased toreduce uncertainty in climatic simulations (e.g.,
Sperber and Yasunari 2006). Therefore, to clarify
and understand governing mechanisms which determine the phase and amplitude of diurnal variation of precipitation is one of the important issues
not only in pure scientiﬁc interests but in climate
projection ﬁeld.
Indochina occupies a special position in thetropics. It is a land mass in tropics, lies in tropicalmonsoon region accompanied by large amount of
rainfall and is occupied by nearly developed countries. Thus rather dense networks using modern
meteorological observation instruments operated
by nearly developed countries in Indochina provide
precious data to scientists interested in tropical meteorology.
The diurnal variation of convective activity inthe central Indochina observed by infrared emission (geostationary meteorological satellites) and asatellite-borne radar (TRMM-PR; Tropical Rain-Journal of the Meteorological Society of Japan

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Fig. 1. Left: Topography of Indochina, Vientiane radar site (marked by a small ‘þ’) and observation radii of
120 km, 270 km, and 400 km. Terrain height contours interval is 500 m. Right: Observation range at 3 km
height. Circles indicate radii of 100 km, 200 km, and 300 km.

fall Measuring Mission Precipitation Radar) showsa peculiar characteristic: the peak of convective
activity is midnight to early morning (Ohsawa et al.
2001; Takahashi et al. 2010). Also the central
Indochina is situated near the northern border of‘‘landside coastal’’ type of diurnal variation wherelandward propagating precipitation systems govern
the diurnal variation (Kikuchi and Wang 2008).
Considering above aspects of the diurnal variationin the central Indochina, it is interesting to study
behavior of precipitation systems in this region focusing on their diurnal variation.
The purpose of this study is to report ﬁrst resultsof diurnal variation of radar echo area observed at
Vientiane, Lao PDR, in the middle of Indochina.
Owing to the lack of rain gauge network dense inspace and time, characteristics of diurnal variationof rainfall such as spatial and seasonal di¤erences
or common points are not yet documented in detailin Indochina. This study only focuses radar echoarea at 3 km height. However, considering that thelack of dense rain gauge network in this region andthat diurnal variation of radar echo area at 3 km
height is qualitatively related with diurnal variationof rainfall at the surface, the diurnal variation ofradar echo area will give important information on
the diurnal variation of rainfall in this region.
2. Data
The C-band Doppler radar used in this study
locates at 17 58 0 N, 102 34 0 E at the riverside of
the Mekong River in the city of Vientiane, Lao
PDR. The location of the radar is shown in Fig. 1
with the observable area at 3 km height. The basic

Table 1.

Basic information of Vientiane radar.

Latitude17 58 0 15.9 00 N

Longitude102 34 0 14.2 00 E

Height (AGL)

45 m

Wave Length

5.66 cmBeam Width1

Observation Radius

Long: 400 km
Short: 120 km

Radial Resolution

Long: 500 m
Short: 250 m

No. of Elevation Angle

Long: 2
Short: 12

information of the radar is indicated in Table 1.
One volume scan includes both long and shortobservation ranges and repeats every 7.5 min(8 volume scans per one hour).
In this study, only the echo intensity data areused from April 1 to October 23, 2008. Though theradar was installed and in operation from the rainyseason in 2007, radar data is not well archived in2007. Thus, the data observed in the rainy season
in 2008 is analyzed in this study. Climatic rainyseason in this region around Vientiane is from theend of April to the end of September (Matsumoto1997). The analysis period of this study covers thewhole rainy season in this region.
All the echo intensity data is interpolated into ahorizontal Cartesian grid system at height of 3 km
using a weighted interpolation method of Cressman

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Fig. 2. Monthly averaged diurnal variation of radar echo area for (a) April 2008 and (b) July 2008. Bars indicate total echo area with larger than 10 dBZ. Solid lines are echo area per one echo.

(1959). The Cartesian grid system has a horizontal
interval of 1 km and 800  800 grids, which covers
the ‘‘long’’ observation range of the radar.
The JCDAS*1 data is used for meteorological
data analysis. For topography, GTOPO30 data
whose resolution is 30 00 both in longitude and latitude is used.
3. Results
3.1 Averaged diurnal variation of radar echoes
Figures 2a and 2b show diurnal variations of theradar echo area averaged over April and July 2008,
respectively. Months of April and July are chosen
as a typical month of pre-monsoon and monsoon
season, respectively. The total echo area had theminimum at 10 LT in April and 11 LT in July.
The maximum echo area is observed at 16–17 LT
in April while it is observed 01 LT in July with asecond maximum at 15 LT. The midnight maximum of total echo area in July around Vientiane isconsistent with previous studies using infrared emission data (e.g., Ohsawa et al. 2001).
Averaged echo area per one echo manifests another aspect. In this paper, only echoes whose areas
are greater than 20 km 2 are counted. In April, theecho area per one echo is comparatively small andis about 600 km 2 at the time of the afternoon maximum of total echo area. Then, while total echoarea decreases in the same time, the echo area per*1 JCDAS (JMA Climate Data Assimilation System)
is a follow-on project of and uses the same system
as the JRA-25 long-range reanalysis (Onogi et al.
2007).

one echo increases its size toward 21 LT, which indicates rapid decrease of echo number in the radarobservation range in the evening in April. In July,
the echo area per one echo at the time of the second
peak of total echo area at 15 LT is about 400 km 2
and is smaller than that in April. In contrast, at thetime of the main peak (01 LT) echo area per one
echo increases to about 1,200 km 2 . These facts
suggest that a number of smaller (i.e., about 400–
600 km 2 per one echo in average) and possibly isolated precipitation systems appear in the afternoon
in April and July, and that meso-b scale or larger
precipitation systems (i.e., about 1,200 km 2 per
one echo in average) appear at the time of themidnight maximum in July. Hirose and Nakamura(2005) analyzed that similar tendencies of diurnal
variation of echo size are common feature in Asia
by use of TRMM-PR data. Our analysis using
ground-based radar data reveals that the midnight
peak of echo area per one echo in average in July
includes ‘‘medium’’ system in their classiﬁcation,
which was not shown clearly in previous studies. It
is also noticed that diurnal variation of averaged
echo sizes per one echo shows no signiﬁcant di¤erence between in April and July: they have minima
in late morning of about 300–400 km 2 and maxima
in midnight of about 1,200 km 2 . This fact suggeststhat the dramatic di¤erence of diurnal variation of
total echo area between in April and July relateswith the number of echoes in nighttime is larger inJuly compared to April.
3.2 Regularity of the diurnal cycle of radar echo
That the diurnal variation of radar echo coverage was a persistent phenomenon, observed each

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Fig. 3. Time series of echo are in (a) April 2008 and (b) July 2008. Echo area whose intensity is greater than
10 dBZ is counted. Shaded periods indicate lack of data.

day during the experiment, and not just the resultof a few days of strong diurnal variability averaged
with many days of little or no diurnal variation, isshown by plotting the areal coverage of radar echo
greater than 10 dBZ as functions of local time
(Fig. 3). From this plot, it is evident that the echoarea reached a maximum once every day in theafternoon in April except the ﬁrst 2 days. In July,
the cycle was more reliable and clearly reached amaximum in the midnight.
3.3

Regional phase di¤erences of diurnal
variations
Radar observation area is divided into 6 bands of
100 km width as shown in Fig. 4. Because the prevailed wind is southwesterly and west-southwesterly
in April and July, respectively, at 850 hPa level
over 17.5 N, 102.5 E near the Vientiane (Fig. 5),
the bands are oriented in the southwest to northeast
direction. Since regions 1 and 6 are too small tocompare their diurnal variation with those in otherregions, regions 1 and 6 are omitted discussing thecharacteristics of diurnal variation.
Figure 6 shows diurnal variation of radar echoarea averaged in April and July in each bandshaped region deﬁned in Fig. 4. In April, echo areain every region has its peak around 15–18 LT.

Fig. 4. Six band-shaped regions in the Vientiane radar observation area. Contours are
terrain height in 500 m interval.

After the peak, echo area shrinks quickly in mountainous region 5 while it decreases slowly in practically plain regions (regions 2–3). The decrease ofecho area in these regions continues until aroundFebruary 2011

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peak at 04 LT. Thus, the peak ‘‘phase’’ of diurnal
cycle of echo area propagates 300 km in 6 hours:
i.e., about 14 m s1 . From a similar estimation using north-south 6 bands (not shown), the ‘‘phasespeed’’ of the propagation of echo-area diurnal
variation is also estimated about 14 m s1 (300 km
in 6 hours). This speed is faster than troposphericwind averaged in July 2008 at the nearest grid pointof JCDAS 2.5 resolution data set (Fig. 7).
4. Discussion and conclusions
Fig. 5. Frequency of 16 wind directions at
850 hPa level over 17.5 N, 102.5 E in
April (solid) and July (dotted) 2008 basedon 6 hourly 1.25 resolution JCDAS data.

08–10 LT next morning except slight increase ofregion 5 from 01–07 LT. Assuming that upslope
phase of mountain-plain circulation in the daytime contributes afternoon maximum of echo area
in mountainous regions, downslope phase of
mountain-plain circulation in the evening probably
causes the quick decrease of echo area in mountainous regions in the evening and early night.
In July, beside in April, diurnal variation of echoarea in each region is strongly enhanced. Furthermore, it is clear that the peak time of diurnal variation in each region di¤ers systematically, which is
not observed in April. Among regions 2–5 showing
clear diurnal variation, region 2 has the earliest
peak at around 22 LT, and region 5 has the latest

Radar echo data from the Vientiane radar in the
middle of Indochina for April and July 2008 wasanalyzed. Averaged radar echo area revealed clear
diurnal variation with the midnight peak by echoes
whose averaged areas per one echo is about@1,200 km 2 in July while the late afternoon peak
was observed by echoes whose averaged areas per
one echo is about @600 km 2 in April.
The diurnal cycle of echo area in bands with100 km width exhibited northeastward phase delay
in July. It is possible that precipitation systems
move northeastward in the radar observation range
and cause the phase delay. It is also theoreticallypossible, for example, that stationary precipitation
systems in bands which separate from other systems
in other bands may make the observed northward
phase delay, if each individual system develops systematically later in more northward bands thanin more southwestern bands. To examine whether
echoes are stationary or moving, echo translation
vectors are calculated by a cross-correlation method(e.g., Asai et al. 1977). For each CAPPI data, 64

Fig. 6. Diurnal variation of radar echo in each band-shaped regions shown in Fig. 3 averaged in (a) Apriland (b) July 2008.

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Fig. 7. Vertical proﬁles of wind speed averaged in July 2008 at 102.5 E, 17.5 N. Solidand dashed lines are southwesterly andsoutheasterly wind, respectively.

translation vectors are calculated for 64 boxes
with 40 km  40 km in a radar observation area of
320 km  320 km centered at Vientiane. The calculated directions from which echo comes and winddirections at 850 hPa level near Vientiane are indicated in Fig. 8.
It is clearly shown in Fig. 8 that echo moves inthe directions almost the same as the wind directions. Combining three results that monthly mean
wind at 850 hPa in July is south-southwesterly(Fig. 5), echoes move almost the same directions as
the wind direction (Fig. 8), and the phase of diurnal
variation delays as the region number increases
from region 2 (southwest) to region 5 (northeast)(Fig. 6b), it is probable that moving precipitation
systems caused the spatial phase di¤erence around
Vientiane. The moving direction consistent with thelarger scale wind is consistent with the case in thewestern Indochina (Okumura et al. 2003) but differs from the case in Bangladesh where the systemmoves against the southerly monsoon wind (Kataoka and Satomura 2005).
The ‘‘phase speed’’ of diurnal echo variation inthis study was about 14 m s1 , which was fasterthan the large scale wind around Vientiane in thelower troposphere. Similar fast spatial phase di¤erence of diurnal rainfall cycles in July is also (butfaintly) suggested in Fig. 8a of Takahashi et al.
(2010) by using TRMM-PR 10 years climate pre-Vol. 89A

Fig. 8. Time series of wind directions and directions from which echo comes (color)
in July, 2008. Open circles, crosses andopen rectangles indicate wind direction at
850 hPa, 700 hPa and 600 hPa level over
17.5 N, 102.5 E near Vientiane. Colorshade indicates ratio (%) of directions
from which echo comes.

cipitation data, which also gives about 14 m s1(102 E at 22 LT to 104.5 E at 03 LT). In the western Indochina, the phase velocity of diurnal cycle isclose to the lower tropospheric wind as shown by
Satomura (2000) and Okumura et al. (2003). Bothof squall line systems faster than western Indochina
and internal gravity waves may cause fast phaseshift found in present analysis. A study to determine which process would be important in the diurnal echo variation in this area is left in the future.
Acknowledgements
This study was partly supported by Heiwa Nakajima Foundation, CREST/JST and Grant-in-Aid
for Scientiﬁc Research B-21310120 from MEXT.
The JCDAS data sets are provided by the Japan
Meteorological Agency. Some of ﬁgures are drawn
by DCL Dennou Library. Support by Mr. Masaru
Wakabayashi in Lao PDR is appreciated.
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