GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 20, 2073, doi:10.1029/2003GL018302, 2003

Diurnal variation of precipitation by moving mesoscale systems:
Radar observations in northern Thailand
K. Okumura1,2 T. Satomura,2 T. Oki,3 and Warawut Khantiyanan4
Received 1 August 2003; accepted 22 September 2003; published 30 October 2003.

[1] The diurnal cycle of radar echo data observed at Om
Koi, northern Thailand, during three rainy seasons (May to
October, 1998, 1999 and 2000) are analyzed. From May to
July when southwesterly monsoons blow over theobservation area, the diurnal cycles in the leeward regions(inland of Thailand) show a phase delay that corresponds tothe distance from the mountains. In October when the winddirection reverses and blows from the east or northeast,
inland regions correspond to the upwind side of themountains and do not show clear phase shifts. The regions
near the coast of the Andaman Sea, which is the leeward in
October, show a similar phase shift of the diurnal cycle as theinland area during May – July. The echo tracking analysis
reveals that echoes, which are triggered near the mountains
and moved to leeward regions, caused phase delays, which
INDEX TERMS: 3309
depended on the lee distance.
Meteorology and Atmospheric Dynamics: Climatology (1620);
3329 Meteorology and Atmospheric Dynamics: Mesoscalemeteorology; 3354 Meteorology and Atmospheric Dynamics:
Precipitation (1854); 3374 Meteorology and Atmospheric
Dynamics: Tropical meteorology; 3360 Meteorology andAtmospheric Dynamics: Remote sensing. Citation: Okumura,K., T. Satomura, T. Oki, and W. Khantiyanan, Diurnal variation of
precipitation by moving mesoscale systems: Radar observations innorthern Thailand, Geophys. Res. Lett., 30(20), 2073, doi:10.1029/
2003GL018302, 2003.

[Oki and Mushiake, 1994; Ohsawa et al., 2001]. Researchhas determined that the diurnal cycle of cloud activity
reaches a maximum between 1800 to 0000 local time anda minimum early in the morning over the land. The largescale numerical simulations, however, have failed to simulate such a diurnal cycle of precipitation in tropical lands.
The model predicts maximum precipitation early in theafternoon over tropical land areas [e.g., Randall et al.,1991]. Recently, Satomura [2000] used his numerical model
results to propose a working hypothesis. His model indicatedthat precipitation systems triggered near the lee-side foot of
the mountains, which move towards the lee areas in theevening, cause a maximum in the diurnal cycle of precipitation late in the afternoon to evening in the Indochina
Peninsula. Rain guage networks suggest a similar mechanism for mid-latitudes [Riley et al., 1987; Asai et al., 1998].
In tropics, however, there is no observational evidence,
which directly supports a phase shift of the precipitation
diurnal cycle related to the distance from the mountains.
[3] In this paper, we analyzed the radar data observed innorthern Thailand for three rainy seasons (May to October
1998, 1999 and 2000). The diurnal variations and regional
seasonal differences of the phases are investigated toelucidate the mechanism of the diurnal cycle of precipitationin tropical lands.

2. Radar Data
1. Introduction[2] Atmospheric general circulation is driven by theearth’s surface, which transfers vast quantities of incoming
solar energy to the atmosphere. Precipitation systems in thetropics strongly affect the general circulation by releasing
latent heat. In addition, clouds that accompany precipitation
systems affect the general circulation by modifying theradiation budget because clouds have a high reflectivity tosolar radiation and low reflectivity to infrared radiation fromthe earth’s surface. Therefore, studying the space-time
distribution of precipitation systems in the tropics is important for water circulation in the tropics and for understanding large-scale atmospheric circulation. The diurnal cycle isone of the most significant variations in cloud activity in thetropics. Therefore, it is studied by researchers using infrared
radiation data from satellites [e.g., Murakami, 1983; Duvel,
1989; Nitta and Sekine, 1994] and rain gauge network data
1

Present at Nomura Research Institute, Otemachi, Chiyoda, Japan.
Department of Geophysics, Kyoto University, Sakyo, Kyoto, Japan.
Research Institute for Humanity and Nature, Kyoto, Japan.
4
Bureau of the Royal Rainmaking and Agricultural Aviation, Bangkok,
Thailand.
2[ 4 ] Three-dimensional echo intensity, which was
obtained from a meteorological radar located at Om Koi(17°4705300N, 98°2505700E) in northern Thailand, is analyzed. Figure 1 depicts the observational area of the Om Koi
radar, which is located on top of a mountain, where the
Chiang Mai radar is also shown as a reference. The Bureauof the Royal Rainmaking and Agricultural Aviation, Thailand, operates the Om Koi radar for routine observations.
The data was obtained at approximately 5 min intervals witha 250 km observation range, 1 km radial resolution, and 1°
azimuthal resolutions. The data from three rainy seasons(May – October 1998, 1999 and 2000) contributes to GAME(GEWEX Asia Monsoon Experiment). Since the Om Koi
radar is at the top of a high mountain, it does not suffer fromground clutter. All the echo intensity data was interpolated
into a horizontal Cartesian grid system at height of 3 km
using a weighted interpolation method of Cressman [1959].
The Cartesian grid system has a horizontal interval of 2 km
and 200  200 grids.

3

Copyright 2003 by the American Geophysical Union.
0094-8276/03/2003GL018302$05.00

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3. Results
3.1. Averaged Diurnal Variation of Radar Echoes[5] Figure 2 shows the diurnal variations of the radar
echo area, which the Om Koi radar observed, averaged over

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OKUMURA ET AL.: DIURNAL VARIATION OF PRECIPITATION

Figure 1. Observation areas of the Chiang Mai and Om
Koi radars.

three rainy seasons (May to July in 1999 – 2001). The totalecho area had a minimum and maximum at 10 LT and at
15 LT, respectively. The significant diurnal variations arealso found for the medium (20 –30 DBZ) and the strong(30 – 40 DBZ) echoes. These characteristics of the diurnal
variation are consistent with previous studies using Tbb[e.g., Nitta and Sekine, 1994].
3.2. Regional Phase Differences of Diurnal Variations[6] Figure 3 shows the eleven narrow stripe-shaped
regions in the southern part of the Om Koi radar observation
area. Each stripe is about 30 km wide and nearly parallels
the mountains between Thailand and Myanmer.
[7] Figure 4 shows the diurnal variation of the radar echoarea in each region, which is defined in Figure 3, from May
to July, 1999. Based on the NCEP/DOE AMIP-II Reanalysis [Kistler et al., 2001], moist westerly wind (the so-called‘‘southwesterly’’ monsoon) observed over the Indochina
Peninsula in the lower half of the troposphere during thefirst half of the monsoon rainy season (May to July) in 1999
(Figures 5a and 5b). Therefore, in this southwesterly mon-Figure 2. Diurnal variation of radar echo area observed by
Om Koi radar averaged over three rainy seasons (May to
October, 1999 – 2001). Bars indicate echo area of
10– 20 DBZ, 20– 30 DBZ and 30– 40 DBZ, respectively.

Figure 3. Eleven narrow stripe-shaped regions in thesouthern part of the Om Koi radar observation area. Regions
1– 4 are east of the mountains, Regions 5 – 7 are in themountains, and Regions 8 –11 are west of the mountains.

soon season, the western regions, 1 –4, are on the windwardside and the eastern regions, 8– 11, are leeward side of themountains. Regions 5 – 7 and the Om Koi radar are in themountains.
[8] On the windward side, the diurnal cycle of echo area
has a midnight or early morning maximum except in the
Region 4, which is near the mountains and has theafternoon maximum (Figure 4a). Since the Regions 1 and 2
are over the Andaman Sea, their morning maxima probably
reflect the diurnal cycle characteristics over the ocean. In
the mountainous region (Figure 4b), the diurnal cycles
reach afternoon maxima around 15– 16 LT, which are adirect response to the increase in vertical instability by
insolation.
[9] A significant feature emerges leeward of the mountains (Figure 4c). The phase of diurnal cycle is delayed as
the region number increases from 8 to 11. Since the distanceof each leeward region from the mountains is nearly inproportional to the region number, this result indicates thatthe phase is delayed according to the distance from themountains.
[10] Figure 6 shows the diurnal variation of the radar
echo area in October, 1999. In October 1999, northwesterly
monsoons blew (Figure 5c) which was the oppsite directionof May – July and the same wind reversal occurred in 1998
and 2000. Under this wind direction, west of the mountains
is the leeward side (Figure 6a) and the east of the mountains
is the windward side (Figure 6c). Systematic phase change
were not noted on the windward side, east of mountains(Figure 6c), whereas east of mountains from May – July
(Figure 4c) displayed a clear diurnal cycle phase shift. By
contrast, the regions near the coast (Figure 6a), which was
the lee side in October clearly showed a similar phase shift
in the diurnal cycle as the inland regions during May – July
(Figure 4c). It is remarkable that the diurnal cycle phase

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13 - 3Figure 4. Diurnal variations of the relative radar echo area in each narrow region shown in Figure 3, averaged from May
to July 1999. a) west of the mountains, b) in the mountains, c) east of the mountains.

delay in the echo area was dependent on the distance fromthe mountains and occurred during both southwesterly andnortheasterly monsoon seasons.
3.3. Origin of Moving Echoes[11] To understand the mechanism of the phase delay ofthe diurnal cycle, the 11 regions (Figure 3) were redefined
into 28 narrower regions as shown in Figure 7 and theechoes focused around 12 hour periods, which included thepeak of the diurnal cycle in a region. The echoes weregrouped according to size, large (800 km2) and small(<800 km2), in a narrow region.
[12] Figure 8 shows the contributions of average echoarea in Region 12 for the large echo group during May to
July 1999, when the southwesterly monsoon blew. It isnoteworthy Regions 10 and 11 contributed more to theechoes in Region 12 than Region 12 itself. From May to
July, Regions 10 and 11 are upwind to Region 12. Similarly,
echoes from upwind regions constituted the main part of thediurnal cycle for both Regions 11 and 13 (not shown). On
the other hand, small echoes contributed to the region where
they appeared (not shown).
[13] The phase shifts of the leeward narrow regions weredependent on the distance from the mountains. Therefore, it
is concluded that large echoes, which are triggered near themountains and moved into leeward regions, governed
the diurnal cycle of leeward regions. These results showthat the real atmosphere is consistent with the mechanism
for diurnal variations of precipitation in the Indochina
Peninsula proposed by 2D simulations.

distance from the mountains. Assuming that moving precipitation systems caused the phase difference among leeward regions in Figure 4c, the drifting speed of systems was
about 6 ms1, which is nearly equal to the moving speed ofprecipitation systems simulated by 2D numerical model[Satomura, 2000]. These observational results confirm theworking hypothesis proposed by Satomura [2000] using 2D
numerical model. It should be noted that the estimatedmoving speed here is similar to that observed in the tropical4. Conclusion[14] Radar echo data from the Om Koi radar in northern
Thailand for three rainy seasons was analyzed. The diurnal
cycles of echo area in the narrow regions revealed thatleeward side exhibited a diurnal cycle phase delay, which
increased the further away a region was from the mountains,for both southwesterly (May to July) and northeasterly(October) monsoon seasons. Back-tracking the radar echo
indicated that large echoes, which were triggered near themountains and moved leeward, governed the diurnal cyclesof leeward regions and the phase shift depended on the

Figure 5. Vertical profiles of horizontal velocity, potential
temperature and relative humidity at 100°E, 15°N. a) zonal(solid line) and meridional (dashed line) wind velocity, andb) potential temperature (solid line) and relative humidity(dashed line) averaged over May to July 1999. c) and d) are
the same as a) and b) except for the monthly average ofOctober 1999.

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OKUMURA ET AL.: DIURNAL VARIATION OF PRECIPITATION

Figure 6. Same as Figure 4 except for the observation period is October, 1999.

Australia as monsoon flow systems [Keenan and Carbone,1992].
[15] Recently, similar evidence that in the summer moving
precipitation systems govern the diurnal cycle of precipitation in leeward areas of the mountains was found by
Carbone et al. [2002] over a wide region of the UnitedStates. Although the systems observed by Carbone et al.
[2002] had a long lifetime (>1 day) and propagation distance(>103 km), comparing the two studies (our study was 1 day
and 102 km) suggests that it is plausible for moving
precipitation systems to play a crucial role in the diurnal
variation of precipitation during summer or tropical lands.

Figure 7. Redefined narrow stripe-shaped regions in the
Om Koi radar observation area. Regions 1– 5 and 15– 17
are east of mountains, Regions 6 – 9 and 18 –21 are in themountains, and Regions 10– 14 and 22 –28 west of themountains.

Figure 8. Contributions of the large echo group to Region
12 averaged from May to July 1999. The abscissa is theregion number where the echo appeared. Two vertical lines
indicate the position of Region 12. The ordinate is the area
where the echoes occupy Region 12.

OKUMURA ET AL.: DIURNAL VARIATION OF PRECIPITATION[16] Acknowledgments. This research was partly supported by aGrant-in-Aid for Scientific Research B-2-11440141 of the former Ministryof Education, Science and Culture, Japan. BRRAA, Thailand, in collaboration with the GAME-Tropics project performed the observation andcollected the data.

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K. Okumura, Nomura Research Institute, Otemachi, Chiyoda, 100-0004,
Japan.
T. Satomura, Department of Geophysics, Graduate School of Science,
Kyoto University, Kitashirakawa Oiwakecho, Sakyo, Kyoto, 606-8502,
Japan. (satomura@kugi.kyoto-u.ac.jp)
T. Oki, Research Institute for Humanity and Nature, Kyoto, 602-0878,
Japan.
W. Khantiyanan, Bureau of the Royal Rainmaking and Agricultural
Aviation, Bangkok 10900, Thailand.