Space-time characteristics of diurnal variation in convection
and precipitation over the Tibetan Plateau during the Summer Monsoon*Hatsuki Fujinami1, 2, Shigeyuki Nomura3 and Tetsuzo Yasunari1(1: HyARC, Nagoya University, 2: JST/ CREST, 3: Japan Weather Association)* HyARC, Nagoya University, Japan, 464-8601, Furo-cho,
Chikusa Nagoya, Japan. e-mail: hatsuki@hyarc.nagoya-u.ac.jp
Abstract
We focused on the time-space characteristics of diurnal cycle of convection over the mountain-valley complex terrain in
the southern part of the plateau for the transition from noon to midnight. Data used in this study are GMS-IR (0.05 x 0.05grid) and TRMM PR (0.05 x 0.05 grid) for the summer (JJAS) from 1998 to 2002. Composite diurnal cycle of convection
showed that active convection area moves from the mountain ranges (along 28.5N and 30.5N) to valley area (along 29N)
from 09 to 18 GMT (from 15 to 00 LST at 90E). This progress was also confirmed in precipitation fields observed byTRMM.
Keyword: Tibetan Plateau, convection, precipitation, diurnal variation, topography.

1. Introduction
Diurnal variation of convection is a prominent
phenomenon over the Tibetan Plateau during the northernsummer (e.g., Murakami 1983; Fujinami and Yasunari2001). Some previous studies have shown the relationshipbetween convection and topography over the plateau
during summer. Convection (or precipitation) prevails overthe mountain ranges in the afternoon, while it is enhanced
over the valley in the night over the southern part of theplateau (85°–95°E, 27°–33°N) (Ueno 1998; Kurosaki andKimura 2002). The major mountain ranges and major
valleys over the region are aligned east-west direction (c.f.
Fig. 1). The width of mountain ranges is about 100–300
Km, which is very effective topographic scale for water
vapor transport by thermally-induced circulation as
predicted by a numerical model (Kuwagata et al. 2001).
In this study, we focused on the time-spacecharacteristics of diurnal cycle in convection over themountain-valley complex terrain for the transition fromnoon to midnight during summer.

2.

Fig. 1: Topography of the southern part of theTibetan Plateau. The contour interval is 1,000m.
Shading denotes areas higher than 5,000m.

Data

We mainly used GMS-IR Tbb data of 0.05° grids as aproxy for cloud activity. Cloud index (Ic) is defined as Ic =260-Tbb(K) if 260-Tbb(K) is positive; Ic = 0 if 260-Tbb isnegative. Threshold value of 230K is also used to detect
high clouds. Cloud-cover frequency (CCF) is also definedas the percentage of cloud cover (Ic ≥ 1) times to the total
number of available data for each grid at each hour.
TRMM-PR data (2A25) were also used for the analysis ofprecipitation. We made 0.05 x 0.05 dataset of PR data. To
examine atmospheric circulation fields, we used GAME
reanalysis version 1.5 on 0.5° x 0.5° grid in this study.
Analysis period in Tbb and game reanalysis is from July toAugust 1998. In TRMM PR, data for the period from 1998
to 2002 (5 years) were used.

3.

Results

Figure 2 shows the spatial distribution of CCF at 09
GMT (15 LST at 90°E) and 15 GMT (21LST) for August
1998. Zonally-elongated areas of high CCF appear along
30.5°N and 28.5°N at 09 GMT (15 LST), which

Fig.2: Cloud-cover frequency (%) at 15LST (upper)and 21LST (bottom) in August 1998. The 3,000m
topographic contour is shown as a solid line.

correspond to areas of major mountain ranges (see Fig. 1).
By contrast, at 21LST (Fig.2-bottom), highest CCF isobserved over a wide region centered at valley area along
about 29.5°N. The relationship between convections andtopography is not clear to the north of 32°N. In order toexamine the time evolution of CCF associated with the
topography in detail, the latitude-time section of composite
diurnal cycle in CCF along 90°E is shown in Fig. 3. CCF
becomes higher at around 8–9 GMT (14–15 LST) over themountain ranges (28.5°N and 30.5°N) and reaches its
maximum at 11–12 GMT (17–18 LST). The high CCF area
moves from the mountain ranges to valley area (along29.5°N), and then stays from 13 to 18 GMT (19 to 00 LST).
Figure 4 displays a typical case of diurnal variation inconvection on 13 August 1998. At 09 GMT (15LST),
meso-β (a few 10 Km) scale convective clouds develop
over the mountain ranges. The convections grow andcombine to extend at 12 GMT (18LST). By 15 GMT(21LST), the convections move and develop into
convective cells of about 100 Km over the valley area.
Throughout the sequence, convection systems move
eastward because the westerly is dominated in the middle
through upper troposphere on this day. Circulation fields
for 13 August 1998 are shown in Fig. 5. At 200hPa, a deep
trough is located along 93°E to the north of 32°N. A centerof Tibetan high is observed around 82°E, 28°N. The
upper-level westerly is dominated to the north of 28°N
over the plateau. In 500 hPa, to the east of 95°E,
northwesterly wind prevails north of 32°N, whilesouthwesterly flow is dominant south of 32°N. Over thesouthern part of the plateau, however, relatively weaksynoptic-scale winds are observed, indicating that theatmospheric environment is favorable to induce
thermally-driven local circulation.
The characteristics of diurnal variation in clouds arealso confirmed in precipitation fields observed by TRMM.
Figure 6 indicates the 5-year averaged distribution inprecipitation for 06–12GMT (12–18LST) and 12–18GMT(18–00LST) in August. Precipitation amount is lager overthe mountain ranges (along about 28.5°N and 30.5°N)
during 06–12GMT (12–18LST), while it is enhanced overvalley area (along 29.5°N) during 12–18GMT (18–00LST).
In the daily mean, precipitation amount is largest over thevalley.

References
Fujinami, H. and T. Yasunari, 2001: The seasonal andintraseasonal variability of diurnal cloud activity over theTibetan Plateau. J. Meteor. Soc. Japan, 79, 1207-1227.
Kurosaki, Y. and F. Kimura, 2002: Relationship between
topography and daytime cloud activity around Tibetan
Plateau. J. Meteor. Soc. Japan, 80, 1339-1355.
Kuwagata, T., A. Numaguti and N. Endo, 2001: Diurnal variationof water vapor over the central Tibetan Plateau during thesummer. J. Meteor. Soc. Japan, 79, 401-418.
Murakami, M., 1983: Analysis of the deep convective activity
over the western Pacific and Southeast Asia. Part I:
Diurnal variation. J. Meteor. Soc. Japan, 61, 60-76.
Ueno, K., 1998: Characteristics of plateau-scale precipitation in
Tibet estimated by satellite data during 1993 monsoon

season. J. Meteor. Soc. Japan, 76, 533-548.

Fig. 3: Latitude-time section of cloud-cover frequency
along 90°E. The left side figure indicates the crosssection of topography along 90E.

Fig.4: Time evolution of Tbb at 06GMT (12LST), 09GMT(15LST), 12GMT (18LST) and 15GMT (21LST) on 13August 1998. The 3,000m topographic contour is shown asa solid line.

Fig.5 Geopotential height and wind fields at 200 hPa(upper) and wind fields at 500 hPa (bottom) on 12GMT(18LST) 13 August 1998. Shading denotes the TibetanPlateau.

Fig.6 Distribution of 5-year averaged (1998–2002)
precipitation for 06–12GMT (12–18LST) (left) and
12–18GMT (18–00LST) (right). A unit is mm/hour.