SOLA, 2005, Vol. 1, 049 052, doi: 10.2151/sola. 2005 014

49

Characteristics of Diurnal Variations in Convection and Precipitation
over the Southern Tibetan Plateau during Summer
Hatsuki Fujinami†1, Shigeyuki Nomura2 and Tetsuzo Yasunari3
1
Japan Science and Technology Agency, Kawaguchi, Japan
2
Japan Weather Association, Tokyo, Japan
3
Hydrospheric Atmospheric Research Center, Nagoya University, Nagoya, Japan

Abstract
This study investigated diurnal cycles in convection
and precipitation over the complex mountain-valley terrain of the southern Tibetan Plateau (TP) during the mature phase of the summer monsoon. Cloud-coverfrequency (CCF) for high cloud increased after 13 LST(07 UTC) over the mountain ranges along 28.5°N and
30.2°N, reaching a maximum near 18 LST (12 UTC).
Areas of high CCF subsequently moved towards the valley area along 29.3°N; relatively high CCF persisted
there until early morning. Tropical Rainfall Measuring
Mission (TRMM) PR data show a nearly identical variation in rainfall frequency. Formation and developmentof convective-type clouds and phase differences in thediurnal cycle were strongly affected by TP topography.
Possible mechanisms for convective enhancement overthe southern TP are also discussed.

1. Introduction
Diurnal variation of convection is a notable phenomenon over the Tibetan Plateau (TP) during the summer (e.g., Murakami 1983; Fujinami and Yasunari 2001).
Convective activity over the TP is a major heat sourcein the Asian monsoon region (e.g., Yanai et al. 1992;
Ueda et al. 2003). Yanai and Li (1994) noted that a vertical circulation and associated convective activity over
and around the TP in the summer are maintained by athermally induced plateau-scale circulation that isdriven by thermal contrasts between the plateau andsurrounding regions. Sasaki et al. (2003) used resultsfrom a numerical model and showed that diurnal variations in precipitable water (PW) at Lhasa (29.7°N,
91.1°E) in October were associated with a plateau-scale
diurnal circulation system. Recent studies have discussed the relationship between convection (or precipitation) and topography over the TP during summer.
Ueno (1998) showed that satellite-estimated precipitation is more likely to be over the major mountain ranges
during the day (06 18 LST at 90°E) and over major
valleys at night (18 06 LST). Kurosaki and Kimura(2002) used Geostationary Meteorological Satellite(GMS) visible and infrared data for both pre-monsoon
and monsoon periods in 1998 to show that daytime (0915 LST) clouds prevail over major mountain ranges of
the TP. Such variations could be related to thermallyinduced local circulations driven by the topography of
the TP. The major mountain ridges and valleys of the
TP are aligned east-west (c.f., Fig. 1a), and valleys are
100 300 km wide; Kuwagata et al. (2001) used a numeri†
Corresponding author: Hydrospheric Atmospheric ResearchCenter, Nagoya University, Nagoya 464-8601, Japan. E-mail:hatsuki@hyarc.nagoya-u.ac.jp ©2005, the Meteorological Societyof Japancal model to show that this width is a very effective
topographic scale for transport of water vapor by thermally induced circulation during the summer. They also
noted that the diurnal cycle of PW over the TP resultsfrom water vapor transport by the local circulation.
Past studies have not investigated the complete diurnal cycle in convection and precipitation during themature phase of the monsoon. The present study examines diurnal characteristics, such as the transition fromnoon to midnight, over the complex mountain-valley
terrain of the southern TP. The study focuses on Augustwhen a maximum in local convection over the southern
TP appears in the seasonal cycle (Fujinami andYasunari 2001). Clarifying this diurnal cycle over thesouthern TP will yield information on the water cycle
for this region of active convection.

2. DataThis study used hourly GMS-IR equivalent blackbody temperature (Tbb) data on a 0.05° × 0.05° grid as aproxy for cloudiness. High clouds suggest strong convection, and they were inferred from the cloud index
Ic: Ic = 230-Tbb(K) if 230-Tbb(K) > 0; Ic = 0 if 230-Tbb <0. Reanalysis data from the Global Energy and Water
Cycle Experiment-Asian Monsoon Experiment (GAME)
show that the 230-K threshold is the air temperature
about 12 km above sea level over the southern TP.
Cloud-cover frequency (CCF) is the percentage of cloud
cover (Ic > 0) times the total number of available data
points in each grid at each hour. Tropical Rainfall
Measuring Mission (TRMM) precipitation radar (PR)data (product 2A25) diagnosed the diurnal precipitation
cycle. PR algorithm version 5 was used. Rain types considered were convective, stratiform, and other (Awaka
et al. 1997). Rainfall frequency (RF) for each 0.05° ×
0.05°-grid was determined from the number of rainfall
events relative to the total number of samples. A rainflag marked the occurrence of rain. Version 1.5 GAME
reanalysis data, on a 0.5° × 0.5°-grid, diagnosed atmospheric circulations (Yamazaki et al. 2000; Ueda et al.
2003). The present study examined Tbb and GAME
reanalysis data for August 1998. Five years (1998 2002)of TRMM PR data for August reduced any samplingbias that could arise because of the non-sun-synchronous orbit of TRMM.

3. Results
Figure 1a shows the topography of the central andsouthern TP. Dotted lines A and B indicate major valleys (unnamed and Yarlung-Zangbo River). Shortdashed lines C and D denote major mountain ranges(Nyainqentanglha Range and the northern Himalayas).
Valleys A and B have meridional widths of 〜300 km

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Fujinami et al., Diurnal Variation over the Tibetan Plateau

Fig. 2. (a) Latitude-Time section of cloud-coverfrequency averaged between 88° and 92°E for August
1998. Contour interval is 10%. The left-hand figure
shows surface altitude averaged over the same longitudes. (b) As in (a) but for rainfall frequency averaged
for five Augusts (1998 2002). The solid line denotes the
5% contour.

Fig. 1. (a) Tibetan Plateau topography. The solid line isthe 3000-m topographic contour. Dotted lines indicate
major valleys A (unnamed) and B (Yarlung-Zangbo
River). Dashed lines indicate major mountain ranges C(including Nyainqentanglha Range) and D (northernpart of the Himalayas). (b) Average distribution ofcloud-cover frequency at 15 LST for August 1998. (c) Asin (b) but for 21 LST.

and 〜150 km, respectively; they have average depths of〜500 m and 〜1000 m along each dotted line between
88° 92°E. Figure 1b shows the distribution of CCF at 15
LST (09 UTC) for August 1998. Zonally elongated regions of high CCF are found near 28.5°N and 30.2°N,
close to major mountain ranges C and D. High clouds
are suppressed over valley B at 29.3°N. The CCF distribution is consistent with that shown by Kurosaki andKimura (2002). At 21 LST (Fig. 1c), in contrast, the highest CCF values are over a wide region centered on valley
B. The greatest difference in CCF between 15 LST and
21 LST is over valley B.
Figure 2a shows a latitude-time section for CCF averaged between 88° to 92°E to highlight the time evolution of topographically influenced CCF. The left-handfigure shows the surface altitude averaged over thesame longitudes. Small values of CCF are observed over
the TP from 9 to 12 LST. CCF increases after 13 LST
over mountain ranges C and D, reaching a maximum at
17 18 LST over the summits. A CCF maximum also develops over the mountain range along 34°N (theTanggula Range), although CCF there is smaller thanover C and D. Subsequently, areas of high CCF movefrom the two mountain ranges and merge over valley B,
remaining there between 19 and 06 LST. Relatively

high values of CCF also persist until 06 LST over thebottom of valley A. A valley-mountain contrast is distinct over the southern TP.
Figure 2b is similar to Fig. 2a, but for rainfall frequency (RF). Features in the diurnal variation show contrasts between mountain ranges and valleys that are
similar to variations in CCF. RF reaches a maximum
over mountain ranges C and D around 17 LST, after
which time RF decreases rapidly although high clouds
still persist. In valley B, RF peaks between 20 and 22
LST and persists until 03 LST, like CCF. Daytime precipitation starts a few hours earlier than high cloud overthe plateau except in the valley bottoms.
Diurnal variations in PW have been observed by
GPS at Lhasa during the pre-monsoon and monsoon(Takagi et al. 2000). During the summer monsoon, PW
decreases from morning until 15 LST and thenincreases until 21 LST. Subsequently, PW is nearly constant until 03 LST when it decreases. Such an evolution
is consistent with the CCF and RF sequence over valley
B.
Figure 3 shows a typical diurnal variation to illustrate the evolution of convection during the transition
phase from afternoon to night. Figure 3a shows the Ic
distribution at 15 LST (09UTC) on 13 August 1998.
Meso- scale (a few tens of kilometers) convective
clouds develop along mountain ranges C and D. Convection grows and merges so that by 18 LST (12UTC), convection is centered over the mountains. Some newconvective cells evolve to larger cells over valley B as
the valley-mountain contrast becomes ambiguous. At
19 LST (Fig. 3c), high clouds cover the southern TP, andthere are some enhanced convective cells. Convective
cells with scales of about 100 km are over valley region
B by 21 LST (15 UTC). One cell near 30°N, 90.5°E at 18
LST moves southeastward and develops near 29.5°N,

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Fig. 3. Ic distribution at (a) 15 LST, (b) 18 LST, (c) 19
LST, and (d) 21 LST on 13 August 1998. Wind vectors
at 500 hPa are shown in (b). The reference arrow is 10
m s 1 . The thick solid line denotes the 3000-m topographic contour.

91.5°E at 21 LST. A second cell near 29.5°N, 88°E develops in place over valley B. These convective cells remain
nearly stationary until 02 LST (20 UTC) (not shown).
Wind fields over the southern TP are characterized byweak southerlies at 500 hPa (Fig. 3b) and weak
westerlies (<〜10 m s 1 ) at 200 hPa (not shown). Weakambient winds over the TP are consistent with an atmospheric environment there that favors the development of thermally driven local circulations.
Figure 4a shows mean 6-hour RF and total 6-hournear-surface rainfall averaged between 88° and 92°E
and between 12 18 LST (06 12UTC) for five years(1998 2002). Gray bars indicate total precipitation,
while black bars denote total convective rainfall. The
difference between the two bars represents the amountof stratiform rain. High RF occurs over mountain
ranges C and D. Large precipitation totals (exceeding 2
mm in 6 hours) occurred over the mountain ranges. On
average, convective rains account for more than 60 percent of the afternoon precipitation over mountain
ranges, although stratiform rainfall amounts dominate
at some locations even in the mountains. RF is low overvalley B between 12 18 LST. In contrast, between 18
and 24 LST (12 18 UTC), RF is high over valley B,
where precipitation exceeds 2 mm (6 hr) 1. During thesehours, convective rains deposit more than half of thetotal precipitation over the valley bottom.
Shimizu et al. (2001) used TRMM PR data in thesummer of 1998 to show that precipitation associatedwith deep convection develops between 15 and 18 LST
over the central TP (valley A). In contrast, stratiform
precipitation intensity peaks between 18 and 24 LST.
Comparison between Figs. 4a and 4b suggests that, on
average, convective rains prevail more during the day
than at night. Convective rains remain more common atnight over the bottom of the major valleys. However,stratiform rains do become more common at night thanduring the day.
Figure 5 shows the spatial evolution of 6-hour total
precipitation. The distribution of precipitation during
the day (Fig. 5a) and at night (Fig. 5b) is consistent with
CCF distributions (Figs. 1b and c, respectively). During
the day, heavier rains (>2.0 mm per 6 hours) correspondwell with mountain ranges C and D; less precipitation
falls in valley B. Heavy rains also fall over the southeastern TP (around 30°N, 98°E). In contrast, heavier rain
occurs at night in valley B.

Fig. 4. (a) Mean 6-hour rainfall frequency (solid line, leftaxis) and integrated 6-hour precipitation (gray bar; rightaxis) averaged for five Augusts (1998 2002) andbetween 88° and 92°E for 12 18 LST. Black bar indicates the amount of convective-type rain. (b) As in (a)but for 18 24 LST. (c) Topography averaged between
88° and 92°E.

Fig. 5. Spatial distribution of 6-hour total near-surface
rainfall (a) from 12 to 18 LST and (b) from 18 to 24 LST.
The dotted line indicates valley B (Yarlung-Zangbo
River). The thick solid line is the 3000-m topographic
contour.

4. Discussion
High clouds are common over the southern TP
during the mature phase of the summer monsoon.
Uyeda et al. (2001) showed that convection in thecentral TP can reach the tropopause during the monsoon because the ambient wind is weaker than during

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the pre-monsoon season. In August 1998, a core of the
Asian subtropical jet was at 40°N at 200 hPa. Weakambient winds (<10 m s 1) from the surface to 200 hPa
occurred over the areas between 28° and 31°N, wherehigh clouds were enhanced (not shown). The atmospheric structure there allowed convection to reach theupper troposphere.
The atmospheric structure also supported thermallyinduced local circulations. Afternoon convection overthe mountain ranges suggests convergence induced bya local circulation (Kuwagata and Kimura 1997). The
meridional width (〜150 km) and depth (〜1,000 m) ofvalley B are similar to the most effective topographic
scales for water transport (160 km width and 1000 m
depth) shown in Kuwagata et al. (2001). Characteristic
valley scales over the southern TP efficiently trigger
afternoon moist convection over the mountain ranges.
Active convection and precipitation increased aftersunset in valley B. Dynamic lifting is necessary to initiate convection. A possible trigger for nocturnal convection is low-level convergence of down-slope flow frommountain ranges C and D. Surface cooling could causesuch a flow. Cold outflow from deep precipitating convection over the mountains may strengthen low-level
convergence over valley B. Yang et al. (2004) showedthat convergence between pre-existing up-slope flow
and down-slope flow forced by evaporative cooling of
precipitation over the mountain could trigger deep convection that would subsequently propagate from mountain to valley in the central TP. Valley B (the YarlungZangbo River) is aligned roughly east-west, merging
with the Brahmaputra valley east of 94°E. Moisturetransport associated with a wind system flowing along
the Brahmaputra to valley B may enhance convection.
The formation and development of convective-type
clouds and phase differences in the diurnal cycle overthe southern TP are strongly influenced by mountainvalley terrain. Thermally induced mountain-valley
circulations help regulate active convection over the TPin summer. However, the effect of a plateau-scale diurnal circulation system cannot be neglected in a detailed
description of the diurnal convective cycle. A plateauscale circulation may enhance southerly moisture
inflow towards the TP. Such a moisture inflow helps tomaintain high specific humidity and a convectively
unstable stratification over the southern TP. Further
study will reveal interactions between plateau-scale andregional-scale circulation systems and how they influence the diurnal cycle of convection over the southern
TP in summer.

5. Conclusion
This study examined characteristics of the diurnal
cycle in convection and precipitation over the complex
mountain-valley terrain over the southern TP in
August. Special attention was given to the evolution
between noon and midnight.
Cloud-cover frequency (CCF) for high cloud increased after 13 LST (07 UTC) over the mountain
ranges along 28.5°N and 30.2°N, reaching a maximum
near 18 LST (12 UTC). High values of CCF subsequently
moved over the valley along 29.3°N. Relatively large
CCF persisted over the valley until early morning. A
similar evolution occurred in rainfall frequency derived
from TRMM PR.
Over the TP as a whole, the ratio of convective-typerain to total rainfall is larger during the day (12 18
LST) than at night (18 24 LST). However, mesoscaleconvective systems are common over major valleys atnight.

Acknowledgements
We thank Dr. K. Ueno of the University of Shiga
Prefecture for his helpful comments. The authors thankMr. H. Ichikawa of Nagoya University for processing
TRMM PR data. Dr. T. Kikuchi of Kochi University provided GMS-IR data.

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(Manuscript received 12 January 2005, accepted 1 April
2005)