Transition of the Precipitation Process over the Central Tibetan Plateau
during the Summer of 1998
*Hiroyuki YAMADA1 and Hiroshi UYEDA1, 2[1: Institute of Observational Research for Global Change (IORGC), Japan Agency for
Marine-Earth Science and Technology (JAMSTEC), 2: HyARC, Nagoya University]* IORGC, JAMSTEC, 236-0001, 3173-25 Shouwa-machi,
Kanazawa-ku, Yokohama, Japan. e-mail: yamada@jamstec.go.jp
Abstract
This paper describes the transition of precipitation characteristics, which occurred four weeks after the monsoon onset
over the central Tibetan Plateau. This transition occurred when the ground surface became wet and vegetation became
active. The daily rainfall amount associated to a plateau-scale heat low increased with the change in the surface
condition. This increase in rainfall related to the moistening of sub-cloud layer that suppress the evaporation ofprecipitation particles. Therefore, this study suggests that the precipitation characteristics over the plateau are sensitive
to the change in the ground conditions.
Keyword: Tibetan Plateau, precipitation, land-atmosphere interaction.

1. Introduction
The summer monsoon over the Tibetan Plateau, whichusually occurred from June to August, is characterized bydeep convective activity (e.g., Murakami 1983; Fujinamiand Yasunari 2001). To clarify the precipitation
characteristics related to the deep convection, theGAME-Tibet project carried out an intensive field
experiment using an X-band Doppler radar during thesummer of 1998. Early studies of the GAME-Tibet
suggested that there was an interesting transition of
precipitation characteristics in early July, which was fourweeks after the monsoon onset. The analysis on strong
radar echoes by Shimizu et al. (2001) suggested that theactivity of deep convection decreased after the middle ofJuly. In contrast, Ueno et al. (2001) showed that monthly
rainfall increased remarkably from June (51 mm) to July
(134 mm). The cause of the transition was, however, notdiscussed in these two studies. To clarify the cause of thetransition, characterized by the decrease of convective
activity and the increase of surface rainfall, this paper
describes the transition of precipitation characteristics
more in detail, using the data collected during the field
experiment in 1998.

2. Change in the surface conditions
The transition mentioned above occurred when thesurface conditions changed. Figure 1 shows the timeseries of surface meteorological elements. To emphasize
the change at the early half of July, the analysis period
was separated into three sub-periods (labeled as Phases
I-III). The mean values in Phase I (dashed lines) werecompared with those in Phase III (dash-dotted lines).
From Phase I to III, air temperature (Ta) decreased from12.1 to 9.6 oC (in Phase III). Both the decrease in airtemperature and the increase in mixing ratio (q, from 7.5to 8.3 g kg-1) caused a remarkable increase in relative
humidity (RH, from 48 to 63 %). These changes inatmosphere occurred with the change in the groundconditions: the decrease of ground temperature (Tg) from25 to 15 oC, and the increase of soil water content from
16 to 42 %. These changes were consistent with thedecrease of sensible heat flux and the increase of latent
heat flux, which are shown in Tanaka et al. (2001). The
observation of ground condition using video camera

Fig. 1: (a-d) Time series of ground temperature (Tg),air temperature near surface (Ta), mixing ratio (q),relative humidity (RH), soil-moisture content at 0.04m
below the surface, at Amdo from 10 June to 31 August
1998. (e) Area-mean daily rainfall amount. Theseelements were averaged in the daytime (00-12 UTC).
The dashed line (a dash-dotted line) in each panel isthe averaged value during phase I (phase III),respectively.

showed that the color of the grass changed from brownto green (not shown), indicating the activation ofvegetation. These changes indicate that the groundsurface and the air near the ground became moist during
the transition of precipitation characteristics.

Fig. 2: Monthly variation of daily rainfall amountaccumulated for the UH-, TR-, and NL-type rain
events.

3. Change in surface rainfall
3.1.
Separation of rain events
Many studies have recognized the importance ofconvective instability due to strong solar heating for thedevelopment of convective clouds (e.g., Kuo and Qian
1981; Yanai et al. 1992). On the other hand, Ueno et al.
(2001) pointed out that a large-scale frontal disturbancealso affects the development of convective clouds. It is,therefore, necessary to separate rainfall events according
to the large-scale flow pattern before discussing theprecipitation characteristics.
In this study, all rain events were separated into thefollowing three types using the GAME-reanalysis
version 1.5 dataset.
z UH-type, which is characterized by a surface heat
low and a Tibetan high in the upper troposphere.
z TR-type, characterized by a surface low and atrough in the upper troposphere.
z NL-type, characterized by no surface low.
This separation was accomplished through twomethods. First, a surface low was detected using 500-hPa
geopotential height. If no low was found, a rain event
was classified as NL-type. Second, the detected low wasclassified according to vortex advection at 250 hPa. If
vortex advection was positive, which means thepropagation of a trough, a rain event was classified asTR-type. Otherwise (i.e. negative vortex advection), arain event was classified as UH-type.

3.2.

Change in rainfall of three rain types

The differences in rainfall amount among the threetypes are shown in Fig. 2. Total amounts during themonsoon season (from 13 June to 31 August 1998) are
shown under the graph. The amounts of both the UHand TR-types exceeded 100 mm, and the sum of themwas 84 % of the whole amount (305.7 mm). In contrast,
the amount by the NL-type was small (48.6 mm), whichindicates that most of the rainfall occurred during rain
events of both the UH- and TR-types.
The graph of monthly variation in the daily amountshows that intraseasonal change in UH-type rain wasremarkable. The daily amount increased from 1.9mm·day-1 in June to 5.2 mm·day-1 in August. Such achange is not remarkable for TR-type rain. It is, therefore,necessary to describe the change in the precipitation
characteristics associated with UH-type rain in detailFig. 3: (a) Diurnal cycles of six-hourly integrated
rainfall for UH-type rain events in June and July. An
arrow shows remarkable change in amount from theprevious month. (b) The same as (a), but for thehourly echo area 7.5 km ASL (3.0 km ARL). The unitof echo is represented as a cover rate (%) to the wholearea within the observational range (128 km inradius).

Fig. 4: Histograms of hourly rainfall in the daytime(00-12 UTC) during UH-type events in June and July.
Each bar is stratified according to the maximum valueof area-averaged reflectivity at 7.5 km ASL in thesame hour. The cases observed at six stations within
the radar observational area were used.

4. Transition of precipitation characteristics inUH-type rain
Since convective activity over the plateau in summer
shows obvious diurnal variation (e.g., Uyeda et al. 2001),
it is necessary to examine the change in the diurnal
cycles of rainfall and radar echo during UH-type rain
events. Diurnal cycles of six-hourly integrated rainfall(Fig. 3a) show that rainfall in the afternoon (i.e., between
6 and 12 UTC, marked by arrows) increased from Juneto July.
The diurnal cycles of hourly radar-echo area areshown in Fig. 3b. The areas of convective echo wereseparated using the method similar to the one used inSteiner et al. (1995). Diurnal cycle of convective echoarea is characterized by a peak in the afternoon andalmost zero in the late half of the night. By comparing
the diurnal variations between June and July, it is foundthat convective echo in the afternoon (6-12 UTC)decreased after June.

To clarify the relationship between the decrease of
convective-echo area and increase of rainfall amount in
the afternoon, hourly-measured rainfall amount wascompared with reflectivity above the rain gauge (Fig. 4).
The important point to emphasize here is the cases thatno rainfall was measured whereas radar echoes existed
over the rain gauge. Frequency of occurrence of no-raincases decreased from June to July. This result indicatesthat precipitation particles from convective clouds
sometimes did not reach the ground in June. It issuggested that the atmospheric conditions in June weresuitable for the evaporation of precipitation particles.
This evaporation is probably the reason why the daily
rainfall was very small in June. The moistening of
sub-cloud layer after June was confirmed from theanalysis of upper-air sounding data (not shown).

5. Summary and discussion
This study describes the transition of precipitation
characteristics that occurred over the central plateau after
the monsoon onset. This transition is characterized by thefollowing points:
z Change in the ground condition (i.e., moisteningof ground surface and activation of vegetation),and moistening of air near the surface.
z The increase of daily rainfall amount in theUH-type rain, which occurred under thecondition characterized by a heat low near thesurface and a high in the upper troposphere.
These results suggest that the transition of precipitation
characteristics associated with the UH-type rain eventswas related to the changes in the surface conditions.
Figure 5 shows a schematic illustration of the transition.
Both the moistening of the ground and the activation ofgrass may have caused the moistening and cooling of air
in the sub-cloud layer. This moistening of lower
troposphere may have suppressed evaporation of
precipitation particles that fall from deep convective
clouds in the afternoon. As a result of the suppression,
precipitation particles could reach to the ground, andrainfall amount measured by rain gauge may haveincreased. Therefore, this study suggests the importanceof the ground conditions as a prevailing factor for theconvective activity and precipitation characteristics overthe plateau in summer.

Acknowledgments
The authors would like to express their gratitude to themembers of the GAME-Tibet project for theirFig. 5: Schematic illustration showing the transitionof the convective precipitation process in a UH-typerain event due to changes in the ground conditions.

cooperation in the field experiment. Thanks are extended
to Dr. N. Yamazaki of the Meteorological ResearchInstitute, Japan Meteorological Agency, for providing theGAME reanalysis dataset. The GAME-Tibet project wassupported by the Ministry of Education, Science, Sports,and Culture of Japan, the Science and Technology
Agency of Japan, the Chinese Academy of Science, theFrontier Research System for Global Change (FRSGC),
the National Space Development Agency of Japan, and
the Asian Pacific Network.

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