The isotopic composition of water vapor and the concurrent
meteorological conditions around the northeast
part of the Tibetan Plateau*AkiyoYATAGAI1, Atsuko SUGIMOTO 2, Masayoshi NAKAWO1(1: Research Institute for Humanity and Nature,
2: Graduate School of Environmental Earth Science, Hokkaido University)* Research Institute for Humanity and Nature (RIHN), Takashimacho, Kamigyo-ku, Kyoto 602-0878, Japan.
e-mail: akiyo@chikyu.ac.jp

Abstract

The stable isotopic ratio of water vapor (δ18O and δD) was measured near a glacier in the northeast part of the Tibetan
Plateau in the summer of 2003. A clear difference of the isotopic composition was found with the passing of a deep trough,which showed lighterδD (andδ18O ) and higher d-excess compared with those of the previous period. The dry air from thenorthwest, which came after the passing of the trough, seems to come from the upper troposphere/lower stratosphere. We
also found a diurnal change in the isotopic composition. As the air from the northern desert has relatively small δ18O,
relatively small δD values and relatively high d-excess values, the valley wind that prevails in afternoon through evening
must be bringing air from the northern desert to our site. We believe that analysis of the isotopic ratio of water vapor should
be useful to understand the continental and local hydrological circulation of complicated terrains, and to interpret the icecore records in paleo-climatological studies.
Keyword: water vapor, atmospheric branch of the hydrological cycle, isotope, arid/semi-arid region, paleo-climate.

1. Introduction
Several methods have been used to investigate therelation between water vapor transport and precipitation to
the arid regions of the Eurasian Continent. Recently
atmospheric general circulation models (GCMs) were used
to investigate the moisture source for precipitating wateron the Eurasian continent, in addition to diagnostic analysis(e.g. moisture budget analysis) using
meteorologicalreanalysis dataset.
However, these methods are not sufficient to understandthe hydrological circulation in the central part of theEurasian continent, because of the following reasons.
1. Since arid regions and mountainous regions aregenerally unpopulated, the quality of the objective
analysis data is relatively low because of the sparse
data network.
2.
Modeling
theatmospherichydrologicalcirculations
overcomplicatedtopography,
especially for steep mountains, is very difficult.
The Taklimakan desert, one of the driest regions in
the continent, is surrounded by steep mountains(e.g. northern periphery of the Tibetan Plateau andTianshan mountains, etc).
3. The region where moisture from the northwest,
south of the Tibetan Plateau, and monsoonal flowfrom Chinese plain area converge locates theNorthwest and North China. Therefore, theatmospheric circulation field during precipitation isvery different from that during the mean state andon dry days. (Yatagai and Yasunari, 1998).
An alternative tool to understand the hydrologicalcirculation should be stable isotope. Actually, stable
isotope techniques of precipitating water as well as terrainwater (river flow, soil moisture, ground water, etc) areoften used to better understand hydrological circulation.
However, it is thought that understanding atmospherichydrological circulation over complicated terrain like
Northwest China requires analysis of the isotope ratio of
the water vapor because;
1. There is too little precipitation to sample becauseof its arid environment.
2. As written in the previous point 3, moisture source
is not single for this region.
3. Investigating isotope information on water vapor
over the high mountains of this region is useful
because ice core samples of glaciers are sometimes
used to determine the paleo-environment.
To determine the past atmospheric condition and climatechanges in this area, it is very important to clarify therelationship between the moisture source, transport, andatmospheric circulation over the glaciers in this region.
Therefore, our field experiment to collect atmosphericmoisture for isotope analysis was performed on July 1st(Qiyi) Glacier in the Qilianshan Mountains in the northeast
part of the Tibetan Plateau in Northwest China. After
collected and analyzed water vapor near this glacier, results
are compared with the concurrent meteorological fields.

2. Field observation and analysis
2.1. Location
From 11 August 2003 to 17 August 2003, we collected
atmospheric samples in the Qilianshan Mountains.
Location from a large-scale point of view will be shown inFig. 3. Two stations were chosen: 1) near the terminal ofQiyi glacier at an automatic weather station (AWS),hereafter Qiyi (39.15.22N, 97.45.14E, 4250 m), and 2) atbase camp, hereafter BC (39.16.35N, 97.42.50E, 3672 m).
These two sites are separated by about 4 km.

2.2 Meteorological events and focuses
At the terminal of the Qiyi glacier (4250-m a.s.l.), an
automatic weather station (AWS) has been in operation
since August 2002. Figure 1 shows the time series of a)temperature at 2 m, b) relative humidity at 2 m (Rh), c)
wind speed at 10m (WS), and d) wind direction at 10m(WD), and e) short wave radiation (QSD) from 11 August
2002 to 17 August 2003. During the period, snowfall
occurred from 13:00 LT to 13:30 LT on 14 August at Qiyi,
although the amount was very small and was not recordedat the AWS. During the show event, we had relatively highRh and less QSD as shown in Fig.1. A sample of snow
water was collected in addition to air samples. After thesnow event, a strong northerly wind was observed from 16
LT to 18 LT at both Qiyi and BC (Fig.1).
We observed hail at BC for 10 minutes at about 16:30
LT, but we could not collect a sample because the amountof hail was very small and potential hail samples
evaporated fast. In the late evening of 14 August and thenext day, we had a very dry atmosphere and cloudless sky(Fig.1).
Mountain-valley winds commonly occur on mountainslopes. Valley of both Qiyi and BC aligns in north-south
direction. Therefore, prevailed wind is, roughly speaking,either north (0 or 360 degree) or south (180 degree) as
shown in Fig.1. The south of the stations is Tibetan Plateau,
while that of north is Gobi desert. A relatively strong northwind from the valley occurred from near noon to near
midnight, and then a relatively weak south wind from themountain occurred until just before noon of the next day.

2.3 Observational method
Figure 2 shows the apparatus for collecting
atmospheric samples at Qiyi. The sampling altitude isapproximately 1-m and 2-m above the ground at BC andQiyi, respectively. Air samples were pumped into theTedlar Bags for 30minutes to obtain mean values for theperiod. At the same time, the air temperature, relativehumidity, and atmospheric pressure were measured, andthen we used the measured values to estimate the specifichumidity. This estimate was then used to determine thebottle size to bring the air back to Japan for the isotope
measurements. We need between 4 and 8μL of water in asample to analyze the δ18O andδD content. Evacuatedglass bottles of sizes 0.6, 1, 1.2, and 1.5L were brought tothe field. A sample includes either one or more than twobottles, depending on the specific humidity and the need tohave a certain amount of water.

Fig. 2: Apparatus for collecting air at Qiyi
station. The view is to the north. Only this time,noontime of 15th August, we used two bags/tubes.

Fig 1: Time series of temperature, relative humidity,
wind speed and direction, and short wave radiationfrom 11 to 17 August, 2003 at Qiyi (the terminal of theJuly-1st Glacier). Dots represent 30 minutes average.
Numbers on the abscissa show day in August 2003,
and the time at the center of the day is 12 (noon)
local time.

2.4 Isotope analysis of atmospheric water vapor
Samples were brought to Center for EcologucalResearch, Kyoto University, and then water vapor wasextracted with liquid nitrogen in vacuum. Extracted water
was equilibrated in a 6mm glass tube with CO2, which wasmeasured for oxygen isotope ratio. Water was then reducedwith pre-treated Zn shot. Produced H2 was measured forhydrogen isotope ratio. Isotope analysis was made with
MAT252 (Thermoquest, USA, manufactured in Germany).
Details on sampling and analysis will be described inelsewhere (Sugimoto et al., in prep).

3. Results

18

Figure 3 showsδ O, δD, and d-excess from 11
August to 17 August. Measurements at BC and Qiyi aremarked with circles and triangles, respectively. As
mentioned in section 2.1, the distance between the twostations is only 4 km even thought the altitude difference is578 m. Some samples were rejected either because of an
accident or because the sample had less than 4μL of water.
For the latter samples, we plot only theδD values.
During the observation period, I tried to collectatmospheric water vapor samples in the morning andevening at BC every day. In addition, to understand thediurnal change of the isotopic composition, I madeobservations every three hours from 13 to 14 August. A
trial of collecting mountain wind was made at aboutnoontime on 15 August. Two aluminum bags, wereattached to the fence of the AWS (Fig. 2),then one bagcollected the atmosphere when a southerly (mountain)
wind was observed and another collected when a northerly(valley) wind was observed.
3.1 Snow and dry air event
As written in section 2.2, snowfall occurred in theafternoon of 14 August. The δ18O and δD values of thesnow were –4.97‰ and –29.4‰, respectively. They are
not shown in Fig. 3 because they are much higher thanthose of the water vapor. The d-excess (10.4‰) of thesnow is marked as a cross in the bottom panel of Fig. 3.
Comparison of the d-excesses of the vapor and snow fromthe same time indicates that the snow is from the same
source as the water vapor.
The dotted vertical line in the afternoon of 14 Augustof Fig. 3 shows the time when a strong northerly wind
started blowing. This wind may explain why the evening of
the 14th has a lower δD and higher d-excess comparedwith those of the previous period. A sample was takenfrom 19:00 to 19:30 of 14 August, but the water amount
was not enough to obtain a reliable δ18O value. For thisreason, we do not show the δ18O value in the figure;however, the estimated δ18O value was low (–27.1‰)
and d-excess was high (67.8‰). This is consistent with amore reliable sample obtained on 15 August.
As the time series of the meteorological parameters
derived from the AWS shown in Fig. 1, after the snowfallin the late evening of 14th, abnormally dry periods occurred
when there was a strong northerly wind. Since the snowfall
and strong wind seem to pass of a cold front, we will see
large-scale atmospheric fields in the next sub-session.
3.2 Large-scale atmospheric fields
Figure 4 shows the atmospheric circulation field at
500 hPa from operational objective analysis data ofEuropean Centre for Medium-range Weather Forecasts(ECMWF). At 12 UTC (20 LT) of the 14th, there is a deep
trough to the north of BC, and very dry air to the west of
the trough is heading toward the stations. There is a clear
difference in Rh (shown by color in Fig.4) between the twoair masses, and we can identify that the troughaccompanied a cold front. Figure 5 shows a VIS image at
2 UTC (10 LT) of 15 August. Our site was covered with avery dry air at this time, and we can see cloud system thatbrought snow event on 14th to the east of the cold front.
Before the trough’s arrival, the stations receive
moisture from the south. The snow event was dynamicallyinduced by westerly trough, and moisture source isconsidered to be air mass, which evaporated from theTibetan Plateau.
A trajectory analysis using the ECMWF data revealed
that the dry air came from lowermost stratosphere over thewest Siberia (not shown). SinceδD is around –170‰,
the air mass with a very low δD and high d-excess is
considered to be mixed with the air of the mid-troposphere
before we observed.

Fig 3: Isotopic ratios of the collected water vapor.
Upper panel: ratio of the oxygen isotope (δ18O (‰)).
Middle panel: ratio of deuterium δD (‰). Lower
panel: d-excess ( ‰ ). Open circles mark samples
collected at BC during valley winds, closed circles
mark samples collected at BC during mountain winds,open triangles mark samples collected at Qiyi during
valley winds, and closed triangles mark samples
collected at Qiyi during mountain winds. The cross in
the lower panel on 14 August marks the d-excess ofthe observed snow at Qiyi. Long tick marks on thehorizontal axis mean 0 LT, whereas short tick marks
are 12 LT.

3.3 Diurnal variation
We now discuss the diurnal changes of isotopes fromnoon on the 11th to noon on the 14th, all in August (Fig. 3).
Based on the wind measurements (Fig.1), samples obtained
when there was primarily mountain wind (valley) areshown as closed (open) circles and triangles. The datashows a diurnal change of isotopic composition, with lower

Fig 4: Relative humidity Rh, horizontal wind V, andgeopotential height Z at 500 hPa at 12 UTC (20 LT) of
14 August.
and lowerδ18O and δD values and higher and higher
d-excess values as noon progresses to the late evening.
This time period corresponds to the time that a northerly
valley wind was observed. Roughly speaking, valleys ofboth Qiyi and BC align north-south, so that the mountainwind is weakly southerly and valley wind is strong
northerly (Fig. 1). Moisture from the southern mountains in
the Tibetan Plateau is from southerly winds and that fromthe desert is from northerly winds.
In general, the isotopic ratio of precipitation has alower value (lighter water) in the interior of a continent orin higher mountains. However, the relation may bedifferent if there is a large body of water nearby that has
much re-evaporation. In our case, since the surface of theeastern Tibetan Plateau is wet and strong re-evaporation
can occur, atmospheric moisture over the Tibetan Plateau
is thought to be heavier comparing to that over the arid
regions in the interior of the Eurasian continent.
Therefore, we speculate that this change was observed
because BC is located in a marginal or transition area of
the atmosphere, a region where the Tibetan Plateau air
mass mixes with air from the arid region to the north. TheTibetan Plateau has heavier δ 18O and δD and lower
d-excess whereas the arid region has lighter δ18O and δ
D and higher d-excess. The lighter atmosphere from thenorthern desert comes to BC and gradually mixes with theplateau’s atmosphere when the valley wind prevails.

3. Conclusion and remarks
The stable isotopic ratio of water vapor was measured
near July-1st Glacier from 11 to 17 August, 2003. The
difference of the isotopic ratio between the twostations (BC and Qiyi, 4km in distance) are small
comparing to those variation caused from diurnal
cycle and change of the air mass.
The largest change of isotopic ratio was found fromAugust 14th to August 15th with passing a cold front,
which showed lowerδ 18O (and δD) and higher

d-excess compared with those of the previous period.
The snow event on August 14th was dynamicallyinduced by westerly trough, and moisture source isconsidered to be air mass, which evaporated from theTibetan Plateau.
A trajectory analysis using ECMWF data suggeststhat the air mass observed after passing of a cold front
came from the lowermost stratosphere or upper
troposphere.
During August 11th to 13th, we found a diurnal change
in the isotopic composition. As the day progressedfrom noon to late evening, the values ofδ18O and δ
D became lighter and the values of d-excess becamehigher. As the air from the northern desert has
relatively lowδ18O and δD values and relatively
high d-excess values, the valley wind that prevails inafternoon through evening must be bringing air fromthe northern desert to our site.
This case study strongly suggests that analysis of theisotopic ratio of water vapor should be useful tounderstand the following issues;
Continental scale moisture transport,
Local circulation,
Ice core interpretation,
Stratosphere-Troposphere Exchange (STE) withmonsoon circulation around the Tibetan Plateau.

Fig 5: A visual image from GOES-9 at 2 UTC of 15
August, 2003. Shade represented in the bar shows
albedo. A blue cross indicates of the place of our basecamp (BC), which is 4km from the terminal of theJuly-1st Glacier.

References
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precipitation in the arid/semi-arid regions in China andMongolia: Their regionality and relation to the Asiansummer monsoon. J. Meteor. Soc. Japan, 73, 909-923,
1995.