71

Bulletin of Glaciological Research ,. (,**1) 1+ῌ12
ῌJapanese Society of Snow and Ice

Meteorological observations during ,**,ῌ,**. in Lunana region, Bhutan Himalayas
Ryohei SUZUKI+, Koji FUJITA+, Yutaka AGETA+, Nozomu NAITO,, Yoshihiro MATSUDA+
and KARMA+ Graduate School of Environmental Studies, Nagoya University, c/o HyARC, Chikusa-ku, Nagoya .0.ῌ20*+, Japan, Department of Global Environment Studies, Hiroshima Institute of Technology, Hiroshima 1-+ῌ/+3-, Japan- Geological Survey of Bhutan, P.O. Box +1-, Thimphu, Bhutan(Received September -*, ,**0; Revised manuscript accepted November +-, ,**0)

Abstract
Meteorological observations were carried out in the Lunana region, Bhutan Himalayas, from
September ,**, to October ,**.. An automatic weather station (Lugge AWS) was installed at ./,. ma.s.l. on the end moraine of Lugge Glacier. In addition, air temperatures, ground temperatures andrelative humidity were observed simultaneously around the AWS from late September to early
October in ,**- and ,**. to investigate spatial variation of these components. Characteristics of
seasonal and diurnal variations of meteorological elements are shown using the data observed by the
AWS. Relatively large spatial variability of surface temperature is shown by the data observed at
di#erent sites compared to that of air temperature and relative humidity.

+.

Introduction

Supraglacial lakes, which expand in the terminiof debris-covered glaciers due to the glacier shrinkage, have a possibility of Glacial Lake Outburst Floods (GLOFs) that are caused by the collapse of moraines. Such possibilities in northern Bhutan havebeen pointed out in previous studies (e.g., Ageta et al.,,***; Richardson and Reynolds, ,***; Iwata et al., ,**,;
Komori et al., ,**.). However, no study has been
published on meteorological conditions in northern
Bhutan.
Surface temperature as well as atmospheric condition is also basic information on debris-covered glaciers. On a debris-covered glacier, the ice melt rate
under a debris layer varies with thickness and thermal properties of the debris (e.g., Fujii, +311; Nakawo
and Rana, +333; Mattson et al., +33-; Benn and Lehmkuhl, ,***). Generally, the melt rate increases withthickness of debris, having a maximum at several
centimeters thickness, and then decreasing towards a
thickness at which the melt rate equals the rate for abare ice surface without debris. The melt rate decreases further with a further increase in thickness.
However, the range of such variation also depends on
the thermal property of debris, which is not uniform
among glaciers. Thus, surface temperature is a key
element for mass balance studies through investigation of thickness and thermal properties of debris

layers (Nakawo and Young, +32+; +32,).
Meteorological observations were carried out inthe Lunana region of Bhutan under the Japan-Bhutan
joint research project from ,**, to ,**.. The outlineof this project and activities has been reported in
Yamada et al. (,**.). Climatic conditions around debris-free glaciers in the Bhutan Himalayas were also
investigated under the same project, and the results
have been reported in Naito et al. (,**0). The present
paper aims to report the meteorological conditions
around debris-covered glaciers in the Lunana regionof Bhutan.

,.

Locations and methods

An automatic weather station (AWS) was installed at ./,. m a.s.l. on the end moraine of Lugge
Glacier in the Lunana region of Bhutan. The AWS
started to record 3 meteorological elements on +1 September in ,**, (Yamada et al., ,**.). The observed
meteorological elements, heights of each sensor andobservation periods are shown in Table +. Air temperature was measured with a natural-draft type radiation shield. A platinum resistance thermometer
was used for measuring ground surface temperatures.
Sand with a thickness of about + cm covered the sensor for insulating solar radiation. Ground surface
temperature was recorded at +-hour intervals, and theother meteorological data were recorded at -*-minuteintervals. Locations of each measurement are shown

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Bulletin of Glaciological Research

Table +.

Measurements at Lugge automatic weather station.

Fig. +. Locations of meteorological observation sites
in a satellite image of Lunana region, Bhutantaken by ASTER band -N on ,* January, ,**+.
Contour lines drawn every /** m are based on
ASTER-DEM. The position of Lugge automaticweather station is shown as AWS. Other observation points are also shown in this image using
symbols described in Table ,.

Fig. ,. Elevation angle of mountain skyline at LuggeAWS obtained from digital elevation model of
ASTER products. Trajectories of sun on summer
and winter solstices are shown with two thin lines.

in Fig. +. Figure , shows surrounding topography at
the AWS, which was obtained from a digital elevation
model (DEM) provided as ASTER products. Trajectories of the sun on summer and winter solstices showthat direct solar radiation in the morning is shaded
about + hour both in summer and winter. The valley
where the AWS was located runs east and west, andmountains are located on the north and south. The
mean elevation angle is roughly +-῎, which corresponds to about ,,ῌ of the whole sky in solid angle.
Multipoint observations of air temperature andground surface temperature were carried out from ,3September to 2 October in ,**-. In addition to thesecomponents, relative humidity was measured from +October to ++ October in ,**.. Single vinyl chloride
pipes were used as radiation shields for air temperature measurements under natural draft conditions.
An infrared thermometer was used for measuring
surface temperature of debris on the Thorthormi Glacier (Fig. +). Thermistor sensors which were covered
with sand of + cm thickness were used for measuring
ground surface temperatures at the other sites. Total
precipitation was measured on the right-bank of
Lugge Glacier (LG) at .0.3 m a.s.l. and at a peak of /*.m a.s.l. whose positions are shown in Fig. +. Precipitation was measured from ,3 September ,**, through- October ,**-, and from + October ,**, through 2
October ,**-, respectively. Brightness temperature
images around glaciers were obtained using an infrared thermal imager (inframetrics MODEL 10*). The
accuracy of the temperature is ῍,῏ and the minimum detectable temperature di#erence is within*.,῏. Observed brightness temperature varies with
air temperature and amount of precipitable water
through a column between a target and the infrared
thermal imager. Thus, the accuracy as well as spatial
resolution depends on the distance to a given target.
The images were taken in the daytime on only ﬁnedays October +ῌ++ October in ,**. to avoid the abovementioned problem, because the air is relatively dry
and thin due to high altitudes. Descriptions of above
multipoint observations are shown in Table ,.

-.

Results and discussion-.+ Lugge AWS-.+.+ Temperatures
Figure -a shows the seasonal variation of dailymean air temperature and ground surface temperature. They show typical sinusoidal-shape annual
variations. The air temperatures ranged from ῌ+.῏
to 3῏. The annual mean air temperature was approximately *῏, and the mean air temperature fromthe beginning of June to the end of September was
0῏. From the end of May to the beginning of October, the daily mean air temperature was higher than*῏. As seen in the air temperature variations, dis-Suzuki et al.

73

Table ,. Description of observation points shown in Fig. +. Air temperature (Ta), ground surface temperature (Ts) andrelative humidity (Rh) were measured , m above ground. Di#erences in altitudes among AWS, TD and LO are lessthan /* m.

Fig. -. Daily mean air temperature, ground temperature (a) and relative humidity (b) at Lugge AWS
from +2 September, ,**, to ++ October, ,**..

tinct seasonal variations were also found in the ground surface temperature, which was above *῍ fromMarch to October and ranged from ῌ0῍ to +2῍.
Relative humidity
The daily mean relative humidity was higher andrather stable in summer, and lower and rather variable in winter (Fig. -b). The relative humidity ranged
from ,*ῌ to 3/ῌ throughout the year. The variability in relative humidity may be due to the frequency of precipitation. Mean relative humidityfrom June to September during the summer monsoon
season was 23ῌ, against 01ῌ in other months.
-.+.,

Precipitation
Figure . shows daily precipitation from +2 September in ,**, to +* October in ,**.. The precipitation increases in summer monsoon season and decreases in winter. Over 1*ῌ of the total precipitation
was observed from June to September in ,**-. The
amount of winter precipitation is uncertain due toundetected snowfall and freezing of the precipitation
gauge.
Total precipitation for each observed period is
also shown in Fig. .. Naito et al. (,**0) reported fromobservations in +332ῌ+333 (-.* days) that the annual-.+.-

Fig. .. Daily precipitation at Lugge AWS from +2
September to -+ December, ,**,, January to December in ,**- and + January to ++ October, ,**..
Totals through each observation period in eachyear are shown at upper left.

precipitation beside the Jichu Dramo Glacier, which islocated at /,./ m a.s.l. ,* km south of Lugge AWS, wasgreater than +-.3 mm. On the other hand, the annual
precipitation was 3** mm in this region (Fig. .b).
Since the observed periods were di#erent, it is di$cult
to explain these di#erences. However, these data imply spatial variability of precipitation.
Radiation
Daily mean downward shortwave radiation isshown in Fig. /a. Di#erences between the observedand calculated values at the top of the atmosphere
increase in summer due to cloud cover as suggested in
-.+..

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Bulletin of Glaciological Research

Fig. /. Daily mean downward shortwave radiation (a)and albedo (b) at Lugge AWS from +2 September,,**, to ++ October, ,**.. Daily mean downward
shortwave radiation at top of atmosphere (TOA) is
also shown by dashed line in a).
Fig. 1. Daily mean wind speeds (a) and histogram ofwind directions (b) at Lugge AWS from +2 September, ,**, to ++ October, ,**.. Total number ofwind direction data is -0,.*.

Fig. 0. Daily mean net radiation (a) at Lugge AWS
and calculated downward longwave radiation (b)using observed net radiation, shortwave radiationand surface temperature at each time interval.

Fig. ..
Albedo was calculated from the upward shortwave radiation divided by downward shortwave radiation (Table +). Abnormal values above +.*, whichwere considered to be due to snow accretion on thesensor, were eliminated from albedo. Figure /bshows variations in daily mean albedo from +2 September ,**, to +* October ,**.. Increase of albedo inwinter indicates the occurrence of snow cover at
Lugge AWS in winter. Snow cover disappearance
dates, when variation of ground temperatures weremild at around *῎, increases from October ,**, to thefollowing May. These data show that snowfall andmelting of snow covers often occurred at the AWS in
the winter.
Figure 0a shows daily mean values of net radiation. Downward radiation to the ground corresponds to a positive value. In winter, the net radiation is relatively small due to the increase of albedo(Fig. /b). The data show net radiation in spring isrelatively large independently of albedo. A similarseasonal variation to Fig. /a was observed.
Balance of shortwave radiation and upward longwave radiation were subtracted from the observed net
radiation to obtain downward longwave radiation.
The upward longwave radiation was calculated using
the ground surface temperature shown in Fig. -a, assuming that the ground can be regarded as a black
body. Figure 0b shows daily mean downward longwave radation from +2 September in ,**, to +* October in ,**.. The mean downward longwave radiation from June through September ,**- was ,3+ Wmῌ,, while the mean downward shortwave radiation
was ,*1 W mῌ, for the same period. The data showthat the downward longwave radiation decreases in
winter and increases in summer. The cause of suchvariation is considered to be the seasonal variation of
air temperature and cloud cover.
Wind speed and direction
Figure 1a shows daily mean wind speeds and Fig.
1b indicates the frequency distribution of wind directions calculated for every +2῍. Relatively large windspeeds were observed in winter, but no deﬁnite seasonality was observed. About 1*ῌ of all observed
data were concentrated in the east and west directions, which corresponds to the direction of the valley(Fig. ,). The data show that the air ﬂows up anddown along the valley. The diurnal cycle of windspeed in summer is presented in the following section.
-.+./

Diurnal variations
Figure 2 shows monthly mean diurnal variations-.+.0Suzuki et al.

Fig. 2. Monthly mean diurnal variations of downward
shortwave radiation (a), net radiation (b), air temperature (c), relative humidity (d) and wind speed(e) from June to September, ,**- and monthly
precipitations (f) for each local time at Lugge AWS
from June to September, ,**-. Dashed line in Fig. 2adenotes downward shortwave radiation at top of
atmosphere (TOA) averaged for June to September.

of downward shortwave radiation, net radiation, airtemperature, relative humidity and wind speed from
June to September in ,**-. The monthly precipitation for each local time in the same period is plotted inFig. 2f. All components show deﬁnite diurnal variations in association with solar insolation.
The observed shortwave radiation rapidly decreases in the afternoon compared to the value at the
top of the atmosphere (Fig. 2a). This is not due to thee#ect of shading by surrounding mountains but
rather to the attenuation of solar radiation by cloud
cover in the afternoon which is caused by local circulation (Ageta, +310), because the surrounding topography cannot shade the AWS from direct solar radiation
during most of the daytime hours (Fig. ,). Figure 2b
shows that the net radiation varies with the downward shortwave radiation, because the exchange ofshortwave radiation controls the diurnal variation in
net radiation (e.g., Mattson and Gardner, +33+).
The variations in air temperature and relative
humidity show an inverse relationship (Fig. 2c; Fig. 2d). Figure 2e shows that relatively warm and dry
wind increase in the daytime. The phases of windspeeds delay slightly compared to the other elements(Fig. 2e). Since the mountain and valley wind systemis dominant in this region as shown in Fig. 1, the delay
could be due to the time lag of heating between ground surface and atmosphere. However, no clear diurnal cycle of wind direction was recognized from Juneto September ,**-. Figure 2f shows increases of precipitation from afternoon to early morning due to the
local circulation. The same characteristic was also
reported in previous studies (e.g., Ageta, +310; Ueno etal., ,**+).
-.,

Distribution of brightness temperature

75

Surface temperature is one of the key physicalvariables to investigate thermal property of a debris
layer (Nakawo and Young, +32+; +32,). However, extensive measurement of surface temperature was notavailable on debris-covered glaciers due to the large
size and heterogeneity of such glaciers. Brightness
temperature, which can be obtained remotely using
airborne and spaceborne thermal sensors (e.g., Yasunari, +32*; Wessels et al., ,**,), also represents the energy amount on ground surfaces; while it is distinguished as a di#erent physical variable from surface
temperature (Kobayashi et al., ,**+).
Ground-based measurements of brightness temperature were carried out to investigate thermal environment around the glaciers in this region using athermal infrared camera. Figure 3a is a pseudo color
image of brightness temperature in the daytime on
the terminal area of Thorthormi Glacier taken fromthe left side moraine, and Fig. 3b is a photograph ofthe corresponding area. The brightness temperature
varies on the glacier in association with thickness andthermal properties of debris cover. The ice at thelower left of Fig. 3a was covered with thick debris,and the brightness temperature on the area was relatively high. Furthermore, the brightness temperature in this area ranged from *ῌ to +1ῌ due to thepresence of ice cli#s and warmed boulders of varied
sizes (Fig. 3c; Fig. 3d). The brightness temperature
image of Lugge and Thorthormi Glaciers shown in
Fig. +*a is taken from a point near /*.- m a.s.l. Figure +*b shows the view around the correspondingarea. Since the end moraine of Lugge Glacier is composed of relatively thick debris-covered ice as shownin Figs. +*c and +*d, the brightness temperature inthis area is higher than on Thorthormi Glacier. As
mentioned above, the thickness distribution of debris
cover a#ects the surface temperature and could resultin a complex ice melt pattern in this area (e.g., Rana etal., +331; Nakawo et al., +333).
Multipoint observations
Table - shows a comparison of the total precipitation observed at three points around the glaciers (Fig.
+, Table ,). The precipitation at /*.- m was roughly
nine- tenths of those at the other points. Ageta (+310)
reported characteristic distribution of precipitation in
the Nepal Himalayas due to the orographic e#ect on
precipitation. Three observation points in this region were relatively close to each other, and the di#erences in observed precipitations are smaller thanthose reported in Ageta (+310).
Daily ﬂuctuation of air temperature and groundsurface temperature was compared using the dataobtained at Lugge AWS and /*.- m a.s.l. in ,**-. No
time lags between the points were recognized in these
data. The observed air temperatures at the AWS
were -.,ῌ higher on average. The mean lapse rate of
-.-

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Bulletin of Glaciological Research

Fig. 3. a) Pseudo color image of brightness temperature on Thorthormi Glacier taken at 3:-* on / October, ,**.. b) Photo with corresponding view angle in Fig. 3a shown byviolet dashed line. Black dashed lines in Figs. 3a and 3b denote corresponding view
angle of close-up pictures shown in Figs. 3c and 3d, respectively.

Fig. +*. a) Pseudo color image of brightness temperature around end moraine of Lugge
Glacier (left) and Thorthormi Glacier (right) taken at +-:** on - October, ,**.. b) Photowith corresponding view angle of Fig. +*a shown by the violet dashed line. Photos c)and d) are ice cli#s at end moraine of Lugge Glacier.

Suzuki et al.

Table -.

77

Comparison of total precipitations.

Fig. ++. Ground surface temperature decrease at .
observation points (Fig. +, Table ,) from sunset ,October, ,**. until early the following morning.

air temperature in this period was 0.+῍ kmῌ+. On theother hand, the ground surface temperature at AWS
averaged only *.3῍ higher. Since di#erences inmean albedo at the two observation points were negligible, the energy absorbed through shortwave radiation, a dominant energy source of ground surfaces,
was not a cause of the di#erent surface temperatures.
Figure ++ shows the temperature decrease on groundsurface observed at . points (Fig. +) from sunset on ,October ,**. to early the following morning. The
decrease of temperature at Thorthormi Glacier (TD)and Lugge AWS (AWS) was nearly .῍ lower than at
the other , points. The data need to be analyzed
based on the heat balance method to identify thecause of this di#erence, but it implies a possibility ofthe presence of ice under the AWS because the amountsof decrease are considered to be due to di#erent thermal properties among the observation points (Kondo,,***).
The data obtained from the multipoint observations in ,**. (Table ,) about air temperature, groundsurface temperature and relative humidity were compared. A lapse rate of air temperature observed between Lugge AWS and /*.- m a.s.l. in this period
resulted in /..῍ kmῌ+, which is smaller than that obtained in ,**-. The cause of this di#erence is notclear, but the bad weather conditions in the beginningof October in ,**. may be related. The mean ofabove lapse rates is approximately 0῍ kmῌ+, which isthe same value reported by Naito et al. (,**0) around
Jichu Dramo Glacier in Bhutan.
The ground surface temperatures observed at .
points (Table ,, Fig. +) were a#ected by snow cover
during the observed period. The weather was fair in
the ﬁrst half and snowy in the last half of the period.
The maximum di#erence among the temperatures
observed at . points averaged 2῍. In the ﬁrst half,
the di#erences decreased in the daytime and increased in the nighttime. This characteristic was explained by the rapid temperature decrease at Thorthormi Glacier after sunset, and is considered to be dueto the relatively thin debris layer on ice. As shown in
Fig. ++, ground surface temperatures on debriscovered ice decreased rapidly and largely compared tothose on other surfaces. Similar results were obtained in previous studies, which observed higher air
temperature in the debris-covered area than in thedebris-free area due to the e#ect of heat storage ofdebris (e.g., Fujita and Sakai, ,***; Takeuchi et al.,,***).
The relative humidity observed at /*.- m a.s.l.
showed saturated condition during the observation
period. On the other hand, no clear di#erence in relative humidity was observed among the other points.
The data ranged from -0ῌ to 32ῌ, and it varied
inversely with air temperature and ground surface
temperature as shown in Fig. 2d.

..

Summary

Investigations of meteorological conditions
around debris-covered glaciers and glacial lakes in the
Lunana region of Bhutan were carried out from September, ,**, to October, ,**.. The characteristics ofvariations and average values of each meteorologicalelement were shown using the data observed by
Lugge AWS. The data observed at di#erent sites
indicated that spatial variability of surface temperatures was relatively large compared to the air temperature and relative humidity. These data will be
helpful for mass balance and glacial lake studies inthis region.

Acknowledgments
We would like to express thanks to everyone who
so generously cooperated in obtaining data from the78

Bulletin of Glaciological Researchﬁeld observations. In particular, we are grateful to
the Geological Survey of Bhutan and Dr. Akiko Sakaiof Nagoya University. The manuscript was much
improved by useful comments from Dr. Takeshi
Yamazaki and Dr. Keiko Konya. The copyright for
ASTER data is reserved by the Ministry of Economy,
Trade and Industry of the Japanese Government.
This study was supported by the Announcement ofResearch Opportunity (ASTER ARO, AP-***0) and a
Grant-in-Aid for Scientiﬁc Research (Project: +--1-**0;+-/1-**.; G-., the ,+st Century COE Program) from
the Ministry of Education, Culture, Sports, Scienceand Technology of Japan.

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