Surface energy balance of Keqicar Glacier, Tianshan Mountains, China, during ablation period

Jing Li , ShiYin Liu, Yong Zhang, DongHui Shangguan

State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences, Lanzhou, Gansu 730000, China

Surface energy balance of Keqicar Glacier, Tianshan Mountains, China, during ablation period

Jing Li*, ShiYin Liu, Yong Zhang, DongHui Shangguan

State Key Laboratory of Cryospheric Sciences, Cold and Arid Regions Environmental and Engineering Research Institute,Chinese Academy of Sciences, Lanzhou, Gansu 730000, China

The meteorological data of ablation season in 2005 were recorded by two automatic weather stations on Keqicar Glacier, in the southwest Tianshan Mountains of China. One is operated on the glacier near the equilibrium line with an altitude of 4,265 m (Site A) and another is operated on the glacier ablation area with an altitude of 3,700 m (Site B). These data were used to analyze the meteorological conditions and the surface energy balance (SEB) of Keqicar Glacier. Net radiation was directly measured, and turbulent heat fluxes were calculated using the bulk aerodynamic approach, including stability correction. The ablation value of 0.68 m w.e. derived from four ablation stakes is in close correspondence to the modeled value of 0.71 m w.e. During the observation period, net radiation accounts for 81.4% of the total energy with its value of 63.3 W/m2. The rest energy source is provided by the sensible heat flux with a value of 14.4 W/m2. Energy is consumed mainly by melting and evaporation, accounting for 69.5% and 29.7% of the total energy with their values of 54.0 and 23.0 W/m2, respectively. Radiative energy dominates energy exchanges at the glacier-atmosphere interface, governed by the variation in net shortwave radiation. Net short-wave radiation varies significantly due to the effects of cloudiness and the high albedo caused by solid precipitation. Wind speed influences the turbulent heat fluxes distinctively and sensible heat flux and latent heat flux are much larger in July with high wind speed.

Keqicar Glacier; meteorology; surface energy balance; net radiation; ablation

1. Introduction

Under the global warming background, glaciers in China suffer intensive ablation and more than 80% of the glaciers are in a state of retreating (Liuet al., 2006a), but there are also some advancing glaciers in the Tarim Basin, southwestern Tianshan Mountain of China (Liuet al., 2006b).Studies of glacier surface energy balance have improved our understanding of the relationship between glaciers and climate. Also, recent glacier melt models are being run through the use of the energy balance method (Hock and Holmgren,2005). Detailed energy balance studies were carried out in the early 1950s (e.g., Hoinkes, 1953). The energy balance of the world’s glaciers was already conducted in Sierra Nevada(Marks and Dozier, 1992), Antarctica (Bintanjaet al., 1997),the Alps (Klok and Oerlemans, 2002; Oerlemans and Klok,2002), and in the tropics (Wagonet al., 1999, 2001, 2003;Favieret al., 2004; Molg and Hardy, 2004). Based on energy balance calculations, distributed glacier energy-balance models have been developed and most models perform well when simulating the timing and magnitude of glacier meltwater and runoff (e.g., Brunet al., 1989; Arnoldet al., 1996;Hock and Noetzli, 1997; Brock and Arnold, 2000;Escher-Vetter, 2000). Energy balance studies on glaciers in western China have been performed since the late 1960s(e.g., Bai and Xie, 1965; Zeng and Kou, 1975; Bai and Zhang,1980; Kouet al., 1985; Kang and Ohmura, 1994; Xie, 1994;Zhanget al., 1996). However, most of these studies were restricted to a short period and very few glaciers (e.g., Urumuqi No.1 Glacier, Xiao Dongkemadi Glacier).

Considering, a new glacier-climate experiment was conducted on Keqicar Glacier in northwestern China. Keqicar Glacier is located on the south slope of Tuomuer summit.Tuomuer summit is the highest summit in the Tianshan Mountains where numerous large glaciers develop. Keqicar Glacier is of great concern in this study because (1) it is one of the largest glaciers in the Tianshan Mountains of China, and its meltwater is the main source of fresh water for daily life of the local people and wildlife; (2) the climate of this region has shown an apparent transition from warm-dry to warm-wet since the late 1980s (Shiet al., 2003). This study focused on the process of the glacier surface energy balance. First, the glacier’s microclimate was analyzed. Second, an analysis of the components of surface energy flux is given and the change characteristic of each component was analyzed.

2. Location and measurement program

Keqicar Glacier is located in the Tuomuer region,southwest Tianshan Mountains, on the south slope of Tuomuer summit which is the highest summit in the Tianshan Mountains (Figure 1).

Figure 1 The observation network system of Keqicar Glacier in the Tianshan Mountains, northwestern China.

Keqicar Glacier is a typical continental glacier with a length of 26 km, ranging in altitude from 3,020 to 6,342 m a.s.l. The area of the glacier is 83.6 km2, and the ablation area is approximately 30.6 km2. There are three typical surface types over the glacier: firn, ice and rock debris. Most of the ablation area is covered by a deep debris layer (Figure 1).The thickness of the debris layer is up to 2.5 m and varies from 1 to 250 cm. Two Automatic Weather Stations (AWS)from Campell Corporation were operated on the glacier(Figure 1). One is situated at Site A in the ablation area at 4,265 m a.s.l. Air temperature and relative humidity (MP101A TP&RH), wind speed and direction (05103 Wind Monitor)are measured at 1.0 and 2.0 m above the surface. Net all-wave radiation (NR LITE) is measured at 0.8 m and incoming short-wave radiation and outgoing short-wave radiation (CM3) is measured at 1.0 m. Ice temperature(111 TEMP PROBE) is measured at depths of 0.2, 0.4, 0.6 and 0.8 m. Data were sampled every 10 seconds, and data were stored as hourly averages. Another AWS is situated at Site B in the ablation area at 3,727 m a.s.l. (Figure 1).The measurement items and the data record mode of this station are the same as the AWS at Site A. These two AWSs were visited regularly for maintenance at two weeks intervals depending on weather conditions. Because of the harsh environment, some AWS data were missing.We acquired AWS data during the period of June 16 to July 5 and August 7 to September 7 at Site A and June 16 to July 5 at Site B in 2005.

There is also a manual meteorological observation field station situated near the terminus of the glacier where wind speed and direction, air temperature, relative humidity and precipitation are recorded.

3. Meteorological characteristics

Keqicar Glacier is situated on the south slope of the Tuomuer region. The snowline of the Tuomuer region is between 3,900 and 4,500 m a.s.l. (Lanzhou Institute of Glaciology and Geocryology, 1987). The average temperature is between -7 and -11 °C and precipitation is 750–1,000 mm close to the snowline. The climate of this region is mainly affected by westerly currents coming from the Atlantic Ocean and the Arctic Ocean and more than 70%of the yearly total precipitation falls from May to September in this region (Lanzhou Institute of Glaciology and Geocryology, 1987).

Daily mean relative humidity, air temperature and wind speed fluctuant for both sites during the observation period(Figure 2). The range of daily mean air temperature is between -2.9 and 6.0 °C at Site A and between 1.7 and 7.4°C at Site B. The air temperature always exceeded 0 °C for both sites, which means that the air always offer the energy for ablation as the snow or ice surface temperature never exceeded 0 °C. Daily mean relative humidity varies between 20% and 90%, and the relative humidity is always higher at Site B than at Site A when compared at the same time. Relative humidity and air temperature shows the same trend for both sites, but the variation of wind speed is not coincident in the two sites. The highest daily mean wind speed of both sites is near 4 m/s. One cold spell appeared between August 27 and September 1 (Figure 2). In this spell, the relative humidity evidently did not rise and wind speed became high, which means that this westerly current was cold and dry.

Figure 2 Daily mean relative humidity (a), air temperature (b) and wind speed (c) at sites A and B. The period for Site A from June 16 to July 1 and August 7 to September 7 in 2005, the period for Site B from June 17 to July 5 in 2005.

The average air temperature gradient is 0.5 K per 100 m altitude. Considering the short available compared data, this gradient is relatively small. In fact, the air temperature gradient of the glacier is smaller compared with other glaciers in China (Zhanget al., 2004).

The daily process of air temperature and wind speed for both sites appears very different as their controlling factors are different. Site A is mainly affected by the westerly and the calculation shows that the ratio of west and south-west wind direction is more than 50% for this observation period.Air temperature shows a clear daily cycle with one high value around 16 (Figure 3a). Wind speed evidently does not vary for the entire day (Figure 3b). Site B is situated on the ablation area, most of which is covered by a debris layer and surrounded by nearby mountains. Site B is controlled by a well developed local circumfluence. Air temperature changes very slowly in the morning and remains at a high value in the afternoon and night (Figure 3a). The valley and mountain wind is well developed; the valley wind reaches its highest value at 6:00 in the morning and the mountain wind reaches its highest value at 18:00 at night(Figure 3b).

Figure 3 Mean daily fluctuation in air temperature (a) and wind speed (b) at sites A and B. The period for Site A from June 16 to July 1 and August 7 to September 7 in 2005, the period for Site B from June 17 to July 5 in 2005.

4. Surface energy balance

For a melting glacier surface, the surface energy balance can be written as:

whereQmis the heat for melting,Ris the net all-wave radiation,His the turbulent sensible heat flux (in W/m2),LEis the turbulent latent heat flux (in W/m2),Qpis the heat advection by precipitation andGis the conductive heat flux in the snow/ice. During the ablation period, the glacier surface can be considered isothermal soGis very small. The heat advection by precipitation is insignificant compared to the other terms (Wagonet al., 1999). Considering the small value ofGandQpcompared to other energy components, they are disregarded in this study. Consequently, the energy balance of the glacier surface is described by the sum of the radiative component and the turbulent heat fluxes.

4.1. Net radiation

The net radiation is the balance of the incident and reflected short-wave and the incoming and outgoing long-wave radiations. In this study, the net all-wave radiation, the incident short-wave and the reflected short-wave were directly measured, but the incoming and outgoing long-wave radiations are lacking.

4.2. Turbulent heat fluxes

For estimation of the turbulent heat fluxes, the bulk aerodynamic approach was adopted. This approach derives bulk transfer coefficients from empirical air temperature,wind speed and humidity measurements at one or two heights above the glacier surface and from estimated flux-gradient relationships. During both monitoring periods,surface temperatures were typically at or near melting point which nearly required further assumptions (amplified by Moore, 1983) to be accepted, including the validity of the equations for neutral (stable) conditions and similarity of exchange coefficients for water vapor, heat and momentum.Specific humidity was estimated from relative humidity measurements for the Bottom, Middle and Top AWSs (or from dry- and wet-bulb temperatures for the East and West AWSs) and atmospheric pressure measurements (Moore,1983). Surface roughness was estimated from the logarithmic wind profile between the glacier surface, or lower sensor level, and the upper sensor level (Hay and Fitzharris, 1988)but also directly using the micro-topographic approach(Munro, 1989). Monitored net radiation and the turbulent heat flux estimates were summed to give the energy available for melt (Qm).

If the turbulent heat fluxes are not measured directly by complex eddy correlation systems, they are preferably estimated from methods that are based on the Monin-Obukhov similarity theory. In this paper, the turbulent heat fluxes were calculated using the bulk aerodynamic approach, including stability correction. This method is usually used for practical purposes because it approximates the turbulent heat fluxes from one level of measurement (Arck and Scherer, 2002). In this method, a constant gradient is assumed between the level of measurement and surface and thus surface values have to be evaluated. The stability of the surface layer is described by the bulk Richardson numberRiB, which relates the relative effects of boundary to mechanical forces (Brustaert, 1982;Moore, 1983; Oke, 1987):

whereTanduare the mean values of air temperature (in K)and horizontal wind speed (in m/s), respectively, at the level of measurementZ;gis the acceleration of gravity (g=9.81 m/s2);Tsis the surface temperature (in K). Since a lack of outgoing long-wave radiation, and air temperature exceeded 0 °C throughout most of the day during the observation period, it is assumed thatTsis equal to melting point for calculating.Z0mis the surface roughness length for momentum (in m). By definition,Z0mis the fictitious height where, taking into account a semilogarithmic profile,the horizontal component of the wind speed is zero,U(Z0m)=0.RiBis positive in a stable atmosphere. Assuming that local gradients of mean horizontal wind speedu, mean temperatureTand mean specific humidityqare equal to the finite differences between the measurement level and the surface, it is possible to give analytical expressions for the turbulent heat fluxes (Oke, 1987):

whereqsis the mean specific humidity at the surface (in g/kg);ρis the air density at a certain altitude and it can be calculated;Cpis the specific heat capacity for air at constant pressure,Cp=Cpd(1+0.84q) withCpd=1,005 J/(kg·K),the specific heat capacity for dry air at constant pressure;Lis the latent heat of vaporization of snow or ice(L=2.514×106J/kg), andKis the von Karman constant(K=0.4);Z0m,Z0tandZ0qare the surface roughness lengths for momentum, temperature and humidity, respectively. It is assumed that the horizontal component of the wind speed is zero atZ0m, it is also assumed that air temperature is equal to that of snow/ice surface atZ0t, and that the air is saturated with respect to snow/ice surface temperature atZ0q. This last assumption is used to calculate the surface specific humidityqs. The nondimensional stability functions for momentum(m), heat (h) and moisture (v) can be expressed in terms ofRiB:

In this study,Z0m=Z0t=Z0q=Z0is assumed and the roughness lengthsZ0can be determined by profile measurements and the assumption of "near-neutral" profiles. The equation is as follows (Louis, 1979; Oke, 1987):

whereu1andu2are the wind speed at 1 and 2 m, respectively (in m/s),Z1andZ2represent the different height, respectively.

5. Results

5.1. Comparison between calculated and measured ablation

There are four ablation stakes near the automatic weather station at Site B. The average specific mass balance value from these four ablation stakes is 0.60 m water equivalent (w.e.) between June 16 and July 6 assuming ice density of 900 kg/m3, compared with modeled ablation value of 0.62 m w.e. between June 17 and July 4. For meaningful comparison, the specific mass balance data needed to be adjusted to the ablation data, and the comparable period should be the same. The missing data of June 16, July 5 and July 6 was obtained from the days with most similar weather in the same month by analyzing the data from the meteorological observation field station near the glacier terminus. As a result, the modeled ablation value is 0.71 m w.e. between June 16 and July 6 around Site B. The accurate ablation value should be adjusted by deriving the snow precipitation from the total precipitation.In this study, snow and rainfall are differentiated according to a temperature divider of 1.5 °C. Finally, the modeled ablation value is 0.71 m w.e., and the measured ablation value is 0.68 m w.e. with a smaller value of 0.03 m w.e.during the same comparable period. Considering the large spatial variation of glacier surface condition and the uncertainty of turbulent heat fluxes calculation, this result is adequate and acceptable.

5.2. Daily value of the SEB terms

All energy fluxes presented in this study are defined as positive when directed towards the surface. The daily mean net radiation, turbulent sensible and latent heat fluxes of the two sites are presented in Figures 4 and 5, respectively. The ablation heat is the sum of these energy balance terms.

Figure 4 Daily means of the net radiation (R), sensible heat flux (H) and latent heat flux (LE) during the period of June 16 to July 1 and August 7 to September 7 in 2005.

The daily variability of the energy fluxes is large during the observation period (Figures 4 and 5). Daily net radiation is always positive and it is the main energy source for ablation. The net radiation is mainly controlled by net short-wave radiation which is greatly affected by cloudiness and surface albedo. Take Site A for example, the daily net radiation of a very clear day (July 26) can reach 201 W/m2,but on a cloudy day with new fallen snow (August 17), the net radiation appears negative and the value is -47 W/m2.Although the incoming long-wave radiation increases with cloudiness, the net long-wave radiation is usually negative and cannot distinctly change the trend of daily net radiation.Sensible heat flux is usually positive which is the secondary energy source for ablation. Latent heat flux is usually negative which means that the surface loses mass through sublimation. The wind speed directly influences the turbulent heat fluxes. The absolute value of sensible heat flux and latent heat flux are relatively higher for July with high wind speed compared with August with low wind speed at Site A(Figure 4). In conclusion, the radiative energy dominates energy exchanges at the glacier-atmosphere interface, governed by the variation in net shortwave radiation.

Figure 5 Daily means of the net radiation (R), sensible heat flux (H) and latent heat flux (LE) during the period of June 17 to July 5 in 2005.

5.3. Daily cycle of the SEB terms

5.3.1 Short-wave radiation

The mean diurnal cycle of main SEB terms of both sites are shown in Figures 6 and 7, respectively, during the observation period. For both sites, due to obstruction of the surrounding hill, distinct record of incident short-wave radiation appears approximately at 8:00 LT, which is about 2 hours after sunrise. Incident short-wave radiation mainly follows the cycle of STOA, but cloudiness significantly affects the incident short-wave, especially for Site B,which causes it to nearly stop at 13:00 LT. The surface of Site A is covered by snow, while the ice surface is exposed at Site B. Due to surface differences, the reflected short-wave radiation is much higher at Site A than at Site B, which caused a relatively small value of net radiation at Site A. The incident and reflected short-wave of both sites reach their maximum value at 14:00 LT and disappears at about 22:00 LT. The reflected short-radiation shows a similar cycle with the incident short-wave radiation, which means that the change of albedo is regular and mainly controlled by the altitude angle of the sun.

5.3.2 Net radiation

Net radiation shows a regular daily cycle for both sites,but the effect of cloudiness on net radiation is more distinct for Site B according to a little roughness of its daily cycle(Figure 7). Net radiation is negative and basically remains constant with a value of -50 W/m2which equals to the net long-wave radiation at night. During the day, net radiation is mainly governed by short-wave radiation balance and reaches its maximum value at 16:00 LT, which is about 1 hour slower compared with short-wave radiation. Because of the relative high value of net short-wave radiation, the maximum of net radiation is significantly higher at Site B(Figure 7) than at Site A (Figure 6).

5.3.3 Turbulent heat fluxes

HandLEmaintained a similar daily trend.His always positive and reaches its maximum at around 15:00 LT.LEis negative the whole day and appears at a high value during the night. At night, as the air is slowly cooled by the colder surface, the small temperature gradient results in a relatively smallH, while the westerly flows down the glacier’s slopes,carrying little humidity causing the surface to sublimate intensively. Consequently,HandLEare opposite and of more or less equal amplitude and the turbulent heat fluxes were balanced at night. This condition is more obvious for Site B(Figure 6). During the day, the surface temperature ceases to rise after reaching the melting temperature of snow/ice, and the high wind speed and large temperature gradient causes the large value ofH. At the same time, due to the saturation of air vapor, the evaporation or sublimation tends toward zero.

Figure 6 Mean daily cycle in incident (Sin) and reflected short-wave radiation (Sr), net radiation (R) and sensible heat flux (H) and latent heat flux (LE). The daily fluctuations are averages over the period of June 16 to July 1 and August 7 to September 7 in 2005.

Figure 7 Mean daily cycle in incident (Sin) and reflecte short-wave radiation (Sr), net radiation (R) and sensible heat flux (H) and latent heat flux (LE). The daily fluctuations are averages over the period of June 17 to July 5 in 2005.

6. Summary and conclusion

The purpose of this study was to characterize the SEB of a large continental glacier located in the southwest Tianshan Mountains of China during the ablation period. From the close correspondence between modeled and measured ablation value, it is clear that our instrument and the bulk method are suitable for turbulent heat fluxes calculation during the ablation period. It is also reliable enough to provide robust conclusions regarding the surface energy balance. Considering the limited data of Site B, and Site A is much closer to the equilibrium line, the surface energy balance of Site A is much more representative of the glacier. Consequently, the following conclusions about the surface energy balance composition of Keqicar Glacier are based on the data of Site A.

The relative importance of the energy fluxes for the ablation period in 2005 can be inferred from Figure 4. The sum of the positive fluxes is scaled to 100%. Daily net radiation is always positive and net radiation is the main energy source for ablation, accounting for 81.4% of the total energy source with its value of 63.3 W/m2over the observation period. Sensible heat flux is the secondary energy source for ablation, which offers the leftover energy source with its value of 14.4 W/m2. Latent heat flux is usually negative which means that the surface loses mass through sublimation, consuming 30% of total acquired energy. The variabil-ity of each energy flux is distinct during the observation period. Net short-wave radiation varies significantly due to the effect of cloudiness and the high albedo caused by fallen snow. The wind speed influences the turbulent heat fluxes distinctively and sensible and latent heat flux is much larger on July with high wind speed. In conclusion, the radiative energy dominates energy exchanges at the glacier-atmosphere interface, governed by the variation in net short-wave radiation during the ablation period.

These findings are of interest for modeling the spatial distribution of the energy and meltwater for Keqicar Glacier and need further investigation. To date, incoming radiation in mountain terrain is relatively accurate but the modeling of turbulent fluxes and spatial and temporal variability in albedo remains uncertain. Further studies should focus on the spatial variation in air temperature and wind speed over the glacier and the relationship between such air temperature and wind speed and those outside the glacier for a distributed energy-balance melt model.

This work was supported by the Knowledge-Innovation project (No. KZCX2-YW-GJ04) and the project of National Natural Science Foundation of China (Grant No. 41071010,40501007), and the China International Science and Technology Cooperation Program (Grant No. 2008DFA20400).The authors would like to thank all members of the field exploring team of Keqikar glacier and the referees.

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10.3724/SP.J.1226.2011.00197

*Correspondence to: Dr. Jing Li, PhD Candidate of Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences. No.320, West Donggang Road, Lanzhou, Gansu 730000, China. Tel: 13919275343;Email: Jingli@lzb.ac.cn

20 October 2010 Accepted: 3 January 2011