Tanguang Gao

Early onset of rainy season suppresses glacier melt: a case study on Zhadang glacier, Tibetan Plateau

Alpine glaciers have retreated dramatically during the last few decades as global warming has impacted the Tibetan Plateau (e.g. Shi and Liu, 2000; Yao and others, 2004; Sakai and others, 2006; Ye and others, 2006; Kang and others, 2007). Glacier shrinkage over the Tibetan Plateau is mostly caused by a continuous deficit in mass balance of glaciers (Yao and others, 2004; Ye and others, 2005; Pu and others, 2008). Both temperature and precipitation affect glacier mass balance, and much research has focused on the temperature effect on glacier change (e.g. Oerlemans and Fortuin, 1992; Liu and others, 1998; Ye and others, 2005; Sakai and others, 2006; Pu and others, 2008). Using a numerical approach, a recent study revealed that the effects of precipitation seasonality and its concentration on glacier mass balance are more important than those of the annual precipitation amount (Fujita, 2008a,b), especially for glaciers located in a summer-precipitation climate. Due to the difficulty of monitoring precipitation at high elevations, few studies have discussed the effect of precipitation seasonality on glacier mass balance using observed data in the Tibetan Plateau. We demonstrate the seasonal influence of precipitation on glacier mass balance for a Tibetan glacier that is mainly influenced by the Indian monsoon during the summer and westerlies during the winter (Bryson, 1986; Yanai and Wu, 2006). Zhadang glacier (30828.570 N, 90838.710 E; area 2.0 km, length 2.5 km) is located on the northeastern slope of Nyainqêntanglha mountain, southern Tibetan Plateau. Massbalance and glacier terminus variations have been recorded since September 2005, when the Nam Co Station for Multisphere Observation and Research (NAMOR: 30846.440 N, 90859.310 E; 4730ma.s.l.) was set up by the Institute of Tibetan Plateau Research, Chinese Academy of Sciences. The glacier faces north-northwest and spans an elevation range of 5515–6090ma.s.l. It is debris-free, with a fan-shaped terminus. Two automatic weather stations (AWSs) have operated, one at the pass (5800ma.s.l.) and the other in the terminal area (5400ma.s.l.) of the glacier, since September 2005. A rain gauge was set up beside the terminus AWS in May 2007, and a hydrological observation site has operated at 5400ma.s.l. since May 2006, close to the terminus. Since the meteorological data (e.g. air temperature and precipitation) do not cover the whole of 2005, we only report data for 2006–08. A large deficit mass balance occurred during the massbalance years 2005/06 and 2006/07 (Table 1; Fig. 1), but in 2007/08 there was a surplus mass balance and retreat of the glacier terminus slowed in 2008 (Table 1). Furthermore, variations in the glacier runoff in July and August 2008 were quite different from those in 2007 (Table 2; Fig. 1). Since the mass balance had been continuously in deficit for the previous decade (e.g. Yao and others, 2004; Ye and others, 2005; Pu and others, 2008), we investigated the reason for the mass-balance switch in 2007/08. Meteorological data from NAMOR (located 50 km from Zhadang glacier) show that around 90% of the precipitation is deposited between May and September (You and others, 2007). Daily temperatures above 08C commence in late May in the Zhadang glacier area as recorded by the AWSs (Fig. 1). Thus the accumulation and ablation seasons on the glacier overlap during the summer season (May–September). Mean air temperature during summer was 0.628C higher in 2007 than in 2008 (Table 1), while precipitation was 57.5mm lower in 2007 than in 2008. Monthly temperatures were lower in 2008 than in 2007 (Table 2). Monthly cumulative temperature above 08C, a parameter associated mostly with glacier melt (e.g. Liu and others, 1998; Ohmura, 2001), was less in 2008 than in 2007. We suggest that low summer temperature is a major cause of weak glacier melt in 2008 and the associated surplus mass balance (223mm) for the balance year 2007/08. However, when monthly and daily temperature and precipitation as shown in Table 2 and Figure 1 are investigated, differences in summer precipitation between 2007 and 2008 are clearly seen. Precipitation in May and June of 2008 was more than twice that of 2007 (Table 2). Roughly equal or less monthly precipitation occurred during the other months of 2008 compared with those in 2007. Similar monthly precipitation patterns appeared at NAMOR (Fig. 2), where precipitation in June 2008 was 8 times that in June 2007, and precipitation in July 2008 was 1.5 times that in July 2007. Large precipitation differences in the early stages of the ablation season (May and June) between 2007 and 2008 imply that precipitation might be an important factor for the surplus mass balance in 2008. Generally, the Indian monsoon onset is in mid-June (Chang and Chen, 1995; Wu and Zhang, 1998) and precipitation is concentrated in July and September in the southern Tibetan Plateau (Kang and others, 2000; You and others, 2007). For example, 85% of precipitation occurs between July and September at Bange and Dangxiong stations, which are about 50–100 km from the Nam Co basin. Precipitation amounts in June of 110.0mm in the glacier area (Table 2) and 87.3mm at NAMOR (Fig. 2) suggest that the rainy season onset was earlier by 1 month in 2008 than in 2007. The India Meteorological Department reported that the onset of the monsoon in 2008 was about a week ahead of its normal date in the Bay of Bengal, and the monsoon advanced relatively rapidly due to the interaction of monsoon circulation with the mid-latitude westerly system, resulting in higher monsoon rainfall in June (24% above the long-term averaged value) over all of India (India Meteorological Department, http://www.imd.ernet.in/ section/nhac/dynamic/endseasonreport.pdf). Thus an early Journal of Glaciology, Vol. 55, No. 192, 2009

Light‐absorbing impurities enhance glacier albedo reduction in the southeastern Tibetan plateau

Light-absorbing impurities (LAIs) in snow of the southeastern Tibetan Plateau (TP) and their climatic impacts are of interest not only because this region borders areas affected by the South Asian atmospheric brown clouds but also because the seasonal snow and glacier melt from this region form important headwaters of large rivers. In this study, we collected surface snow and snowpit samples from four glaciers in the southeastern TP in June 2015 to investigate the comprehensive observational data set of LAIs. Results showed that the LAI concentrations were much higher in the aged snow and granular ice than in the fresh snow and snowpits due to postdepositional processes. Impurity concentrations fluctuated across snowpits, with maximum LAI concentrations frequently occurring toward the bottom of snowpits. Based on the SNow ICe Aerosol Radiative model, the albedo simulation indicated that black carbon and dust account for approximately 20% of the albedo reduction relative to clean snow. The radiative forcing caused by black carbon and dust deposition on the glaciers were between 1.0-141 W m(-2) and 1.5-120 W m(-2), respectively. Black carbon (BC) played a larger role in albedo reduction and radiative forcing than dust in the study area, enhancing approximately 15% of glacier melt. Analysis based on the Fire INventory from NCAR indicated that nonbiomass-burning sources of BC played an important role in the total BC deposition, especially during the monsoon season. This study suggests that eliminating anthropogenic BC could mitigate glacier melt in the future of the southeastern TP. Plain Language Summary In this study, we focused on light-absorbing impurities (LAIs), including black carbon, organic carbon, and mineral dust in glacial surface snow from southeaster Tibetan glaciers. This study showed the concentrations of LAIs, and estimated their impact on albedo reduction. Furthermore, we discussed the potential source of impurities and their impact to the study area. These results provide scientific basis for regional mitigation efforts to reduce black carbon.

Chemical Records in Snowpits from High Altitude Glaciers in the Tibetan Plateau and Its Surroundings.

Glaciochemistry can provide important information about climatic change and environmental conditions, as well as for testing regional and global atmospheric trace transport models. In this study, δ18O and selected chemical constituents records in snowpits collected from eight glaciers in the Tibetan Plateau and adjacent areas have been investigated. Drawing on the integrated data, our study summarized the seasonal and spatial characteristics of snow chemistry, and their potential sources. Distinct seasonal patterns of δ18O values in snowpits indicated more negative in the south TP controlled by Indian monsoon, and less negative in the north TP and Tien Shan. Overall increasing concentrations of microparticles and crustal ions from south to north indicated a strength of dust deposition on glaciers from semi-arid and arid regions. Principal component analysis and air mass trajectories suggested that chemical constituents were mainly attributable to crustal sources as demonstrated by the high concentrations of ions occurring during the non-monsoon seasons. Nevertheless, other sources, such as anthropogenic pollution, played an important role on chemical variations of glaciers near the human activity centers. This study concluded that air mass transport from different sources played important roles on the spatial distributions and seasonality of glaciochemistry.

Characteristics of black carbon in snow from Laohugou No. 12 glacier on the northern Tibetan Plateau

Black carbon (BC) emitted from the incomplete combustion of biomass and fossil fuel impacts the climate system, cryospheric change, and human health. This study documents black carbon deposition in snow from a benchmark glacier on the northern Tibetan Plateau. Significant seasonality of BC concentrations indicates different input or post-depositional processes. BC particles deposited in snow had a mass volume median diameter slightly larger than that of black carbon particles typically found in the atmosphere. Also, unlike black carbon particles in the atmosphere, the particles deposited in snow did not exhibit highly fractal morphology by Scanning Transmission Electron Microscope. Footprint analysis indicated BC deposited on the glacier in summer originated mainly from Central Asia; in winter, the depositing air masses generally originated from Central Asia and Pakistan. Anthropogenic emissions play an important role on black carbon deposition in glacial snow, especially in winter. The mass absorption efficiency of BC in snow at 632nm exhibited significantly seasonality, with higher values in summer and lower values in winter. The information on black carbon deposition in glacial snow provided in this study could be used to help mitigate the impacts of BC on glacier melting on the northern Tibetan Plateau.

Light-absorbing impurities on Keqikaer Glacier in western Tien Shan: concentrations and potential impact on albedo reduction

Light-absorbing impurities on glaciers are important factors that influence glacial surface albedo and accelerate glacier melt. In this study, the quantity of light-absorbing impurities on Keqikaer Glacier in western Tien Shan, Central Asia, was measured. We found that the average concentrations of black carbon was 2,180 ng/g, with a range from 250 ng/g to more than 10,000 ng/g. The average concentrations of organic carbon and mineral dust were 1,738 ng/g and 194 μg/g, respectively. Based on simulations performed with the Snow Ice Aerosol Radiative model simulations, black carbon and dust are responsible for approximately 64% and 9%, respectively, of the albedo reduction, and are associated with instantaneous radiative forcing of 323.18 W/m 2 (ranging from 142.16 to 619.25 W/m 2 ) and 24.05 W/m 2 (ranging from 0.15 to 69.77 W/m 2 ), respectively. For different scenarios, the albedo and radiative forcing effect of black carbon is considerably greater than that of dust. The estimated radiative forcing at Keqikaer Glacier is higher than most similar values estimated by previous studies on the Tibetan Plateau, perhaps as a result of black carbon enrichment by melt scavenging. Light-absorbing impurities deposited on Keqikaer Glacier appear to mainly originate from central Asia, Siberia, western China (including the Taklimakan Desert) and parts of South Asia in summer, and from the Middle East and Central Asia in winter. A footprint analysis indicates that a large fraction (>60%) of the black carbon contributions on Keqikaer Glacier comes from anthropogenic sources. These results provide a scientific basis for regional mitigation efforts to reduce black carbon.

Dissolved organic carbon in snow cover of the Chinese Altai Mountains, Central Asia: Concentrations, sources and light-absorption properties

Dissolved organic carbon (DOC) in snow plays an important role in river ecosystems that are fed by snowmelt water. However, limited knowledge is available on the DOC content in snow of the Chinese Altai Mountains in Central Asia. In this study, DOC in the snow cover of the Kayiertesi river basin, southern slope of Altai Mountains, was investigated during November 2016 to April 2017. The results showed that average concentrations of DOC in the surface snow cover (1.01 ± 0.52 mg L-1) were only a little higher than those in glaciers of the Tibetan Plateau, European Alps, and Alaska, but much higher than in Greenland Ice Sheet. Depth variations of DOC concentrations from snowpack profiles indicated higher values in the surface layer. During the observation period, scavenging efficiency for DOC in snow cover is estimated to be 0.15 ± 0.10, suggesting that DOC in snow can be affected more by the meltwater during ablation season than during accumulation season. The average mass absorption cross section at 365 nm and the absorption Ångström exponent of DOC were 0.45 ± 0.35 m2 g-1 and 2.59 ± 1.03, respectively, with higher values in March and April 2017. Fraction of radiative forcing caused by DOC relative to black carbon accounted for about 10.5%, implying DOC is a non-ignorable light-absorber of solar radiation in snow of the Altai regions. Backward trajectories analysis and aerosol vertical distribution images from satellites showed that DOC in the snow of the Altai Mountains was mainly influenced by air masses from Central Asia, Western Siberia, the Middle East, and some even from Europe. Biomass burning and organic carbon mixed with mineral dust contributed significantly to the DOC concentration. This study highlights the effects of DOC in the snow cover for radiative forcing and the need to study carbon cycling for evaluation of quality of the downstreams ecosystems.

Impacts of the active layer on runoff in an upland permafrost basin, northern Tibetan Plateau

The paucity of studies on permafrost runoff generation processes, especially in mountain permafrost, constrains the understanding of permafrost hydrology and prediction of hydrological responses to permafrost degradation. This study investigated runoff generation processes, in addition to the contribution of summer thaw depth, soil temperature, soil moisture, and precipitation to streamflow in a small upland permafrost basin in the northern Tibetan Plateau. Results indicated that the thawing period and the duration of the zero-curtain were longer in permafrost of the northern Tibetan Plateau than in the Arctic. Limited snowmelt delayed the initiation of surface runoff in the peat permafrost in the study area. The runoff displayed intermittent generation, with the duration of most runoff events lasting less than 24 h. Precipitation without runoff generation was generally correlated with lower soil moisture conditions. Combined analysis suggested runoff generation in this region was controlled by soil temperature, thaw depth, precipitation frequency and amount, and antecedent soil moisture. This study serves as an important baseline to evaluate future environmental changes on the Tibetan Plateau.

Dissolved organic carbon in glaciers of the southeastern Tibetan Plateau: Insights into concentrations and possible sources

Dissolved organic carbon (DOC) released from glaciers has an important role in the biogeochemistry of glacial ecosystems. This study focuses on DOC from glaciers of the southeastern Tibetan Plateau, where glaciers are experiencing rapid shrinkage. We found that concentrations of DOC in snowpits (0.16±0.054 μg g−1), aged snow (0.16±0.048 μg g−1), and bare ice (0.18±0.082 μg g−1) were similar across the southeastern Tibetan Plateau, but were slightly lower than those in other glaciers on the Tibetan Plateau. Vertical variations of DOC, particulate organic carbon, black carbon, and total nitrate in snowpit showed no systematic variations in the studied glaciers, with high values of DOC occurring in the ice or dusty layers. We estimated the export of DOC and particulate organic carbon from glaciers to be 1.96±0.66 Gg yr–1 and 5.88±2.15 Gg yr–1 in this region, respectively, indicating that organic carbon released from glacier meltwater may be affecting downstream ecosystems. Potential sources of the air masses arriving at the southeastern Tibetan glaciers include South Asia, Central Asia, Middle East, and northwest China. Emissions from biomass burning of South Asia played an important role in the deposition of DOC to the glacier, which can be evidenced by backward trajectories and fire spot distributions from MODIS and CALIPSO images. Our findings suggest that anthropogenic aerosols contribute abundant DOC to glaciers on the southeastern Tibetan Plateau. The pronounced rate of glacial melting in the region may be delivering increased quantities of relic DOC to downstream rivers.

Stream temperature dynamics in Nam Co basin, southern Tibetan Plateau

Stream temperatures are sensitive to climate change and runoff regime variations. A comprehensive understanding on the effects of glacial melting on the stream temperatures are important in the Tibetan Plateau, of which contains the largest ice volume outside Polar Regions. This study documented the high-resolution stream temperature thermal regimes from glacier-fed and non-glacial rivers at four sites, versus a high-resolution glacier mass balance monitoring at Zhadang glacier, during summer melt seasons from 2007‒2009 in the Nam Co basin of southern Tibetan Plateau. The results showed mean summer stream temperature and magnitude of daily thermal variation were lower at all sites when compared with alpine glacierized environments at lower latitudes. Mean stream temperatures for glacier-fed rivers (4.0°C to 6.5°C) were minimum and least variable near the glacier terminus with increasing toward downstream (+0.13°Ckm–1 to +0.28°C km–1). Meanwhile, stream temperature in 2008 was similar to that in 2007 and 2009. For the non-glacial rivers, mean stream temperatures was about 9.0°C with significantly warmer in summer months in 2009 and 2007 than that in 2008. These differences indicated that stream temperature was strongly influenced by discharge and precipitation. Particularly, the glacier mass balance played a large role on the stream temperature directly when the glacier melt contributed more than 50% of the glacial river runoff. Our results demonstrated the stream thermal variability from southern Tibetan rivers and provided new insight into the influence of glacier mass balance on stream thermal variability in high-altitude river system.