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The impact of lake effects on the temporal and spatial distribution of precipitation in the Nam Co basin, Tibetan Plateau
Quaternary International xxx (2016) 1e7
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The impact of lake effects on the temporal and spatial distribution of precipitation in the Nam Co basin, Tibetan Plateau Yufeng Dai a, b, Tandong Yao a, c, *, Xiangyu Li d, e, Fan Ping a a
Institute of Tibetan Plateau Research, Chinese Academy of Sciences (ITPCAS), Beijing, China University of Chinese Academy of Sciences (UCAS), Beijing, China c CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing, China d Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China e Climate Change Research Center, Chinese Academy of Sciences, Beijing, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
Nam Co (Heavenly Lake) not only responds sensitively to climate change, but also exerts significant influences on climate change. Previous studies revealed that the lake effects from the North American Great Lakes shown interaction with climate changes. However, the lakes effect of more than 1200 lakes with an area of 47,000 km2 on the Tibetan Plateau remains less studied. In this study, we analyzed the spatial distribution of precipitation in the Nam Co Basin during JulyeAugust and NovembereDecember to clarify the lake effect. We found that the lake cooling effect during JulyeAugust and lake heating effect during NovembereDecember resulted in the spatial differences of precipitation within the Nam Co Basin. These effects generally caused more precipitation in the south than in the north during JulyeAugust, and more precipitation in the east than in the west during NovembereDecember. Weakening Indian monsoon and climate warming led to an increased northesouth precipitation difference in the Nam Co Basin. However, the precipitation differences between the west and east during NovembereDecember decreased with climate warming. The intensification of the westerly might be responsible for the particular phenomenon. © 2016 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Nam Co Lake cooling effect Lake heating effect Spatial differences in precipitation
1. Introduction The impacts of lake effects on climate change result primarily from the following two processes: Landelake breezes (the first process we discuss) arise due to the difference of thermal properties between land and lake water. ‘Lake breezes’ form when the sun heats the land under fine weather, warming the land faster than the water and causing an uneven distribution of horizontal temperatures. This results in an airflow from the lake toward the land (as the higher temperatures on land cause the warm air to rise). After sunset, when the land cools more rapidly, the ‘land breeze’ blows back towards the lake (Pielke, 1974; Sills, 1998). Landelake breezes can develop at any time of the year, but their highest frequency occurs during the spring and summer (Sills, 1998). Landelake breezes also increase the intensity of thunderstorms (Sills et al., 2002; Lv et al., 2007,
* Corresponding author. 16-3 Lincui Rd., Beijing 100101, China. E-mail address: [email protected] (T. Yao).
2008). The circulation intensities of landelake breezes relate directly to the horizontal temperature gradient and depth of the highly heated area (Pielke and Segal, 1986). Different studies regarding the Great Lakes (Moroz, 1967; Sills, 1998; and Sills et al., 2002), the Qinghai Lake (Lv et al., 2007), Nam Co (Lv et al., 2008 and 2009), and the Gyaring and Ngoring Lakes (Yang and Wen, 2012) show significant impacts on the local climate resulting from landelake breezes. The second process involves the cooling and heating effect that occurs when an air mass passes over different surfaces. Here, lake water plays a cooling role in summer and warming role in winter. During summer, the decrease of precipitation in the downwind area results mainly from the cooling effect of the lake, which causes less rainfall going across the lake (Scott and Huff, 1996). In winter, the residual heat effect of the lake causes lake-effect snowfall and increases the precipitation in the downwind area (Scott and Huff, 1996). A cold air mass passing over the large lake when the water is ice free receives more water vapor evaporated from lake water and forms rain, snow, and fog in the areas downwind of the lake. When in the form of snow, it can be called
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Please cite this article in press as: Dai, Y., et al., The impact of lake effects on the temporal and spatial distribution of precipitation in the Nam Co basin, Tibetan Plateau, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.075
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Y. Dai et al. / Quaternary International xxx (2016) 1e7
‘lake-effect snowfall’ (Lavoie, 1972; Hjelmfelt Mark, 1990; Scott and Huff, 1996). The mechanisms involve (1) the unstable vertical temperature gradient produced when cold air mass passes over the warm lake surface and becomes more warm and humid, (2) the uplift effect resulting from friction caused by convergence with the shore terrain and a moderate wind speed and small wind shear, and (3) favorable physical cloud processes (Lavoie, 1972: Yang et al., 2009). Considerable research has pointed out that lakeeffect snowfall in the Great Lakes area has increased significantly with global warming over the past few decades (Braham and Dungey, 1984; Burnett et al., 2003; and Kunkel et al., 2009). Based on the data from 1230 weather stations around the Great Lakes area, Norton and Bolsenga (1993) revealed that lake-effect snowfall have exhibited a significant upward trend from 1951 to 1980. Based on synoptic climatology, Leathers and Ellis (1996) found that lake-effect snowfall at Lake Erie and Lake Ontario increased from 1950 to 1981. Burnett et al. (2003) analyzed differences between snowfall caused by lake effects and non-lake effects, in the area of the Great Lakes, and found that lake-effect snowfall increased from 1931 to 2001. Zhang et al. (2011a, 2011b and 2014) identified more than 1200 lakes larger than 1 km2 on the Tibetan Plateau, with a total area of more than 47,000 km2 and shown that most of these lakes are expanding as a result of their sensitive responses to climate warming from 1960 to 2012, with a rate of temperature rise of 0.3e0.4 C per decade (approximately twice the rate of global warming of its surroundings over the same period). What are the characteristics of lake effect in the Tibetan Plateau? What has happened to the lake effect under the background of global warming? The present study is focusing on the cooling effect and heating effect of Nam Co, one of the two largest lakes (over 2000 km2) and the most observed lake on the Tibetan Plateau. The study of the lake effect at Nam Co will help us understand how lake effects on the Tibetan Plateau may impact climate and respond to changes under the background of global warming.
2. Nam Co Nam Co lies at 30 300 e30 560 N, 90160 e91030 E, with an elevation of 4718 m. Nam Co covers 2026 km2 of area, occupying 19% of the basin area. Nam Co lies at the margin of the Indian monsoon dominance. Nam Co used to be the largest lake of the Tibetan Plateau, but became the second largest lake in 2011, after the rapid expansion of Selin Co. The rainy season of the Indian monsoon lasts from June to October, has a dominating influence on Nam Co. The westerly controls the Nam Co during the dry season and last from November to the following May (Chen et al., 2009). In the last ten years, the lake has begun to freeze around January 4th, and has completely frozen up by about February 13th (Kropa cek et al., 2013). Haginoya et al. (2009) analyzed the water and heat balance of Nam Co and found that the air temperature was warmer than the lake surface temperature in spring and summer (FebruaryeAugust), but cooler during autumn and winter (SeptembereJanuary). Consequently, the lake remains a heat sink during spring and summer, and becomes the heat source during autumn and winter (Fig. 1). 3. Data and methods Precipitation data used in this study include the meteorological dataset compiled by the Institute of Tibetan Plateau Research, which was based on TRMM (Tropical Rainfall Measuring Mission) 3B42, GLDAS (Global Land Data Assimilation System), and a high spatial and temporal resolution meteorological element dataset from sites of the China Meteorological Administration, i.e., ITPCAS (Yang et al., 2010; Chen et al., 2011; and http://dam.itpcas.ac.cn/chs/ rs/?q¼data). ITPCAS presents a reanalyzed dataset that contains multiple elements (including near-surface air temperatures, barometric pressures, wind speeds, downward shortwave radiation, and precipitation) with a long time series (1979e2012) and high spatial and temporal resolution (0.1 0.1, 3h). Kan et al. (2013) tested the applicability of the analysis in the upstream portion of the Yarkand
Fig. 1. Location of Nam Co.
Please cite this article in press as: Dai, Y., et al., The impact of lake effects on the temporal and spatial distribution of precipitation in the Nam Co basin, Tibetan Plateau, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.075
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River in the Karakoram, using four precipitation datasets (rainfall data from CMORPH, TMPA 3B42V6, ITPCAS, and APHRODITE). The results of Kan's analysis showed that the spatial distribution provided by ITPCAS appears more reasonable. The wind data at 500 hPa were taken from the NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) monthly reanalysis data from 1979 to 2012 (with horizontal resolutions of 2.5 2.5 ; Kalnay et al., 1996). All reanalysis wind data were aggregated to a horizontal resolution of 0.5 0.5 , using bilinear interpolation. Monthly average temperature data used in this research come from two meteorological stations, the Ban-ge Meteorological Station and the Dang-xiong Meteorological Station. These lie on the western and eastern sides of the Nam Co Basin, respectively. This study calculates average temperature trends for July through August and for November through December.
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4. Results and discussion 1) Lake cooling effect during July and August, as revealed by temporal and spatial characteristics of precipitation. This study focuses on the lake's cooling effect in July and August when the Indian monsoon is strongest during 1979e2012. It also examines the November and December periods, to study the lake's heating effect when the lake is not completely frozen. The study further analyzed the spatial distribution of precipitation underlying the impact of the lake effects, and also analyzed the changes in spatial difference of precipitation under the background of global warming from 1979 to 2012. As can be seen in Fig. 2a, more precipitation falls on the south bank of Nam Co than on the north bank. The southern bank as a boundary divided the whole basin into southern and northern part.
Fig. 2. Precipitation of the Nam Co Basin in July and August: (a) spatial distributions (during 1979e2012), and (b) temporal variations of northern and southern portions (from 1979 to 2012).
Please cite this article in press as: Dai, Y., et al., The impact of lake effects on the temporal and spatial distribution of precipitation in the Nam Co basin, Tibetan Plateau, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.075
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During the 1979e2012 periods, the southern area shows a rising (but fluctuating) precipitation tendency, while the northern part shows a decreasing tendency (Fig. 2b). During 1979e1998, no large differences of precipitation occurred between the north and south; showing only about 2% more average precipitation in the south than in the north. Between 1999 and 2010, precipitation amounts showed significant differences between the north and south, with about 30% more average precipitation in the south than in the north. Differences of the precipitation between the southern and northern parts of the Nam Co Basin exhibit typical characteristics of the lake effect. These differences result mainly from the different effects exerted by the Indian monsoon on the Tibetan Plateau. In summer, the warm and wet Indian monsoon provides the dominant carrier of water vapor in the basin. From the spatial pattern of the winds (Fig. 3a), it can be seen that the main wind is from southwest during July and August in the Nam Co Basin. Water vapor from the Indian monsoon first precipitates in the southern part of the Nam Co, then moves northward. In addition, the effect of the
cold summer lake also results in less precipitation in downwind areas of the lake (Scott and Huff, 1996), as is the northern part of the Nam Co basin. Therefore, more rain falls in the southern part than in the northern part in the Nam Co Basin. From 1979 to 2012, a significant decrease trend of wind speed is shown in Fig. 3b. As pointed out by Yao et al. (2012), the Nam Co Basin is under the Indian monsoon dominance in summer, and the Indian monsoon is weakening now. We therefore suspect that the weakening of the Indian monsoon will lead to a northward decrease in water vapor, and hence decreased precipitation in the northern part of the Nam Co Basin. Lv et al. (2008) suggested that orographic weather effects occur frequently in the southeastern part of the Nam Co region. During daytime, the lake breeze generated at the southern lake shore and the valley wind generated from the ^ntanglha combine and then converge northern slope of Mt. Nyainqe with another valley wind produced on the southern slopes of the high mountain area. This phenomenon results in complex weather in this area during summer. During night, the superimposition of land winds from the southern lake shore and valley winds from the northern slope produce strong southern flows that control the whole area. This strong southern flow not only supplies moisture, but also provides the energy for unstable motion. When lake breezes occur, the increased temperature difference between land surface and lake water will intensify circulation (Sills, 1998). Data from meteorological stations on the west and east sides of the Nam Co Basin (Ban-ge and Dang-xiong) indicate that the summer average temperatures have been rising (Fig. 4). Therefore, we suggest that the rising of summer temperatures will lead to enhanced lake circulation and an increasing tendency for precipitation in the southern areas. Furthermore, the northesouth precipitation differences also positively correlate with average temperatures (with correlation coefficients of 0.13 at Dang-xiong and 0.21 at Bang-ge). This means that the northesouth precipitation differences have increased with the rising average temperatures. 2) Lake heating effect in November and December revealed by temporal and spatial characteristics of precipitation. A pattern exists showing that precipitation increased from the west to the east (Fig. 5a), with greater precipitation in the east than in the west. The western bank as a boundary divided the whole basin into western and eastern part. Precipitations in November and December in both the western and eastern portions of the basin have shown upward trends during 1979e2012, and fluctuations in the east exceed those in the west (Fig. 5b). The differences
Fig. 3. (a) The spatial pattern of wind data at 500 hPa (indicated by the vector, m/s) in July and August (during 1979e2012). The shading color represents the wind speed (m/ s); (b) Variations of wind speed (average wind speed of the Nam Co basin) in July and August (from 1979 to 2012).
Fig. 4. Precipitation differences between the southern and northern parts of the Nam Co Basin and the average temperatures during July and August (from 1979 to 2012). The rdx (rbg) indicates the correlation coefficient of the precipitation differences associated with average temperature in Dang-xiong (Ban-ge).
Please cite this article in press as: Dai, Y., et al., The impact of lake effects on the temporal and spatial distribution of precipitation in the Nam Co basin, Tibetan Plateau, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.075
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in precipitation between the east and west increases firstly and then decreases. The westerly circulations strengthen and dominate the whole basin during November and December (Fig. 6). Airflow that passes over the lake surface (coupling with water vapor resulting from heat of the lake) increases precipitation in the downwind area (eastern of the lake). Due to the rising terrain along the eastern side of the lake, lake-effect snowfall increases. Therefore, there is more precipitation in the eastern part than the western part in the Nam Co Basin. During 1979e2012, Precipitation in the west increased with the intensification of the wind speed. Precipitation also increased during winter in the east, within the region of the lake effect snow belt (Kropacek et al., 2010). As pointed out by Yao et al. (2012), the Nam Co Basin is under the westerly dominance in winter, and the westerly is enhancing now. We speculate that the
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cold weather events from more frequent westerly processes make the lake heating effect stronger, and then increasing precipitation in the east. The data from the meteorological stations (Ban-ge and Dang-xiong) near the Nam Co Basin shows that the temperature in November and December are increasing which is not parallel with the decreases of the differences of precipitation between east and west (Fig. 7). It means that the precipitation differences correlate negatively with the average temperatures (with a correlation coefficient of 0.31 at Dang-xiong and 0.15 at Bang-ge). Therefore we speculate that the decreasing differences in precipitation during November and December may be related to the enhancing westerly too. The water vapor from the westerly circulation might be more than that over the lake water, which cause a greater precipitation increase in the west than the east. Then, the differences of precipitation between east and west decreased.
Fig. 5. Precipitation of the Nam Co Basin in November and December: (a) spatial distribution (during 1979e2012), and (b) temporal variations in the eastern and western sectors (from 1979 to 2012).
Please cite this article in press as: Dai, Y., et al., The impact of lake effects on the temporal and spatial distribution of precipitation in the Nam Co basin, Tibetan Plateau, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.075
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5. Conclusions The lake cooling effect during JulyeAugust and lake heating effect during NovembereDecember result in the spatial differences in the precipitation distribution in the Nam Co Basin, characterized by more precipitation in the south than in the north in summer, and more precipitation in the east than in the west in winter. As the Indian monsoon weakened between 1979 and 2012, precipitation lessened in the north. With rising temperatures, the lake breeze effect strengthened, leading to precipitation increases in the south. Overall spatial differences of precipitation between the south and the north in Nam Co Basin increased. The cold weather events from more frequent westerly processes make the lake heating effect stronger between 1979 and 2012, resulting in increasing precipitation in the east. The enhancing westerly is also bringing more precipitation in the west, which results in a significantly greater precipitation increase in the west. As a result, the overall spatial differences of precipitation between the east and the west in the Nam Co basin decreased.
Acknowledgments This study was funded from the National Natural Science Foundation of China (Grant No.Y2Ia011001 and No. 41401082) and from the China Postdoctoral Science Foundation funded project (Grant No. 2015M581154). Many thanks to the Third Pole Environment Database for the data support of the ITPCAS dataset (http://www.tpedatabase.cn). Constructive comments from anonymous reviewers that improved the quality of this manuscript are greatly appreciated.
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Fig. 6. (a) The spatial pattern of wind data at 500 hPa (indicated by the vectors, m/s) in November and December (during 1979e2012). The shading color represents the wind speed (m/s); (b) Variation of wind speed (average wind speed of the Nam Co basin) in November and December (from 1979 to 2012).
Fig. 7. Precipitation differences in western and eastern parts of the Nam Co basin and the average temperature during November and December (from 1979 to 2012). The rdx (rbg) means the correlation coefficient of precipitation difference associated with average temperature in Dang-xiong (Ban-ge).
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The impact of lake effects on the temporal and spatial distribution of precipitation in the Nam Co basin, Tibetan Plateau Yufeng Dai a, b, Tandong Yao a, c, *, Xiangyu Li d, e, Fan Ping a a
Institute of Tibetan Plateau Research, Chinese Academy of Sciences (ITPCAS), Beijing, China University of Chinese Academy of Sciences (UCAS), Beijing, China c CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Sciences, Beijing, China d Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing, China e Climate Change Research Center, Chinese Academy of Sciences, Beijing, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
Nam Co (Heavenly Lake) not only responds sensitively to climate change, but also exerts significant influences on climate change. Previous studies revealed that the lake effects from the North American Great Lakes shown interaction with climate changes. However, the lakes effect of more than 1200 lakes with an area of 47,000 km2 on the Tibetan Plateau remains less studied. In this study, we analyzed the spatial distribution of precipitation in the Nam Co Basin during JulyeAugust and NovembereDecember to clarify the lake effect. We found that the lake cooling effect during JulyeAugust and lake heating effect during NovembereDecember resulted in the spatial differences of precipitation within the Nam Co Basin. These effects generally caused more precipitation in the south than in the north during JulyeAugust, and more precipitation in the east than in the west during NovembereDecember. Weakening Indian monsoon and climate warming led to an increased northesouth precipitation difference in the Nam Co Basin. However, the precipitation differences between the west and east during NovembereDecember decreased with climate warming. The intensification of the westerly might be responsible for the particular phenomenon. © 2016 Elsevier Ltd and INQUA. All rights reserved.
Keywords: Nam Co Lake cooling effect Lake heating effect Spatial differences in precipitation
1. Introduction The impacts of lake effects on climate change result primarily from the following two processes: Landelake breezes (the first process we discuss) arise due to the difference of thermal properties between land and lake water. ‘Lake breezes’ form when the sun heats the land under fine weather, warming the land faster than the water and causing an uneven distribution of horizontal temperatures. This results in an airflow from the lake toward the land (as the higher temperatures on land cause the warm air to rise). After sunset, when the land cools more rapidly, the ‘land breeze’ blows back towards the lake (Pielke, 1974; Sills, 1998). Landelake breezes can develop at any time of the year, but their highest frequency occurs during the spring and summer (Sills, 1998). Landelake breezes also increase the intensity of thunderstorms (Sills et al., 2002; Lv et al., 2007,
* Corresponding author. 16-3 Lincui Rd., Beijing 100101, China. E-mail address: [email protected] (T. Yao).
2008). The circulation intensities of landelake breezes relate directly to the horizontal temperature gradient and depth of the highly heated area (Pielke and Segal, 1986). Different studies regarding the Great Lakes (Moroz, 1967; Sills, 1998; and Sills et al., 2002), the Qinghai Lake (Lv et al., 2007), Nam Co (Lv et al., 2008 and 2009), and the Gyaring and Ngoring Lakes (Yang and Wen, 2012) show significant impacts on the local climate resulting from landelake breezes. The second process involves the cooling and heating effect that occurs when an air mass passes over different surfaces. Here, lake water plays a cooling role in summer and warming role in winter. During summer, the decrease of precipitation in the downwind area results mainly from the cooling effect of the lake, which causes less rainfall going across the lake (Scott and Huff, 1996). In winter, the residual heat effect of the lake causes lake-effect snowfall and increases the precipitation in the downwind area (Scott and Huff, 1996). A cold air mass passing over the large lake when the water is ice free receives more water vapor evaporated from lake water and forms rain, snow, and fog in the areas downwind of the lake. When in the form of snow, it can be called
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‘lake-effect snowfall’ (Lavoie, 1972; Hjelmfelt Mark, 1990; Scott and Huff, 1996). The mechanisms involve (1) the unstable vertical temperature gradient produced when cold air mass passes over the warm lake surface and becomes more warm and humid, (2) the uplift effect resulting from friction caused by convergence with the shore terrain and a moderate wind speed and small wind shear, and (3) favorable physical cloud processes (Lavoie, 1972: Yang et al., 2009). Considerable research has pointed out that lakeeffect snowfall in the Great Lakes area has increased significantly with global warming over the past few decades (Braham and Dungey, 1984; Burnett et al., 2003; and Kunkel et al., 2009). Based on the data from 1230 weather stations around the Great Lakes area, Norton and Bolsenga (1993) revealed that lake-effect snowfall have exhibited a significant upward trend from 1951 to 1980. Based on synoptic climatology, Leathers and Ellis (1996) found that lake-effect snowfall at Lake Erie and Lake Ontario increased from 1950 to 1981. Burnett et al. (2003) analyzed differences between snowfall caused by lake effects and non-lake effects, in the area of the Great Lakes, and found that lake-effect snowfall increased from 1931 to 2001. Zhang et al. (2011a, 2011b and 2014) identified more than 1200 lakes larger than 1 km2 on the Tibetan Plateau, with a total area of more than 47,000 km2 and shown that most of these lakes are expanding as a result of their sensitive responses to climate warming from 1960 to 2012, with a rate of temperature rise of 0.3e0.4 C per decade (approximately twice the rate of global warming of its surroundings over the same period). What are the characteristics of lake effect in the Tibetan Plateau? What has happened to the lake effect under the background of global warming? The present study is focusing on the cooling effect and heating effect of Nam Co, one of the two largest lakes (over 2000 km2) and the most observed lake on the Tibetan Plateau. The study of the lake effect at Nam Co will help us understand how lake effects on the Tibetan Plateau may impact climate and respond to changes under the background of global warming.
2. Nam Co Nam Co lies at 30 300 e30 560 N, 90160 e91030 E, with an elevation of 4718 m. Nam Co covers 2026 km2 of area, occupying 19% of the basin area. Nam Co lies at the margin of the Indian monsoon dominance. Nam Co used to be the largest lake of the Tibetan Plateau, but became the second largest lake in 2011, after the rapid expansion of Selin Co. The rainy season of the Indian monsoon lasts from June to October, has a dominating influence on Nam Co. The westerly controls the Nam Co during the dry season and last from November to the following May (Chen et al., 2009). In the last ten years, the lake has begun to freeze around January 4th, and has completely frozen up by about February 13th (Kropa cek et al., 2013). Haginoya et al. (2009) analyzed the water and heat balance of Nam Co and found that the air temperature was warmer than the lake surface temperature in spring and summer (FebruaryeAugust), but cooler during autumn and winter (SeptembereJanuary). Consequently, the lake remains a heat sink during spring and summer, and becomes the heat source during autumn and winter (Fig. 1). 3. Data and methods Precipitation data used in this study include the meteorological dataset compiled by the Institute of Tibetan Plateau Research, which was based on TRMM (Tropical Rainfall Measuring Mission) 3B42, GLDAS (Global Land Data Assimilation System), and a high spatial and temporal resolution meteorological element dataset from sites of the China Meteorological Administration, i.e., ITPCAS (Yang et al., 2010; Chen et al., 2011; and http://dam.itpcas.ac.cn/chs/ rs/?q¼data). ITPCAS presents a reanalyzed dataset that contains multiple elements (including near-surface air temperatures, barometric pressures, wind speeds, downward shortwave radiation, and precipitation) with a long time series (1979e2012) and high spatial and temporal resolution (0.1 0.1, 3h). Kan et al. (2013) tested the applicability of the analysis in the upstream portion of the Yarkand
Fig. 1. Location of Nam Co.
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River in the Karakoram, using four precipitation datasets (rainfall data from CMORPH, TMPA 3B42V6, ITPCAS, and APHRODITE). The results of Kan's analysis showed that the spatial distribution provided by ITPCAS appears more reasonable. The wind data at 500 hPa were taken from the NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) monthly reanalysis data from 1979 to 2012 (with horizontal resolutions of 2.5 2.5 ; Kalnay et al., 1996). All reanalysis wind data were aggregated to a horizontal resolution of 0.5 0.5 , using bilinear interpolation. Monthly average temperature data used in this research come from two meteorological stations, the Ban-ge Meteorological Station and the Dang-xiong Meteorological Station. These lie on the western and eastern sides of the Nam Co Basin, respectively. This study calculates average temperature trends for July through August and for November through December.
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4. Results and discussion 1) Lake cooling effect during July and August, as revealed by temporal and spatial characteristics of precipitation. This study focuses on the lake's cooling effect in July and August when the Indian monsoon is strongest during 1979e2012. It also examines the November and December periods, to study the lake's heating effect when the lake is not completely frozen. The study further analyzed the spatial distribution of precipitation underlying the impact of the lake effects, and also analyzed the changes in spatial difference of precipitation under the background of global warming from 1979 to 2012. As can be seen in Fig. 2a, more precipitation falls on the south bank of Nam Co than on the north bank. The southern bank as a boundary divided the whole basin into southern and northern part.
Fig. 2. Precipitation of the Nam Co Basin in July and August: (a) spatial distributions (during 1979e2012), and (b) temporal variations of northern and southern portions (from 1979 to 2012).
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During the 1979e2012 periods, the southern area shows a rising (but fluctuating) precipitation tendency, while the northern part shows a decreasing tendency (Fig. 2b). During 1979e1998, no large differences of precipitation occurred between the north and south; showing only about 2% more average precipitation in the south than in the north. Between 1999 and 2010, precipitation amounts showed significant differences between the north and south, with about 30% more average precipitation in the south than in the north. Differences of the precipitation between the southern and northern parts of the Nam Co Basin exhibit typical characteristics of the lake effect. These differences result mainly from the different effects exerted by the Indian monsoon on the Tibetan Plateau. In summer, the warm and wet Indian monsoon provides the dominant carrier of water vapor in the basin. From the spatial pattern of the winds (Fig. 3a), it can be seen that the main wind is from southwest during July and August in the Nam Co Basin. Water vapor from the Indian monsoon first precipitates in the southern part of the Nam Co, then moves northward. In addition, the effect of the
cold summer lake also results in less precipitation in downwind areas of the lake (Scott and Huff, 1996), as is the northern part of the Nam Co basin. Therefore, more rain falls in the southern part than in the northern part in the Nam Co Basin. From 1979 to 2012, a significant decrease trend of wind speed is shown in Fig. 3b. As pointed out by Yao et al. (2012), the Nam Co Basin is under the Indian monsoon dominance in summer, and the Indian monsoon is weakening now. We therefore suspect that the weakening of the Indian monsoon will lead to a northward decrease in water vapor, and hence decreased precipitation in the northern part of the Nam Co Basin. Lv et al. (2008) suggested that orographic weather effects occur frequently in the southeastern part of the Nam Co region. During daytime, the lake breeze generated at the southern lake shore and the valley wind generated from the ^ntanglha combine and then converge northern slope of Mt. Nyainqe with another valley wind produced on the southern slopes of the high mountain area. This phenomenon results in complex weather in this area during summer. During night, the superimposition of land winds from the southern lake shore and valley winds from the northern slope produce strong southern flows that control the whole area. This strong southern flow not only supplies moisture, but also provides the energy for unstable motion. When lake breezes occur, the increased temperature difference between land surface and lake water will intensify circulation (Sills, 1998). Data from meteorological stations on the west and east sides of the Nam Co Basin (Ban-ge and Dang-xiong) indicate that the summer average temperatures have been rising (Fig. 4). Therefore, we suggest that the rising of summer temperatures will lead to enhanced lake circulation and an increasing tendency for precipitation in the southern areas. Furthermore, the northesouth precipitation differences also positively correlate with average temperatures (with correlation coefficients of 0.13 at Dang-xiong and 0.21 at Bang-ge). This means that the northesouth precipitation differences have increased with the rising average temperatures. 2) Lake heating effect in November and December revealed by temporal and spatial characteristics of precipitation. A pattern exists showing that precipitation increased from the west to the east (Fig. 5a), with greater precipitation in the east than in the west. The western bank as a boundary divided the whole basin into western and eastern part. Precipitations in November and December in both the western and eastern portions of the basin have shown upward trends during 1979e2012, and fluctuations in the east exceed those in the west (Fig. 5b). The differences
Fig. 3. (a) The spatial pattern of wind data at 500 hPa (indicated by the vector, m/s) in July and August (during 1979e2012). The shading color represents the wind speed (m/ s); (b) Variations of wind speed (average wind speed of the Nam Co basin) in July and August (from 1979 to 2012).
Fig. 4. Precipitation differences between the southern and northern parts of the Nam Co Basin and the average temperatures during July and August (from 1979 to 2012). The rdx (rbg) indicates the correlation coefficient of the precipitation differences associated with average temperature in Dang-xiong (Ban-ge).
Please cite this article in press as: Dai, Y., et al., The impact of lake effects on the temporal and spatial distribution of precipitation in the Nam Co basin, Tibetan Plateau, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.075
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in precipitation between the east and west increases firstly and then decreases. The westerly circulations strengthen and dominate the whole basin during November and December (Fig. 6). Airflow that passes over the lake surface (coupling with water vapor resulting from heat of the lake) increases precipitation in the downwind area (eastern of the lake). Due to the rising terrain along the eastern side of the lake, lake-effect snowfall increases. Therefore, there is more precipitation in the eastern part than the western part in the Nam Co Basin. During 1979e2012, Precipitation in the west increased with the intensification of the wind speed. Precipitation also increased during winter in the east, within the region of the lake effect snow belt (Kropacek et al., 2010). As pointed out by Yao et al. (2012), the Nam Co Basin is under the westerly dominance in winter, and the westerly is enhancing now. We speculate that the
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cold weather events from more frequent westerly processes make the lake heating effect stronger, and then increasing precipitation in the east. The data from the meteorological stations (Ban-ge and Dang-xiong) near the Nam Co Basin shows that the temperature in November and December are increasing which is not parallel with the decreases of the differences of precipitation between east and west (Fig. 7). It means that the precipitation differences correlate negatively with the average temperatures (with a correlation coefficient of 0.31 at Dang-xiong and 0.15 at Bang-ge). Therefore we speculate that the decreasing differences in precipitation during November and December may be related to the enhancing westerly too. The water vapor from the westerly circulation might be more than that over the lake water, which cause a greater precipitation increase in the west than the east. Then, the differences of precipitation between east and west decreased.
Fig. 5. Precipitation of the Nam Co Basin in November and December: (a) spatial distribution (during 1979e2012), and (b) temporal variations in the eastern and western sectors (from 1979 to 2012).
Please cite this article in press as: Dai, Y., et al., The impact of lake effects on the temporal and spatial distribution of precipitation in the Nam Co basin, Tibetan Plateau, Quaternary International (2016), http://dx.doi.org/10.1016/j.quaint.2016.01.075
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5. Conclusions The lake cooling effect during JulyeAugust and lake heating effect during NovembereDecember result in the spatial differences in the precipitation distribution in the Nam Co Basin, characterized by more precipitation in the south than in the north in summer, and more precipitation in the east than in the west in winter. As the Indian monsoon weakened between 1979 and 2012, precipitation lessened in the north. With rising temperatures, the lake breeze effect strengthened, leading to precipitation increases in the south. Overall spatial differences of precipitation between the south and the north in Nam Co Basin increased. The cold weather events from more frequent westerly processes make the lake heating effect stronger between 1979 and 2012, resulting in increasing precipitation in the east. The enhancing westerly is also bringing more precipitation in the west, which results in a significantly greater precipitation increase in the west. As a result, the overall spatial differences of precipitation between the east and the west in the Nam Co basin decreased.
Acknowledgments This study was funded from the National Natural Science Foundation of China (Grant No.Y2Ia011001 and No. 41401082) and from the China Postdoctoral Science Foundation funded project (Grant No. 2015M581154). Many thanks to the Third Pole Environment Database for the data support of the ITPCAS dataset (http://www.tpedatabase.cn). Constructive comments from anonymous reviewers that improved the quality of this manuscript are greatly appreciated.
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Fig. 6. (a) The spatial pattern of wind data at 500 hPa (indicated by the vectors, m/s) in November and December (during 1979e2012). The shading color represents the wind speed (m/s); (b) Variation of wind speed (average wind speed of the Nam Co basin) in November and December (from 1979 to 2012).
Fig. 7. Precipitation differences in western and eastern parts of the Nam Co basin and the average temperature during November and December (from 1979 to 2012). The rdx (rbg) means the correlation coefficient of precipitation difference associated with average temperature in Dang-xiong (Ban-ge).
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