Binbin Wang

A 3-year dataset of sensible and latent heat fluxes from the Tibetan Plateau, derived using eddy covariance measurements

The Tibetan Plateau (TP) has become a focus of strong scientific interest due to its role in the global water cycle and its reaction to climate change. Regional flux estimates of sensible and latent heat are important variables for linking the energy and hydrological cycles at the TP’s surface. Within this framework, a 3-year dataset (2008–2010) of eddy covariance measured turbulent fluxes was compiled from four stations on the TP into a standardised workflow: corrections and quality tests were applied using an internationally comparable software package. Second, the energy balance closure (CEB) was determined and two different closure corrections applied. The four stations (Qomolangma, Linzhi, NamCo and Nagqu) represent different locations and typical land surface types on the TP (high altitude alpine steppe with sparse vegetation, a densely vegetated alpine meadow, and bare soil/gravel, respectively). We show that the CEB differs between each surface and undergoes seasonal changes. Typical differences in the turbulent energy fluxes occur between the stations at Qomolangma, Linzhi and NamCo, while Nagqu is quite similar to NamCo. Specific investigation of the pre-monsoon, the Tibetan Plateau summer monsoon, post-monsoon and winter periods within the annual cycle reinforces these findings. The energy flux of the four sites is clearly influenced by the Tibetan Plateau monsoon. In the pre-monsoon period, sensible heat flux is the major energy source delivering heat to the atmosphere, whereas latent heat flux is greater than sensible heat flux during the monsoon season. Other factors affecting surface energy flux are topography and location. Land cover type also affects surface energy flux. The energy balance residuum indicates a typically observed overall non-closure in winter, while closure (or ‘turbulent over-closure’) is achieved during the Tibetan Plateau summer monsoon at the Nagqu site. The latter seems to depend on ground heat flux, which is higher in the wet season, related not only to a larger radiation input but also to a thermal decoupling of dry soils. Heterogeneous landscape modelling using a MODIS product is introduced to explain energy non-closure.

Monitoring and Modeling the Tibetan Plateau's climate system and its impact on East Asia

The Tibetan Plateau is an important water source in Asia. As the “Third Pole” of the Earth, the Tibetan Plateau has significant dynamic and thermal effects on East Asian climate patterns, the Asian monsoon process and atmospheric circulation in the Northern Hemisphere. However, little systematic knowledge is available regarding the changing climate system of the Tibetan Plateau and the mechanisms underlying its impact on East Asia. This study was based on “water-cryosphere-atmosphere-biology” multi-sphere interactions, primarily considering global climate change in relation to the Tibetan Plateau -East Asia climate system and its mechanisms. This study also analyzed the Tibetan Plateau to clarify global climate change by considering multi-sphere energy and water processes. Additionally, the impacts of climate change in East Asia and the associated impact mechanisms were revealed, and changes in water cycle processes and water conversion mechanisms were studied. The changes in surface thermal anomalies, vegetation, local circulation and the atmospheric heat source on the Tibetan Plateau were studied, specifically, their effects on the East Asian monsoon and energy balance mechanisms. Additionally, the relationships between heating mechanisms and monsoon changes were explored.

The regional surface heating field over the heterogeneous landscape of the Tibetan Plateau using MODIS and in-situ data

In this study, a parameterization scheme based on Moderate Resolution Imaging Spectroradiometer (MODIS) data and in-situ data was tested for deriving the regional surface heating field over a heterogeneous landscape. As a case study, the methodology was applied to the whole Tibetan Plateau (TP) area. Four images of MODIS data (i.e., 30 January 2007, 15 April 2007, 1 August 2007, and 25 October 2007) were used in this study for comparison among winter, spring, summer, and autumn. The results were validated using the observations measured at the stations of the Tibetan Observation and Research Platform (TORP). The results show the following: (1) The derived surface heating field for the TP area was in good accord with the land-surface status, showing a wide range of values due to the strong contrast of surface features in the area. (2) The derived surface heating field for the TP was very close to the field measurements (observations). The APD (absolute percent difference) between the derived results and the field observations was <10%. (3) The mean surface heating field over the TP increased from January to April to August, and decreased in October. Therefore, the reasonable regional distribution of the surface heating field over a heterogeneous landscape can be obtained using this methodology. The limitations and further improvement of this method are also discussed.

Air temperature estimation with MODIS data over the Northern Tibetan Plateau

Time series of MODIS land surface temperature (Ts) and normalized difference vegetation index (NDVI) products, combined with digital elevation model (DEM) and meteorological data from 2001 to 2012, were used to map the spatial distribution of monthly mean air temperature over the Northern Tibetan Plateau (NTP). A time series analysis and a regression analysis of monthly mean land surface temperature (Ts) and air temperature (Ta) were conducted using ordinary linear regression (OLR) and geographical weighted regression (GWR). The analyses showed that GWR, which considers MODIS Ts, NDVI and elevation as independent variables, yielded much better results [R2 Adj > 0.79; root-mean-square error (RMSE) = 0.51°C–1.12°C] associated with estimating Ta compared to those from OLR (R2Adj = 0.40−0.78; RMSE = 1.60°C–4.38°C). In addition, some characteristics of the spatial distribution of monthly Ta and the difference between the surface and air temperature (Td) are as follows. According to the analysis of the 0°C and 10°C isothermals, Ta values over the NTP at elevations of 4000–5000 m were greater than 10°C in the summer (from May to October), and Ta values at an elevation of 3200 m dropped below 0°C in the winter (from November to April). Ta exhibited an increasing trend from northwest to southeast. Except in the southeastern area of the NTP, Td values in other areas were all larger than 0°C in the winter.

Recent trends (2003–2013) of land surface heat fluxes on the southern side of the central Himalayas, Nepal

Novice efforts have been made in order to study the regional distribution of land surface heat fluxes on the southern side of the central Himalayas utilizing high-resolution remotely sensed products, but these have been on instantaneous scale. In this study the Surface Energy Balance System model is used to obtain annual averaged maps of the land surface heat fluxes for 11 years (2003–2013) and study their annual trends on the central Himalayan region. The maps were derived at 5 km resolution using monthly input products ranging from satellite derived to Global Land Data Assimilation System meteorological data. It was found that the net radiation flux is increasing as a result of decreasing precipitation (drier environment). The sensible heat flux did not change much except for the northwestern High Himalaya and High Mountains. In northwestern High Himalaya sensible heat flux is decreasing because of decrease in wind speed, ground-air temperature difference, and increase in winter precipitation, whereas in High Mountains it is increasing due to increase in ground-air temperature difference and high rate of deforestation. The latent heat flux has an overall increasing trend with increase more pronounced in the lower regions compared to high elevated regions. It has been reported that precipitation is decreasing with altitude in this region. Therefore, the increasing trend in latent heat flux can be attributed to increase in net radiation flux under persistent forest cover and irrigation land used for agriculture.

Comparison of Different Generation Mechanisms of Free Convection between Two Stations on the Tibetan Plateau

Based on high-quality data from eddy covariance measurements at the Qomolangma Monitoring and Research Station for Atmosphere and Environment (QOMS) and the Southeast Tibet Monitoring and Research Station for Environment (SETS), near-ground free convection conditions (FCCs) and their characteristics are investigated. At QOMS, strong thermal effects accompanied by lower wind speeds can easily trigger the occurrence of FCCs. The change of circulation from prevailing katabatic glacier winds to prevailing upslope winds and the oscillation of upslope winds due to cloud cover are the two main causes of decreases in wind speed at QOMS. The analysis of results from SETS shows that the most important trigger mechanism of FCCs is strong solar heating. Turbulence structural analysis using wavelet transform indicates that lower-frequency turbulence near the ground emerges from the detected FCCs both at QOMS and at SETS. It should be noted that the heterogeneous underlying surface at SETS creates large-scale turbulence during periods without the occurrence of FCCs. Regarding datasets of all seasons, the distribution of FCCs presents different characteristics during monsoonal and non-monsoonal periods.摘要利用珠穆朗玛大气与环境观测站(QOMS)及藏东南高山环境综合观测研究站(SETS)的经质量控制后并被质量评价为高质量的涡度相关观测数据, 对近地层自由对流条件的(FCCs)产生机制及其特征进行了分析. 在QOMS, 较强的加热作用配合低风速容易触发FCCs. 上午, 盛行弱的下山冰川风转换为盛行上坡风l上坡风建立以后, 当有云影响时, 加热小幅减弱(未减弱到使上坡风消失的程度), 风速减小. 这是QOMS局地环流中风速减小的两个主要原因. SETS的分析结果则显示, 该站FCCs的主要触发机制是较强的太阳辐射加热. 利用小波分析方法, 对两个站的近地层湍流结构进行了分析, 两个站的结果均显示存在一些较大尺度的湍流. 另外, SETS的非均匀下垫面导致在没有发生FCCs的时间段里, 出现了许多大尺度湍流, 这可能是造成该站能量不闭合的一个主要原因. 对两个站点全年FCCs发生时间的统计结果显示, 由于环流调整, 季风期与非季风期存在显著区别: QOMS, 季风期比非季风期FCCs发生时间晚, 发生频率低;SETS, 季风期FCCs发生时间不再主要集中在上午.

Physical controls on half-hourly, daily, and monthly turbulent flux and energy budget over a high-altitude small lake on the Tibetan Plateau: Evaporation Over a High-Altitude Lake

Precise measurements of evaporation and understanding of the physical controls on turbulent heat flux over lakes have fundamental significance for catchment-scale water balance analysis and local-scale climate modeling. The observation and simulation of lake-air turbulent flux processes have been widely carried out, but studies that examine high-altitude lakes on the Tibetan Plateau are still rare, especially for small lakes. An eddy covariance (EC) system, together with a four-component radiation sensor and instruments for measuring water temperature profiles, was set up in a small lake within the Nam Co basin in April 2012 for long-term evaporation and energy budget observations. With the valuable measurements collected during the ice-free periods in 2012 and 2013, the main conclusions are summarized as follows: First, a bulk aerodynamic transfer model (B model), with parameters optimized for the specific wave pattern in the small lake, could provide reliable and consistent results with EC measurements, and B model simulations are suitable for data interpolation due to inadequate footprint or malfunction of the EC instrument. Second, the total evaporation in this small lake (812 mm) is approximately 200 mm larger than that from adjacent Nam Co (approximately 627 mm) during their ice-free seasons. Third, wind speed shows significance at temporal scales of half hourly, whereas water vapor and temperature gradients have higher correlations over temporal scales of daily and monthly in lake-air turbulent heat exchange. Finally, energy stored during April to June is mainly released during September to November, suggesting an energy balance closure value of 0.97.