The Third Pole Environment centered on the Tibetan plateau and the Himalayas feeds Asia’s largest rivers which provide water to 1.5 billion people across ten countries. Due to its high elevation, TPE plays a significant role in global atmospheric circulation and is highly sensitive to climate change. Intensive exchanges of water and energy fluxes take place between the Asian monsoon, the plateau land surface (lakes, glaciers, snow and permafrost) and the plateau atmosphere at various temporal and spatial scales, but a fundamental understanding of the details of the coupling is lacking especially at the climate scale.

Based on in-situ measurements, satellite remote sensing and numerical modeling, several main achievements have been acquired to promote the understanding of water and energy cycles over the Tibetan Plateau. (1) In-situ measurements: The warm season characteristics of turbulence structure and transfer of turbulent kinetic energy over alpine wetlands and alpine meadow are analyzed. We found that the turbulence intensities decrease rapidly with increasing wind velocity under conditions of wind velocity smaller than 2 m s^-1 and the pulse of CO₂ flux is very small at noon time because of the high temperatures. We also identified that variations in soil moisture had important effects on carbon exchange in the alpine steppe ecosystem. Both the photosynthesis and respiration were active under high soil moisture content, and suppressed during periods of water shortage. Further, precise measurements of evaporation and understanding of the physical controls on turbulent heat flux at different time scales over a high-elevation small lake are also performed. A total evaporation value of 812 mm is reported for the small lake and the energy budget is generally closed during the open water period. Also we analyzed the variability and trends of daily precipitation extremes over the northern and southern side of central Himalaya. The results suggest that increases in precipitation have been accompanied by an increasing frequency of extremes over the southern central Himalaya while no relation could be established between the precipitation extreme indices and circulation indices for higher altitudes. (2) Remote sensing: an accurate estimate of monthly mean LST based on averaging of the multidaily overpasses of MODIS sensors was established, with RMSE value of 2.65℃ and mean bias of smaller than 1 ℃. Combining satellite remote sensing data and surface meteorological forcing data, land surface heat fluxes at multi-spatiotemporal scales over the Tibetan Plateau have been achieved. The parameterization schemes for diffused and reflected downward shortwave radiation flux of the TESEBS model were improved by introducing the parameters sky-view factor (SVF) and terrain configuration factor (C_t). In addition, a parameterization approach of effective roughness length was introduced into the SEBS model to account for subgrid-scale topographical influences. The results show that sensible heat flux decreased overall while latent heat flux increased over the majority TP over 2001 to 2012. (3) Model simulation: Lake-air interactions at Nam Co lake were analyzed through evaluating two popular lake-air exchange models: a bulk aerodynamic transfer model (B Model) and a multi-layer model (M Model). It was found that both models underestimated turbulent fluxes. This was due to inaccurate values of the Charnock coefficient and the roughness Reynolds number which are both important parameters for calculating the roughness length for momentum over water. A new land surface model (LSM) with coupled snow and frozen soil physics was developed based on a hydrologically improved LSM (HydroSiB2) and the results show significant improvements in snow internal process and soil water phase changes. Also Regional Atmospheric Modeling System (RAMS) was applied to the study of the effect of the topographical altitude of the Tibetan Plateau (TP) on a severe drought event which took place in eastern China from November 2008 to January 2009. (4) Hydrological model: Flow generated from Upper Indus Basin (UIB) originates in Hindukush-Karakoram-Himalaya region, Pakistan. The initial water supply reinstates after winter, depending upon the accumulated snow aggregate and subsequent temperature. Seasonal temperature dictates the state and fate of snow and glacier melt during summer. Recently, developing evidence of warming at high-mountains is accelerated regarded as Elevation dependent warming (EDW). We have identified trends, analyzed variability, and assessed changes in annual and seasonal maximum, minimum, mean and diurnal temperature range. (5) Training of young scientists in the area of climate and environment. Four PhD students have been sent to European partner for joint training. Two of them have got PhD degree in University of Twente under the supervision of European PI (Prof. Z.(Bob) Su) and Chinese PI (Prof. Yaoming Ma). Several European students from our partner also come to China regularly for joint field visiting and academic exchange.

The Third Pole Environment plays a significant role in global atmospheric circulation and is highly sensitive to climate change and its impact on Asia’s largest rivers which provide water to 1.5 billion people across ten countries. A fundamental understanding of intensive exchanges of water and energy fluxes between the Asian monsoon, the plateau land surface and the plateau atmosphere at various temporal and spatial scales especially at the climate scale is crucial to understand the role of TPE on global climate and the impact of climate change on TPE.

The CLIMATE-TPE project aims to improve understanding the interactions between the Asian monsoon, the plateau surface (including its permafrost and lakes) and the Tibetan plateau atmosphere in terms of water and energy budgets in order to assess and understand the causes of changes in cryosphere and hydrosphere, in relation to changes of plateau atmosphere in the Asian monsoon system, and to predict the possible changes in water resources in the Third Pole Environment. A core innovation of the project is to verify or falsify recent hypotheses (e.g. links between plateau heating and monsoon circulation, snow cover and monsoon strength, soil moisture and timing of monsoon) and projections of the changes of glaciers and permafrost in relation to surface and tropospheric heatings on the Tibetan plateau as precursors of monsoon pattern changes and glaciers retreat, and their impacts on water resources in South East Asia.

We use earth observation, in-situ measurements and modelling to advance process understanding relevant to monsoon scale predictions, and improve and develop coupled regional scale hydroclimatic models to explain different physical links and scenarios that cannot be observed directly. Three work-packages (WP) are defined to address three specific objectives. 1) advancement of the understanding of microwave scattering and emission under complex terrains with permafrost and freeze – thawing conditions. The focus is to reduce current uncertainties in microwave satellite observations over complex terrain and improve retrieval accuracies of soil moisture and freeze-thaw states by deploying in-situ observations, laboratory experiment and numerical modelling. 2) Advancement of physical understanding and quantification of changes of water and energy budgets in the TPE. The focus here is to integrate current understandings in the mechanism of changes in water and energy budget in TPE using satellite data products and numerical modelling. Objective 3) Advancement of quantifying changes in surface characteristics and monsoon interactions. All variables related to water and energy budgets in TPE will be subject to systematic analysis to ensure their consistence in terms of climate data records. The variables will include albedo, vegetation coverage, soil thermal and hydraulic properties, LST, soil moisture, lake levels and land use changes among others.

In this contribution we focus on WP1: Observation and modelling of microwave scattering and emission under complex terrains and including permafrost and freeze and thawing.

Since 2006 the Tibetan plateau observatory for soil moisture and soil temperature (Tibet-Obs, Su et al., 2011, HESS) has been in operation and has provided valuable dataset for land-atmosphere process studies. The networks and collected data have been used for calibration and validation of satellite soil moisture retrieval algorithms and data products as well as for improving numerical model parameterizations (Su et al., 2013, JGR; Zheng et al., 2015a, b, JHM; 2017a, JHM, b, JGR) and for understanding passive and active microwave signals (Dente et al., 2015, RSE; Wang et al., 2016, JAG; Lv et al., 2014, RSE). Most recently an in-situ microwave radiometer (ELBARA III from ESA) has been operating at the Maqu site of the Tibet-Obs, as such coherent process observation, process modeling and radiative transfer modeling can be conducted to examine land-atmosphere interactions. We report here recent results of these experiments in combined radiative transfer and heat-water transfer processes (Zheng et al., 2017, TGRS) and in understanding SMOS/SMAP observation signals and data products (Lv et al., 2018, RS).

Young scientists engaged in this project:

1. Dr. Rogier van der Velde, University of Twente, email: r.vandervelde@utwente.nl, remote sensing of soil moisture
2. Dr. Yijian Zeng, University of Twente, email: y.zeng@utwente.nl, data assimilation
3. Dr. Xin Wang, Northwest Institute of Eco-Environment and Resources, Chinese Academy of Sciences, Lanzhou, China, remote sensing of soil moisture
4. Dr. Xuelong Chen (ITP) land-atmosphere interactions, boundary layer processes
5. Dr. Donghai Zheng (ITP) water cycle in the upper Yellow River basin, hydrological modelling
6. Shaoning Lv (PhD student, UT-NIEER) Microwave emission and soil moisture, SMOS/SMAP signals
7. Qiang Wang (PhD student, UT) Soil moisture monitoring using Aquarius data
8. Binbin Wang (PhD student, UT-ITP) Energy balance of high plateau lakes
9. Xu Yuan (PhD student, UT) The role of sea surface salinity in the Indian Ocean and its effect on the South Asian monsoon
10. Hong Zhao (PhD student, UT) Retrieval of soil thermal and hydraulic parameters from satellite observations
11. Dongyu Jia (PhD student, NIEER) land-atmosphere interaction study in cold region
12. Prof. Weiqiang Ma, Institute of Tibetan Plateau, CAS, email: wqma@itpcas.ac.cn, remote sensing and atmospheric modelling of land-atmosphere processes
13. Lian Liu (PhD student, ITP) Retrieval of thermal and hydraulic parameters from satellite observations on the glaciers
14. Dr. Cristina Aguilar, University of Cordoba, e-mail: caguilar@uco.es, ecohydrological modelling
15. Dr. Rafael Pimentel(postdoc, UC)remotely sensed data fusion and assimilation for snow and hydrological modelling
16. Gabriel Delgado Leal(PhD student, UC) Water and energy fluxes regime in snow mountain regions
17. Marta Sáenz de Rodrigáñez (PhD student, UC) Remote sensing data fusion to generate long and high resolution time series of snow and hydrological variables
18. Dr. Jian Peng (LMU/OU) Thermal and microwave remote sensing of surface energy and water fluxes
19. Dr. Christiaan van der Tol, University of Twente, email: c.vandertol@utwente.nl, remote sensing of fluorescence
20. Jan Hofste (PhD student, UT),University of Twente, email: j.g.hofste@utwente.nl, remote sensing of land surface by scatterometry and spectroscopy

A global daily evapotranspiration product without spatial-temporal gaps for 2000-2017 is delivered by using an energy balance (EB) algorithms and MODIS satellite data. A global turbulent exchange parameterization scheme was developed and used in an energy balance model, which uses land-air temperature gradient to estimate the turbulent sensible heat (H), and take the latent heat flux as a residual of the available energy (net radiation minus ground heat flux) and sensible heat. It provides us with the first ever moderate resolution estimates of ET without spatial-temporal gaps on a global scale. The performance of evapotranspiration (ET) data has been evaluated in comparison to 230 flux sites measurements representative of a broad range of biomes and climates at the global scale. The gap-filling algorithm reproduces observed ET with reasonable accuracy. The daily ET product has a mean bias of 0.04 mm/day, with the RMSE value of 1.56 (±0.25) mm/day.

Surface soil moisture (SSM) plays a significant role in various domains of science, including agriculture, hydrology, meteorology and ecology. However, the spatial resolution of microwave SSM products is too coarse for regional applications. This study estimates SSM directly from data of the Chinese geostationary meteorological satellite FY-2E, without establishing empirical relationships between SSM measurements and satellite derived proxies of SSM. The derived SSM has a spatial resolution of 5 km and is based on an elliptical-new SSM retrieval model developed from the synergistic use of the diurnal cycles of Land Surface Temperature (LST) and Net Surface Shortwave Radiation (NSSR). Validation of the model is conducted based on ground measurements over the source area of the Yellow River (SAYR) on the northeastern Tibet Plateau. The FY-2E-derived SSM using the elliptical-new model exhibited good consistency with the ground measurements, with R of 0.845, RMSE of 0.064 m³/m³ and bias of 0.017 m³/m³. In addition, since the spatial resolution of microwave SSM products is too coarse, various downscaling methods are proposed to improve the spatial information. In this study, the CCI SSM product is downscaled to 5 km through a simple vegetation-temperature-condition-index (VTCI) method, which is simpler in terms of input requirements and implementation and has similar accuracy compared to other downscaling methods. The comparison of the VTCI downscaling model against in-situ measurement in Maqu, Luqu and Ruoergai also shows good agreement with R of 0.73, RMSE of 0.08 m³/m³ and bias of 0.04 m³/m³. Furthermore, the spatial patterns of elliptical-new SSM retrieval and VTCI downscaled SSM are also compared. The results show that FY-2E-derived SSM is similar to the downscaled SSM, with SSM over the Eastern region higher where has lakes than SSMs in the west. Lowest SSMs from both methods appear in the northernmost region. In order to provide more accurate SSM for these two methods, high accuracy vegetation indexes, LST and NSSR are still needed.

There are tens of thousands of lakes on the Tibetan Plateau and lakes show significant influences on catchment scale water and heat budget and influence local climate modeling. However, the observation and simulation of the high-elevation lakes are still quite limited. Thus, based on eddy covariance observations and meteorological data over open water periods from a small lake in 2012-2013 on the Tibetan Plateau, we achieved the goals of understanding the characteristics and driving forces of lake-air interaction process, obtaining the evaporation and energy budget, and testing evaporation estimation methods at temporal scale of 10 days over high altitude small lakes. The optimized parameters of roughness length for momentum are suitable for lake-atmosphere heat flux simulation by bulk aerodynamic transfer method. Wind speed shows high correlations at temporal scale of 30 minutes while temperature gradient and water vapor gradient has much higher correlations at temporal scales of daily and monthly. Under neutral conditions, the water vapor gradients have no influence on latent heat flux. The accumulated heat during April to August is fast released during September to November. The average evaporation over the entire open water period is 812 mm and the energy budget is generally closed with a closure ratio of 0.97. The constructed data series provide a good data set for evaporation methods evaluation. The energy budget based method show much better performances than methods of radiation based and Dalton type.