Estimation of Land Surface Temperature over the Tibetan Plateau Using AVHRR and MODIS Data

Estimation of large-scale land surface temperature from satellite images is of great importance for the study of climate change. This is especially true for the most challenging areas, such as the Tibetan Plateau （TP）. In this paper, two split window algorithms （SWAs）, one for the NOAA’s Advanced Very High Resolu-tion Radiometer （AVHRR）, and the other for the Moderate Resolution Imaging Spectroradiometer （MODIS）, were applied to retrieve land surface temperature （LST） over the TP simultaneously. AVHRR and MODIS data from 17 January, 14 April, 23 July, and 16 October 2003 were selected as the cases for winter, spring, summer, and autumn, respectively. Firstly, two key parameters （emissivity and water vapor content） were calculated at the pixel scale. Then, the derived LST was compared with in situ measurements from the Coordinated Enhanced Observing Period （CEOP） Asia-Australia Monsoon Project （CAMP） on the TP （CAMP/Tibet） area. They were in good accordance with each other, with an average percentage error （PE） of 10.5% for AVHRR data and 8.3% for MODIS data, meaning the adopted SWAs were applicable in the TP area. The derived LST also showed a wide range and a clear seasonal difference. The results from AVHRR were also in agreement with MODIS, with the latter usually displaying a higher level of accuracy.

SEBS模型在黄淮海地区地表能量通量估算中的应用

在对SEBS模型的有关参数进行订正的基础上,利用MODIS遥感数据结合地表气象观测数据,对黄淮海地区地表能量通量进行了估算。将SEBS估算结果先与北京顺义定量遥感综合试验(QRSLSP/Shunyi)实测结果进行对比分析,然后与郑州的大孔径闪烁仪(LAS)观测的感热通量进行对比分析。结果表明,SEBS估算的通量与二者的实测结果相当一致。因此,利用改进后的SEBS模型估算的黄淮海地区地表能量通量具有一定的精度,可满足区域日蒸散量计算等的需求。

New evidence for the links between the local water cycle and the underground wet sand layer of a mega-dune in the Badain Jaran Desert, China

Scientists and the local government have great concerns about the climate change and water resources in the Badain Jaran Desert of western China. A field study for the local water cycle of a lake-desert system was conducted near the Noertu Lake in the Badain Jaran Desert from 21 June to 26 August 2008. An underground wet sand layer was observed at a depth of 20–50 cm through analysis of datasets collected during the field experiment. Measurements unveiled that the near surface air humidity increased in the nighttime. The sensible and latent heat fluxes were equivalent at a site about 50 m away from the Noertu Lake during the daytime, with mean values of 134.4 and 105.9 W/m2 respectively. The sensible heat flux was dominant at a site about 500 m away from the Noertu Lake, with a mean of 187.7 W/m2, and a mean latent heat flux of only 26.7 W/m2. There were no apparent differences for the land surface energy budget at the two sites during the night time. The latent heat flux was always negative with a mean value of –12.7 W/m2, and the sensible heat flux was either positive or negative with a mean value of 5.10 W/m2. A portion of the local precipitation was evaporated into the air and the top-layer of sand dried quickly after every rainfall event, while another portion seeped deep and was trapped by the underground wet sand layer, and supplied water for surface psammophyte growth. With an increase of air humidity and the occurrence of negative latent heat flux or water vapor condensation around the Noertu Lake during the nighttime, we postulated that the vapor was transported and condensed at the lakeward sand surface, and provided supplemental underground sand pore water. There were links between the local water cycle, underground wet sand layer, psammophyte growth and landscape evolution of the mega-dunes surrounding the lakes in the Badain Jaran Desert of western China.

Analysis of Turbulence Characteristics over the Northern Tibetan Plateau Area

Based on CAMP/Tibet [Coordinated Enhanced Observing Period （CEOP） Asia-Australia Monsoon Project （CAMP） on the Tibetan Plateau] turbulent data collected at the Bujiao （B J） site of the Nagqu area, the turbulent structure and transportation characteristics in the near surface layer during summer are analyzed. The main results show that the relationship between the normalized standard deviation of 3D wind speed and stability satisfies the similarity law under both unstable and stable stratifications. The relations of normalized standard deviation of temperature and specific humidity to stability only obey the ＂-1/3 power law＂ under unstable conditions. In the case of stable stratifications, their relations to stability are dispersing. The sensible heat dominates in the dry period, while in the wet period, the latent heat is larger than the sensible heat.

Characteristics of land-atmosphere energy and turbulentfluxes over the plateau steppe in central Tibetan Plateau

The land-atmosphere energy and turbulence exchange is key to understanding land surface processes on the Tibetan Plateau(TP). Using observed data for Aug. 4 to Dec. 3, 2012 from the Bujiao observation point(BJ) of the Nagqu Plateau Climate and Environment Station(NPCE-BJ), different characteristics of the energy flux during the Asian summer monsoon(ASM) season and post-monsoon period were analyzed. This study outlines the impact of the ASM on energy fluxes in the central TP. It also demonstrates that the surface energy closure rate during the ASM season is higher than that of the post-monsoon period. Footprint modeling shows the distribution of data quality assessments(QA) and quality controls(QC) surrounding the observation point. The measured turbulent flux data at the NPCE-BJ site were highly representative of the target land-use type. The target surface contributed more to the fluxes under unstable conditions than under stable conditions. The main wind directions(180° and 210°) with the highest data density showed flux contributions reaching 100%, even under stable conditions. The lowest flux contributions were found in sectors with low data density, e.g., 90.4% in the 360° sector under stable conditions during the ASM season. Lastly, a surface energy water balance(SEWAB) model was used to gap-fill any absent or corrected turbulence data. The potential simulation error was also explored in this study. The Nash-Sutcliffe model efficiency coefficients(NSEs) of the observed fluxes with the SEWAB model runs were 0.78 for sensible heat flux and 0.63 for latent heat flux during the ASM season, but unrealistic values of-0.9 for latent heat flux during the post-monsoon period.

两倍的青藏高原蒸散发增长速率

Estimation of evapotranspiration(ET_(a))change on the Tibetan Plateau(TP)is essential to address the water requirement of billions of people surrounding the TP.Existing studies have shown that ET_(a)estimations on the TP have a very large uncertainty.In this article,we discuss how to more accurately quantify ET_(a)amount and explain its change on the TP.ET_(a)change on the TP can be quantified and explained based on an ensemble mean product from climate model simulations,reanalysis,as well as ground-based and satellite observations.ET_(a)on the TP experienced a significant increasing trend of around 8.4±2.2 mm(10 a)^(-1)(mean±one standard deviation)during 1982–2018,approximately twice the rate of the global land ET_(a)(4.3±2.1 mm(10 a)^(-1)).Numerical attribution analysis revealed that a 53.8%TP area with the increased ET_(a)was caused by increased temperature and 23.1%part was due to soil moisture rising,because of the warming,melting cryosphere,and increased precipitation.The projected future increase in ET_(a)is expected to cause a continued acceleration of the water cycle until 2100.

An Air-to-Soil Transition Model for Discrete Scattering-Emission Modelling at L-Band

Topsoil structures and inhomogeneous distribution of moisture in the soil volume will induce dielectric discontinuities from air to bulk soil,which in turn may induce multiple and volume scattering and affect the microwave surface emission.In situ ELBARA-Ⅲ L-band radiometer observations of brightness temperature T_(B)^(p) (p=H or V polarization)at the Maqu site on the Eastern Tibetan Plateau are exploited to understand the effect of surface roughness on coherent and incoherent emission processes.Assisted with in situ soil moisture(SM)and temperature profile measurements,this study develops an air-to-soil transition(ATS)model that incorporates the dielectric roughness(i.e.,resulted from fine-scale topsoil structures and the soil volume)characterized by SM and geometric roughness effects,and demonstrates the necessity of the ATS model for modelling L-band T_(B)^(p).The Wilheit(1978)coherent and Lv et al.(2014)incoherent models are compared for determining the dielectric constant of bulk soil in the ATS zone and for calculating soil effective temperature T_(eff).The Tor Vergata discrete scattering model(TVG)integrated with the advanced integral equation model(AIEM)is used as the baseline model configuration for simulating L-band T_(B)^(p).Whereafter,the ATS model is integrated with the foregoing model for assessing its performance.Results show the ATS-based models reduce the underestimation of T_(B)^(p)(≈20-50 K)by the baseline simulations.Being dynamic in nature,the proposed dielectric roughness parameterization in the ATS model significantly improves the ability in interpreting T_(B)^(p) dynamics,which is important for improving SM retrieval at the global scale.