Evaluation of remote sensing and reanalysis soil moisture products on the Tibetan Plateau
FAN Keke1,2,3,, ZHANG Qiang1,2,3,, SHI Peijun1,2,3, SUN Peng4, YU Huiqian1,2,3
1. Key Laboratory of Environmental Change and Natural Disaster, Ministry of Education, Beijing Normal University, Beijing 100875, China
2. Academy of Disaster Reduction and Emergency Management, Beijing Normal University, Beijing 100875, China
3. Faculty of Geographical Science, Beijing Normal University, Beijing 100875, China
4. College of Territorial Resources and Tourism, Anhui Normal University, Wuhu 241002, Anhui, China
Creative Research Groups of National Natural Science Foundation of China, No.41621061;
National Science Foundation for Distinguished Young Scholars of China, No.51425903;
Soil water is the key link between land surface and atmosphere in water-heat exchange and it is the key element of water cycle. It is also the key control factor affecting the process of surface runoff. The Himalayan-Tibetan Plateau (HTP), also known as the "Asian Water Tower", is the source region of many Asian rivers. Meanwhile, HTP has direct impacts on its surrounding climate via hydro-meteorological processes, and on establishment and maintenance of Asian monsoon. This study collected observed soil moisture data from 100 in-situ soil moisture observatory stations and evaluated applicability of the available remote sensing and reanalysis soil moisture datasets such as ECV, ERA-Interim, MERRA, and Noah at different spatial resolutions (0.25°×0.25°, 0.5°×0.5°, 1°×1°) during different time intervals such as non-freezing and freezing periods. Statistical indicators such as R, RMSE and Bias were used to evaluate the performances of these remote sensing and reanalysis soil moisture datasets. The results indicated that: (1) All remote sensing and reanalysis soil moisture datasets except ERA can well estimate soil moisture changes of the Tibetan Plateau and the soil moisture changes are in generally good line with precipitation changes. In the Naqu region, however, the remote sensing and reanalysis soil moisture datasets substantially underestimate observed soil moisture. In space, MERRA and Noah are mostly consistent with the change of vegetation index, and can well estimate spatial distribution of soil moisture changes. (2) Soil moisture changes across most parts of the Tibetan Plateau are greatly influenced by precipitation changes. In addition, soil moisture changes in the western flank of the Tibetan Plateau and Himalayas are the combined results of melting snow/glaciers and precipitation. (3) Except in the Ngari region, soil moisture during non-freezing period is usually higher than that during freezing period. In the Naqu region, all remote sensing and reanalysis soil moisture datasets overestimate soil moisture amount during freezing periods, while they underestimate it during non-freezing periods. Besides, from a spatial scale viewpoint, at medium and large scales, remote sensing and reanalysis soil moisture datasets can better evaluate soil moisture availability compared with at small scale. This study provides a theoretical basis for selection of the right remote sensing and reanalysis soil moisture datasets for evaluation and analysis of soil moisture of the Tibetan Plateau.
2.3.2 ERA-Interim ERA-Interim数据是由欧洲中尺度气候预报中心（European Centre for Medium Range Weather Forecasts, ECMWF）制作的全球最新大气再分析数据（自1979年1月至今）,可实时获取。它是基于一种变分同化系统且融合了站点实测和卫星遥感数据而获得的同化数据集,包含每日0时、6时、12时和18时的土壤水数据,选择的空间分辨率为0.25°×0.25°。ERA-Interim包含四层土壤水数据（0~7 cm、7~28 cm、28~100 cm、100~289 cm）。多项研究表明,再分析数据与实测数据之间的差异是普遍存在的,在数值上的差异是无法避免的,这种差异主要是数值预报模式和同化方法等带来的系统性误差和观测资料的非一致性共同作用的结果。尽管ERA-Interim/Land数据中路面机制参数有所提高,但其时间跨度为1979-2010年,其不能完全覆盖实测站点数据的时间长度,为了充分利用实测数据,本文使用日平均的ERA-Interim上层土壤水（0~7 cm）数据。
2.3.3 MERRA MERRA（Modern Era-Retrospective Analysis for Research and Application）是由美国航空航天局（National Aeronautics and Space Application）为支持地球科学研究的需要而开发的再分析数据产品。MERRA是通过GEOS-5 DA系统获得,DA系统使用大量对地观测卫星产生长期的水文循环合成数据并使用最先进的GEOS-5流域水文陆面模型来分析陆面过程。本文使用上层（0~2 cm）逐小时土壤水数据,空间分辨率为0.625°×0.5°,首先通过线性插值将数据集采样到0.25°×0.25°,与其他数据集一致,然后通过算术平均获得每月土壤水数据。
Spatial distributions of (a) soil moisture for ECV, (b) soil moisture for ERA, (c) soil moisture for MERRA, (d) soil moisture for Noah, (e) precipitation and (f) NDVI in 2009-2014
Taylor diagram showing the standard deviation and correlation of in-situ observed and estimated soil moisture for all grids in (a) Ngari, (b) Naqu and (c) Maqu. Taylor diagram for different periods of frozen and unfrozen periods in (d) Ngari, (e) Naqu and (f) Maqu
ZhuoGa, DejiZhuoma, NimaJi.Distribution of soil moisture over the Qinghai-Tibetan Plateau and its effect on the precipitation in June and July over the mid-lower reaches of Yangtze River Basin. , 2017, 36(3): 657-666.
ShuklaJ, MintzY.Influence of land-surface evapotranspiration on the earth's climate. , 1982, 215(4539): 1498-1501.
Calculations with a numerical model of the atmosphere show that the global fields of rainfall, temperature, and motion strongly depend on the landsurface evapotranspiration. This confirms the long-held idea that the surface vegetation, which produces the evapotransporation, is an important factor in the earth's climate.
WangChenghai, DongWenjie, WeiZhigang.Anomaly feature of seasonal frozen soil variations on the Qinghai-Tibet Plateau. , 2002, 12(1): 99-107.
1 Introduction Soil freezing-thawing process in cryosphere plays an important role in regional climate and environmental change. Permafrost is a sensitive indicator (Pavlov, 1994). Because the seasonal freeze-thaw layer (activity layer) occurs in the u
Miralles DG, Holmes T R H, De Jeu R A M, et al. Global land-surface evaporation estimated from satellite-based observations. , 2011, 15(2): 453.
This paper outlines a new strategy to derive evaporation from satellite observations. The approach uses a variety of satellite-sensor products to estimate daily evaporation at a global scale and 0.25 degree spatial resolution. Central to this methodology is the use of the Priestley and Taylor (PT) evaporation model. The minimalistic PT equation combines a small number of inputs, the majority of which can be detected from space. This reduces the number of variables that need to be modelled. Key distinguishing features of the approach are the use of microwave-derived soil moisture, land surface temperature and vegetation density, as well as the detailed estimation of rainfall interception loss. The modelled evaporation is validated against one year of eddy covariance measurements from 43 stations. The estimated annual totals correlate well with the stations annual cumulative evaporation (lt;igt;Rlt;/igt;=0.80, lt;igt;Nlt;/igt;=43) and present a low average bias (鈭5%). The validation of the daily time series at each individual station shows good model performance in all vegetation types and climate conditions with an average correlation coefficient of lt;igt;lt;span style=text-decoration: overlinegt;Rlt;/spangt;lt;/igt;=0.83, still lower than the lt;igt;lt;span style=text-decoration: overlinegt;Rlt;/spangt;lt;/igt;=0.90 found in the validation of the monthly time series. The first global map of annual evaporation developed through this methodology is also presented.
DharssiI, Bovis KJ, MacphersonB, et al.Operational assimilation of ASCAT surface soil wetness at the Met Office. , 2011, 15(8): 2729-2746.
Currently, no extensive, near real time, global soil moisture observation network exists. Therefore, the Met Office global soil moisture analysis scheme has instead used observations of screen temperature and humidity. A number of new space-borne remote sensing systems, operating at microwave frequencies, have been developed that provide a more direct retrieval of surface soil moisture. These systems are attractive since they provide global data coverage and the horizontal resolution is similar to weather forecasting models. Several studies show that measurements of normalised backscatter (surface soil wetness) from the Advanced Scatterometer (ASCAT) on the meteorological operational (MetOp) satellite contain good quality information about surface soil moisture. This study describes methods to convert ASCAT surface soil wetness measurements to volumetric surface soil moisture together with bias correction and quality control. A computationally efficient nudging scheme is used to assimilate the ASCAT volumetric surface soil moisture data into the Met Office global soil moisture analysis. This ASCAT nudging scheme works alongside a soil moisture nudging scheme that uses observations of screen temperature and humidity. Trials, using the Met Office global Unified Model, of the ASCAT nudging scheme show a positive impact on forecasts of screen temperature and humidity for the tropics, North America and Australia. A comparison with in-situ soil moisture measurements from the US also indicates that assimilation of ASCAT surface soil wetness improves the soil moisture analysis. Assimilation of ASCAT surface soil wetness measurements became operational during July 2010.
SuZ, WenJ, DenteL, et al.The Tibetan Plateau observatory of plateau scale soil moisture and soil temperature (Tibet-Obs) for quantifying uncertainties in coarse resolution satellite and model products. , 2011, 15(7): 2303-2316.
SuZ, RosnayP, WenJ, et al.Evaluation of ECMWF's soil moisture analyses using observations on the Tibetan Plateau. , 2013, 118(11): 5304-5318.
An analysis is carried out for two hydrologically contrasting but thermodynamically similar areas on the Tibetan Plateau, to evaluate soil moisture analysis based on the European Centre for Medium-Range Weather Forecasts (ECMWF) previous optimum interpolation scheme and the current point-wise extended Kalman filter scheme. To implement the analysis, this study used two regional soil moisture and soil temperature networks (i.e., Naqu and Maqu) on the Tibetan Plateau. For the cold-semiarid Naqu area, both ECMWF soil moisture analyses significantly overestimate the regional soil moisture in the monsoon seasons. For the cold-humid Maqu network area, the ECMWF products have comparable accuracy as reported by previous studies in the humid monsoon period. The comparisons were made among the liquid soil moisture analysis from ECMWF, the ground station's measurements and the satellite estimates from the Advanced Scatterometer sensor. The results show reasonable performances of the ECMWF soil moisture analyses (i.e., both optimum interpolation and extended Kalman filter products) and the Advanced Scatterometer level 2 products, when compared to the in situ measurements.
YangK, Chen YY, QinJ.Some practical notes on the land surface modeling in the Tibetan Plateau. , 2009, 13(5): 687-701.
The Tibetan Plateau is a key region of land-atmosphere interactions, as it provides an elevated heat source to the middle-troposphere. The Plateau surfaces are typically characterized by alpine meadows and grasslands in the central and eastern part while by alpine deserts in the western part. This study evaluates performance of three state-of-the-art land surface models (LSMs) for the Plateau typical land surfaces. The LSMs of interest are SiB2 (the Simple Biosphere), CoLM (Common Land Model), and Noah. They are run at typical alpine meadow sites in the central Plateau and typical alpine desert sites in the western Plateau. <br><br> The identified key processes and modeling issues are as follows. First, soil stratification is a typical phenomenon beneath the alpine meadows, with dense roots and soil organic matters within the topsoil, and it controls the profile of soil moisture in the central and eastern Plateau; all models, when using default parameters, significantly under-estimate the soil moisture within the topsoil. Second, a soil surface resistance controls the surface evaporation from the alpine deserts but it has not been reasonably modeled in LSMs; an advanced scheme for soil water flow is implemented in a LSM, based on which the soil resistance is determined from soil water content and meteorological conditions. Third, an excess resistance controls sensible heat fluxes from dry bare-soil or sparsely vegetated surfaces, and all LSMs significantly under-predict the ground-air temperature gradient, which would result in higher net radiation, lower soil heat fluxes and thus higher sensible heat fluxes in the models. A parameterization scheme for this resistance has been shown to be effective to remove these biases.