ESA CCI SM MODEL-INDEPENDENT Long-term Climate Data Record of Surface Soil Moisture from merged multi-satellite observations

This dataset was produced with funding from the European Space Agency (ESA) Climate Change Initiative (CCI) Plus Soil Moisture Project (CCN 3 to ESRIN Contract No: 4000126684/19/I-NB "ESA CCI+ Phase 1 New R&D on CCI ECVS Soil Moisture").  Project website: https://climate.esa.int/en/projects/soil-moisture/

This dataset contains information on the Surface Soil Moisture (SM) content derived from satellite observations in the microwave domain.

Abstract

The MODEL-INDEPENDENT (or "MODELFREE") product of the ESA CCI SM v9.1 science data suite provides - similar to the COMBINED product - global, harmonized daily satellite soil moisture measurements from both radar and radiometer observations. This product contains soil moisture estimates at 0.25-degree spatial resolution, and covers the period from 2002-2023. Soil moisture is derived from observations of 13 different active and passive satellites operating across various frequency bands (K, C, X, and L-band). Unlike the COMBINED product, for which soil moisture fields from the GLDAS Noah model dataset are used to harmonize individual satellite sensor measurements, the MODELFREE product utilizes a satellite-only scaling reference dataset (Madelon et al., 2022). This reference incorporates gap-filled soil moisture derived from AMSR-E (2002-2010) and from intercalibrated SMAP/SMOS brightness temperature data (2010-2023). The merging algorithm employed is consistent with that of the v9.1 COMBINED product. The new scaling reference leads to significantly different absolute soil moisture values, especially in latitudes above 60 °N. Data from the SMMR, SSMI and ERS missions are not included in this product.

This product is in its early development stage and should be used with caution, as it may contain incomplete or unvalidated data.

Summary

First version of a model-independent version of the ESA CCI SM COMBINED product

2002-2023, global, 0.25 deg. resolution

GLDAS Noah (model) is replaced with a purely satellite-based scaling reference

Different absolute value range compared to the COMBINED product is expected due to the different scaling reference used

Known issues

A temporal inconsistency is observed between the AMSR-E and SMOS period (at 01-2010). This can affect long-term trends in the data

In the period from 01-2002 to 06-2002 no data are available above 37 °N and below 37 °S respectively (all measurements in this period are from the TRMM Microwave Imager)

Technical Details

The dataset provides global daily estimates for the 2002-2023 period at 0.25° (~25 km) horizontal grid resolution. Daily images are grouped by year (YYYY), each subdirectory containing one netCDF image file for a specific day (DD), month (MM) in a 2-dimensional (longitude, latitude) grid system (CRS: WGS84). The file name has the following convention:

ESACCI-SOILMOISTURE-L3S-SSMV-COMBINED_MODELFREE-YYYYMMDD000000-fv09.1.nc

Each netCDF file contains 3 coordinate variables (WGS84 longitude, latitude and time stamp), as well as the following data variables:

sm: (float) The Soil Moisture variable reflects estimates of daily average volumetric soil moisture content (m3/m3) in the soil surface layer (~0-5 cm) over a whole grid cell (0.25 degree).

sm_uncertainty: (float) The Soil Moisture Uncertainty variable reflects the uncertainty (random error) of satellite observations. Derived using triple collocation analysis.

dn_flag: (int) Indicator for satellite orbit(s) used in the retrieval (day/nighttime). 1=day, 2=night, 3=both

flag: (int) Indicator for data quality / missing data indicator. For more details, see netcdf attributes.

freqbandID: (int) Indicator for frequency band(s) used in the retrieval. For more details, see netcdf attributes.

mode: (int) Indicator for satellite orbit(s) used in the retrieval (ascending, descending)

sensor: (int) Indicator for satellite sensor(s) used in the retrieval. For more details, see netcdf attributes.

t0: (float) Representative time stamp, based on overpass times of all merged satellites.

Additional information for each variable is given in the netCDF attributes.

Software to open netCDF files

These data can be read by any software that supports Climate and Forecast (CF) conform metadata standards for netCDF files, such as:

Xarray (python)

netCDF4 (python)

esa_cci_sm (python)

Similar tools exists for other programming languages (Matlab, R, etc.)

Software packages and GIS tools can open netCDF files, e.g. CDO, NCO, QGIS, ArCGIS

You can also use the GUI software Panoply to view the contents of each file

References

R. Madelon et al., “Toward the Removal of Model Dependency in Soil Moisture Climate Data Records by Using an L-Band Scaling Reference," in IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol. 15, pp. 831-848, 2022, doi: 10.1109/JSTARS.2021.3137008.

Related Records

The following records are all part of the Soil Moisture Climate Data Records from satellites community

1

ESA CCI SM RZSM Root-Zone Soil Moisture Record  

10.48436/v8cwj-jk556

2

ESA CCI SM GAPFILLED Surface Soil Moisture Record 

10.48436/hcm6n-t4m35

本数据集由欧洲空间局（ESA）气候变化倡议（CCI）Plus土壤湿度项目资助产生，相关合同为ESRIN合同编号4000126684/19/I-NB，项目编号CCN 3，项目全称：ESA CCI+ Phase 1 New R&D on CCI ECVS Soil Moisture。项目官网：https://climate.esa.int/en/projects/soil-moisture/

本数据集包含基于微波域卫星观测反演得到的地表土壤湿度（Surface Soil Moisture，SM）信息。

摘要
欧洲空间局CCI土壤湿度v9.1科学数据集的无模型（MODELFREE）产品，与组合（COMBINED）产品类似，提供来自雷达和辐射计观测的全球统一化每日卫星土壤湿度测量数据。该产品的空间分辨率为0.25度，时间覆盖范围为2002年至2023年。土壤湿度数据源自13颗运行于不同频段（K、C、X及L波段）的主动及被动卫星观测。与组合产品不同——组合产品采用全球陆面数据同化系统（GLDAS）Noah模型数据集的土壤湿度场来协调各卫星传感器的测量数据——无模型产品使用纯卫星缩放参考数据集（Madelon等人，2022）。该参考数据集包含来自AMSR-E（2002-2010年）的间隙填充土壤湿度数据，以及经过相互校准的SMAP/SMOS亮温数据（2010-2023年）。所采用的融合算法与v9.1版本的组合产品一致。新的缩放参考会导致绝对土壤湿度值出现显著差异，尤其是在北纬60度以上区域。本产品未包含SMMR、SSMI及ERS任务的观测数据。
本产品尚处于早期开发阶段，使用时需谨慎，可能存在不完整或未经验证的数据。

概述
本数据集为ESA CCI SM组合产品的首个独立模型版本，具备以下特点：
- 时间覆盖2002-2023年，全球范围，空间分辨率0.25度；
- 采用纯卫星缩放参考替代GLDAS Noah模型；
- 由于使用了不同的缩放参考，预期其绝对数值范围与组合产品存在差异。

已知问题
1. 2010年1月期间，AMSR-E与SMOS时段存在时间不一致性，这可能影响数据的长期趋势；
2. 在2002年1月至2002年6月期间，北纬37度以北及南纬37度以南分别无可用数据（此时段所有测量数据均来自TRMM微波成像仪）。

技术细节
本数据集提供2002-2023年全球范围的每日估计值，空间分辨率为0.25度（约25km）的水平网格。每日数据按年份（YYYY）分组，每个子目录包含对应某日（DD）、某月（MM）的netCDF图像文件，采用二维（经度、纬度）网格系统（坐标系：WGS84，世界大地测量系统1984）。文件名遵循以下格式规范：
ESACCI-SOILMOISTURE-L3S-SSMV-COMBINED_MODELFREE-YYYYMMDD000000-fv09.1.nc

每个netCDF文件包含3个坐标变量（WGS84经度、纬度及时间戳），以及以下数据变量：
- sm：（浮点型）土壤湿度变量，反映整个0.25度网格单元内表层土壤（约0-5cm）的日平均体积土壤湿度估计值，单位为m³/m³。
- sm_uncertainty：（浮点型）土壤湿度不确定性变量，反映卫星观测的不确定性（随机误差），采用三重配置分析法推导得到。
- dn_flag：（整型）反演所用卫星轨道指示符（昼/夜间），1=昼间，2=夜间，3=昼夜间均有。
- flag：（整型）数据质量/缺失数据指示符，详细说明请参见netCDF属性。
- freqbandID：（整型）反演所用频段指示符，详细说明请参见netCDF属性。
- mode：（整型）反演所用卫星轨道指示符（升轨、降轨）。
- sensor：（整型）反演所用卫星传感器指示符，详细说明请参见netCDF属性。
- t0：（浮点型）基于所有融合卫星过境时间的代表性时间戳。

每个变量的附加信息详见netCDF属性文件。

netCDF文件读取软件
支持符合《气候与预报（CF）》元数据标准的netCDF文件的软件均可读取本数据集，例如：
- Python库：Xarray、netCDF4、esa_cci_sm；
- 其他编程语言亦有对应工具（如Matlab、R等）；
- 软件包及GIS工具：CDO、NCO、QGIS、ArcGIS；
也可使用GUI软件Panoply查看各文件内容。

参考文献
R. Madelon等人，"通过使用L波段缩放参考去除土壤湿度气候数据记录中的模型依赖性"，载于《IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing》，第15卷，第831-848页，2022年，DOI: 10.1109/JSTARS.2021.3137008。

相关记录
以下记录均属于卫星土壤湿度气候数据记录社区：
1. ESA CCI SM RZSM 根区土壤湿度记录，DOI:10.48436/v8cwj-jk556
2. ESA CCI SM GAPFILLED 地表土壤湿度记录，DOI:10.48436/hcm6n-t4m35