DOI: 10.5281/zenodo.17034718
title: A global long-term (2002–2022) C-band vegetation optical depth record retrieved after merging AMSR-E, AMSR2 and WindSat

description: The Version 1.0 AMSR-E/AMSR2/WindSat C-VOD is a global long-term (2002–2022) passive C-band vegetation optical depth (C-VOD) product retrieved from brightness temperature (TB) observations after merging AMSR-E (Advanced Microwave Scanning Radiometer for EOS Aqua, 2002–2011), AMSR2 (Advanced Microwave Scanning Radiometer 2, 2012–present) and WindSat (2003-2020). It exhibited temporal consistency between the three sensors period, and has highly consistent spatial fitting relationships with various vegetation variables including Above Ground Biomass (AGB), canopy height, Normalized Difference Vegetation Index (NDVI) and Enhanced Vegetation Index (EVI).
description: Vegetation optical depth (VOD) based on microwave remote sensing has become an essential indicator for monitoring large-scale vegetation conditions. The relatively long-term record of satellite-based passive C-band observations offers potential for assessing dynamic vegetation changes. Due to the limited operational lifetime of individual microwave sensors, retrieving a long-term C-band VOD (C-VOD) dataset often requires combining multiple sensors, such as Advanced Microwave Scanning Radiometer for EOS Aqua (AMSR-E, 2002-2011), Advanced Microwave Scanning Radiometer 2 (AMSR2, 2012-present) and WindSat (2003-2020). Retrieving a long-term C-VOD from their C-band brightness temperature (TB) observations faces two major challenges: (i) non-negligible systematic biases between the TB observations among the three sensors; (ii) TB from different sensors may not be strongly associated, especially in tropical forests with small seasonal variations. To address these challenges, we employed a combined inter-calibration method, using linear regression in sparse vegetation and linear rescaling in dense vegetation, to merge TB from three sensors. Results showed that: (i) in undisturbed dense forests where minimal emissivity variation can be assumed, the merged TB (2002-2022) exhibited strong temporal correlations with skin temperature (H polarization: R = 0.90, V polarization: R = 0.86); (ii) the merged C-VOD retrieved from the merged TB exhibited substantially improved temporal consistency across three sensors, reducing global discrepancies between AMSR-E and AMSR2 from 6.20% to 0.34%. Furthermore, the merged C-VOD showed stable long-term consistency across sensors, with paired t-tests indicating no significant differences (P-value > 0.01) at the global and vegetation-type scales, confirming reliable cross-sensor continuity. (iii) The merged C-VOD showed stronger temporal correlations with NDVI, EVI, and site-level GPP across more vegetated areas globally, and exhibited higher and more stable spatial correlations with vegetation variables across sensor transitions compared to the existing merged C-VOD product.
description: Data File Information

AMSR-E/AMSR2/WindSat C-VOD provides daily descending (overpass at 01:30 a.m. Local Solar Time) VOD in the NetCDF format with the spatial resolution of 0.25° and with the WGS84 project for 21 years (2002-2022). 

Naming convention

PM1AME_IB_VOD_YYYYMMDD_D_C_V1.ncSatellite: PM1(AQUA)Sensor: AME(AMSR-E)Producer: IB (Fixed Value)Product name: VOD (Fixed Value)Observation time: YYYYMMDD (UTC time)Orbit direction: D (Descending)Frequency channel: C bandVersion: V1

GW1AM2_IB_VOD_YYYYMMDD_D_C_V1.ncSatellite : GW1(GCOM-W1) Sensor: AM2(AMSR2)Producer: IB (Fixed Value)Product name: VOD (Fixed Value)Observation time: YYYYMMDD (UTC time)Orbit direction: D (Descending)Frequency channel: C bandVersion: V1

WindSat_IB_VOD_YYYYMMDD_D_C_V1.ncSensor: WindSatProducer: IB (Fixed Value)Product name: VOD (Fixed Value)Observation time: YYYYMMDD (UTC time)Orbit direction: D (Descending)Frequency channel: C bandVersion: V1

Data Format

All data are saved as 32-bit single point numbers in NetCDF format. Each file contains a 720 by 1440 entry map. 

lat: latitude (°)lon: longitude (°)Optical_Thickness_Nad: Vegetation optical depthQuality_Flag: Quality Flag

Table 1. Quality Flag descriptions.




8-bit flag
Indication


00000000
Data OK


00000001
Missing data because of no valid TB


00000010
Missing data because of no valid ERA5 data


00000100
Missing data because of water polluted or frozen




RMSE: RMSE between measured TB and modeled TB (K)Scene_Flags: Scene FlagsTable 2. Scene Flags descriptions.




8-bit flag
Indication


00000001
Moderate Topo


00000010
Strong Topo


00000100
Polluted scene (water + urban + ice > 10% of the pixel)


00001000
Frozen scene, ERA5 skin temperature < 273.15 K




Soil_Moisture: Soil Moisture (m3/m3)
description: Data Filtering

For VOD applications: RMSE should be < 5 K and QF should be equal to 0, here QF is Quality Flag. The normal range of VOD values is [0, 1.5].Daily VOD values can be slightly negative. This is expected from the point of view of the minimization process. Daily negative values were not set to zero here as they can be used to compute yearly average. After yearly averaging, negative yearly median VOD values, should be set equal to zero. These values are mostly distributed in Barren areas.
description: Questions?

Email: leifan33@swu.edu.cn or wmeng_jia@163.com.