Long-term Continuous Red and Near-infrared Channel Reflectance from AVHRR, 1982-2023 (LCREF-AVHRR)

Usage Notes:This is the updated LCREF dataset (v3.2) consists of calibrated AVHRR surface reflectance record for the red and near-infrared channel. 

Key updates in version 3.2 include:

Improved Calibration: Enhanced consistency in calibration methods, addressing technical limitations in version 3.1 including applying more stringent quality filtering and snow masks.

Quality Flags: New quality flag layer enables users to identify whether a pixel is derived from observed surface reflectance (QA=0), high-quality gap-filled values (QA=1), lower-quality gap-filled based on the mean seasonal cycle (QA=2), or missing entirely (QA=3). We advice the user to rely only on observed and high-quality gap-filled values for their analyses.

Extension to include observations from the year of 2023.

The user can choose between LCREF-AVHRR and LCREF-MODIS for the overlapping period from 2001-2023. The two datasets are generally consistent during this overlapping period, although LCREF-MODIS shows a stronger greening trend between 2001-2023. For studies exploring the long-term vegetation dynamics, the user can either use only LCREF-AVHRR or use a blend dataset of LCREF-AVHRR and LCREF-MODIS as a sensitivity test. 

The LCREF-MODIS v3.2 (2001-2023) is available at 10.5281/zenodo.11657458.

The LCREF-AVHRR dataset was used as the input to generate LCSPP-AVHRR (previously known as LCSIF-AVHRR), and it can also be used to derive temporally consistant records of NDVI, NIRv, kNDVI, and other vegetation indices based on red and NIR surface reflectance variables. The user can access LCSPP products at:

LCSPP-AVHRR v3.2 (1982-2000): 10.5281/zenodo.7916850

LCSPP-AVHRR v3.2 (2001-2023): 10.5281/zenodo.11906675

LCSPP-MODIS v3.2(2001-2023): 10.5281/zenodo.11658088

A manuscript describing the technical details is available at https://arxiv.org/abs/2311.14987, which detailed the uses and limitations of the dataset. All data outputs from this study are available at 0.05° spatial resolution and biweekly temporal resolution in NetCDF format. Each month is divided into two files, with the first file “a” representative of the 1st day to the 15th day of a month, and the second file “b” representative of the 16th day to the last day of a month.

Abstract:

Satellite-observed solar-induced chlorophyll fluorescence (SIF) is a powerful proxy for the photosynthetic characteristics of terrestrial ecosystems. Direct SIF observations are primarily limited to the recent decade, impeding their application in detecting long-term dynamics of ecosystem function. In this study, we leverage two surface reflectance bands available both from Advanced Very High-Resolution Radiometer (AVHRR, 1982-2023) and MODerate-resolution Imaging Spectroradiometer (MODIS, 2001-2023). Importantly, we calibrate and orbit-correct the AVHRR bands against their MODIS counterparts during their overlapping period. Using the long-term bias-corrected reflectance data from AVHRR and MODIS, a neural network is trained to produce a Long-term Continuous SIF-informed Photosynthesis Proxy (LCSPP) by emulating Orbiting Carbon Observatory-2 SIF, mapping it globally over the 1982-2023 period. Compared with previous SIF-informed photosynthesis proxies, LCSPP has similar skill but can be advantageously extended to the AVHRR period. Further comparison with three widely used vegetation indices (NDVI, kNDVI, NIRv) shows a higher or comparable correlation of LCSPP with satellite SIF and site-level GPP estimates across vegetation types, ensuring a greater capacity for representing long-term photosynthetic activity.

使用说明：本数据集为更新至v3.2版本的LCREF数据集，包含红波段与近红外波段的定标AVHRR（先进甚高分辨率辐射计，Advanced Very High-Resolution Radiometer）地表反射率序列。
v3.2版本的核心更新内容如下：

1. 定标优化：优化了定标方法的一致性，解决了v3.1版本中的技术局限，包括采用更严格的质量过滤与雪覆盖掩膜方案。
2. 质量标记：新增质量标志层，支持用户识别像素类型：观测得到的地表反射率（QA=0）、高质量间隙填充值（QA=1）、基于季节循环均值的低质量间隙填充值（QA=2），以及完全缺失的数据（QA=3）。建议用户仅使用观测数据与高质量间隙填充值开展分析。
3. 数据延伸：新增了2023年的观测数据。

用户可在2001-2023年的重叠观测时段内，选择LCREF-AVHRR与LCREF-MODIS两款数据集。两款数据集在该时段内整体一致性较好，但LCREF-MODIS在2001-2023年间呈现出更强的绿化趋势。若开展长期植被动态研究，用户可仅使用LCREF-AVHRR数据集，或混合使用LCREF-AVHRR与LCREF-MODIS数据集进行敏感性测试。

LCREF-MODIS v3.2（2001-2023）的获取地址为：10.5281/zenodo.11657458。

LCREF-AVHRR数据集可作为生成LCSPP-AVHRR（原名为LCSIF-AVHRR）的输入数据，同时也可基于红波段与近红外波段地表反射率变量，推导得到时间序列一致的NDVI（归一化差异植被指数）、NIRv（近红外短波反射率植被指数）、kNDVI（核化归一化差异植被指数）等植被指数产品。LCSPP系列产品的获取地址如下：
- LCSPP-AVHRR v3.2（1982-2000）：10.5281/zenodo.7916850
- LCSPP-AVHRR v3.2（2001-2023）：10.5281/zenodo.11906675
- LCSPP-MODIS v3.2（2001-2023）：10.5281/zenodo.11658088

本研究的技术细节说明论文可访问https://arxiv.org/abs/2311.14987，其中详细阐述了数据集的使用方法与局限性。本研究生成的所有数据产品均为0.05°空间分辨率、双周时间分辨率的NetCDF（网络通用数据格式）文件。每月数据分为两个文件，以“a”为前缀的文件代表当月1日至15日的数据，以“b”为前缀的文件代表当月16日至月末的数据。

摘要：
卫星观测的太阳诱导叶绿素荧光（SIF，Solar-induced chlorophyll fluorescence）是表征陆地生态系统光合特征的有效替代指标。但直接的SIF观测仅局限于近十年，限制了其在生态系统功能长期动态监测中的应用。本研究利用了先进甚高分辨率辐射计（AVHRR，1982-2023年）与中分辨率成像光谱仪（MODIS，2001-2023年）均搭载的两个地表反射波段。关键之处在于，我们在两款传感器的重叠观测时段内，以MODIS反射率数据为基准，对AVHRR波段开展了定标与轨道校正。基于经过长期偏差校正的AVHRR与MODIS反射率数据，我们训练了神经网络，以模拟轨道碳观测站-2（OCO-2，Orbiting Carbon Observatory-2）的SIF数据，进而生成长期连续的SIF辅助光合替代指标LCSPP（Long-term Continuous SIF-informed Photosynthesis Proxy），并完成了1982-2023年全球范围的制图。与此前基于SIF的光合替代指标相比，LCSPP具备相当的反演性能，且可拓展至AVHRR的早期观测时段，这是其显著优势。进一步与三类广泛使用的植被指数（NDVI、kNDVI、NIRv）对比发现，LCSPP与卫星观测SIF及站点尺度总初级生产力（GPP，Gross Primary Productivity）估计值的相关性更高或相当，且在不同植被类型中表现稳定，因此具备更强的长期光合活动表征能力。