European Space Administration Snow Climate Change Initiative (Snow_cci): Snow Water Equivalent (SWE) level 3C daily global climate research data package (CRDP) (1979 – 2020), version 2.0

This dataset contains v2.0 of the Daily Snow Water Equivalent (SWE) product from the European Space Administration (ESA) Climate Change Initiative (CCI) Snow project, at 0.1 degree resolution. Snow water equivalent (SWE) indicates the amount of accumulated snow on land surfaces, in other words the amount of water contained within the snowpack. The SWE product time series covers the period from 1979/01 to 2020/05. Northern Hemisphere SWE products are available at daily temporal resolution with alpine areas masked. The dataset is aimed to serve the needs of users working on climate research and monitoring activities, including the detection of variability and trends, climate modeling, and aspects of hydrology and meteorology. The Finnish Meteorological Institute is responsible for the SWE product development and generation. This version 2 dataset has some notable differences compared to the v1 data. In v2, passive microwave radiometer data are obtained from the recalibrated enhanced resolution CETB ESDR dataset (MEaSUREs Calibrated Enhanced-Resolution Passive Microwave Daily EASE-Grid 2.0 Brightness Temperature (CETB) Earth System Data Record (ESDR) https://nsidc.org/pmesdr/data-sets/), the grid spacing is reduced from 25 kilometers (km) to 12.5 km, and spatially and temporally varying snow density fields are used to adjust SWE retrievals in post processing. The output grid spacing is reduced from 0.25-degree to 0.10-degree WGS84 latitude / longitude to be compatible with other Snow_cci products. The time series has been extended by two years with data from 2018 to 2020 added. The ESA CCI phased product development framework allowed for a systematic analysis of these changes to the input data and snow density parameterization that occurred between v1 and v2 using a series of step-wise developmental datasets. In comparison with in-situ snow courses, the correlation and Root Mean Squared Error (RMSE) of v2 improved 18 percent (%) (0.1) and 12% (5 millimeters (mm)), respectively, relative to v1. The timing of peak snow mass is shifted two weeks later and a temporal discontinuity in the monthly northern hemisphere snow mass time series associated with the shift from the Special Sensor Microwave/Imager (SSM/I) and the Special Sensor Microwave Imager/Sounder (SSMIS) in 2009 is removed in v2. Luojus, K.; Moisander, M.; Pulliainen, J.; Takala, M.; Lemmetyinen, J.; Derksen, C.; Mortimer, C.; Schwaizer, G.; Nagler, T.; Venäläinen, P. (2022): ESA Snow Climate Change Initiative (Snow_cci): Snow Water Equivalent (SWE) level 3C daily global climate research data package (CRDP) (1979 – 2020), version 2.0. NERC EDS Centre for Environmental Data Analysis, 17 March 2022. doi:10.5285/4647cc9ad3c044439d6c643208d3c494. https://dx.doi.org/10.5285/4647cc9ad3c044439d6c643208d3c494本数据集包含欧洲空间局（European Space Administration, ESA）气候变化倡议（Climate Change Initiative, CCI）雪项目的每日雪水当量（Snow Water Equivalent, SWE）产品v2.0版本，空间分辨率为0.1度。雪水当量（SWE）指代陆地表面的积雪总量，亦即积雪层中蕴含的液态水总量。该SWE产品的时间序列覆盖1979年1月至2020年5月时段。北半球SWE产品以日时间分辨率发布，且已对高山区域进行掩膜处理。本数据集旨在满足气候研究与监测相关从业者的需求，涵盖变异性与趋势检测、气候模拟以及水文学与气象学相关研究方向。芬兰气象研究所负责该SWE产品的开发与生成。相较于v1版本数据，本v2版本数据集存在若干显著差异：在v2版本中，被动微波辐射计数据取自重校准增强分辨率CETB ESDR数据集（MEaSUREs校准增强分辨率被动微波每日EASE-Grid 2.0亮温（CETB）地球系统数据记录（ESDR）https://nsidc.org/pmesdr/data-sets/）；网格间距从25千米（km）缩减至12.5 km；并在后期处理中使用时空变化的积雪密度场对SWE反演结果进行校正。输出网格间距从0.25度WGS84经纬度调整为0.10度，以适配其他Snow_cci产品。时间序列已延长两年，补充了2018年至2020年的观测数据。ESA CCI分阶段产品开发框架允许通过一系列逐步迭代的开发数据集，系统分析v1与v2版本间输入数据及积雪密度参数化方案的改进。相较于原位雪道观测数据，v2版本的相关系数与均方根误差（Root Mean Squared Error, RMSE）较v1版本分别提升了18%（0.1）与12%（5毫米，mm）。积雪质量峰值的出现时间延后两周；同时，v2版本消除了2009年因专用传感器微波成像仪（Special Sensor Microwave/Imager, SSM/I）升级为专用传感器微波成像仪/探测器（Special Sensor Microwave Imager/Sounder, SSMIS）所引发的北半球月度积雪质量时间序列的时序断裂问题。Luojus, K.; Moisander, M.; Pulliainen, J.; Takala, M.; Lemmetyinen, J.; Derksen, C.; Mortimer, C.; Schwaizer, G.; Nagler, T.; Venäläinen, P. (2022)：欧洲空间局雪气候变化倡议（Snow Climate Change Initiative, Snow_cci）：雪水当量（SWE）3C级每日全球气候研究数据包（Climate Research Data Package, CRDP）（1979年—2020年），版本2.0。NERC EDS环境数据分析中心，2022年3月17日。doi:10.5285/4647cc9ad3c044439d6c643208d3c494。https://dx.doi.org/10.5285/4647cc9ad3c044439d6c643208d3c494