ESA Snow Climate Change Initiative (Snow_cci): Snow Water Equivalent (SWE) level 3C daily global climate research data package (CRDP) (1979 – 2018), version 1.0

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 to 2018. Northern Hemisphere SWE products are available at daily temporal resolution with alpine areas masked. The product is based on data from the Scanning Multichannel Microwave Radiometer (SMMR) operated on National Aeronautics and Space Administration’s (NASA) Nimbus-7 satellite, the Special Sensor Microwave / Imager (SSM/I) and the Special Sensor Microwave Imager / Sounder (SSMI/S) carried onboard the Defense Meteorological Satellite Program (DMSP) 5D- and F-series satellites. The satellite bands provide spatial resolutions between 15 and 69 km. The retrieval methodology combines satellite passive microwave radiometer (PMR) measurements with ground-based synoptic weather station observations by Bayesian non-linear iterative assimilation. A background snow-depth field from re-gridded surface snow-depth observations and a passive microwave emission model are required components of the retrieval scheme. The dataset was aimed to serve the needs of users working on climate research and monitoring activities, including the detection of variability and trends, climate modelling, and aspects of hydrology and meteorology. The Finnish Meteorological Institute is responsible for the SWE product development and generation. For the period from 1979 to May 1987, the products are available every second day. From October 1987 till May 2018, the products are available daily. Products are only generated for the Northern Hemisphere winter seasons, usually from beginning of October till the middle of May. A limited number of SWE products are available for days in June and September; products are not available for the months July and August as there is usually no snow information reported on synoptic weather stations, which is required as input for the SWE retrieval. Because of known limitations in alpine terrain, a complex-terrain mask is applied based on the sub-grid variability in elevation determined from a high-resolution digital elevation model. All land ice and large lakes are also masked; retrievals are not produced for coastal regions of Greenland.

雪水当量（Snow Water Equivalent, SWE）指陆面积雪的累积总量，换言之即积雪层所蕴含的液态水总量。该SWE产品时间序列的覆盖时段为1979年至2018年。北半球SWE产品以日为时间分辨率提供，且对高山区域进行了掩膜处理。
本产品基于搭载于美国国家航空航天局（National Aeronautics and Space Administration, NASA）雨云7号（Nimbus-7）卫星的扫描多通道微波辐射计（Scanning Multichannel Microwave Radiometer, SMMR），以及搭载于国防气象卫星计划（Defense Meteorological Satellite Program, DMSP）5D系列与F系列卫星的专用传感器微波成像仪（Special Sensor Microwave / Imager, SSM/I）和专用传感器微波成像仪/测声仪（Special Sensor Microwave Imager / Sounder, SSMI/S）的观测数据。卫星观测波段的空间分辨率介于15至69千米之间。
其反演方法通过贝叶斯非线性迭代同化，将卫星被动微波辐射计（Passive Microwave Radiometer, PMR）观测数据与地面天气站的常规观测数据进行融合。反演方案需依赖重网格化的地表积雪深度观测场背景场，以及被动微波辐射发射模型两类核心组件。
本数据集旨在满足气候研究与监测相关用户的需求，包括变异性与趋势检测、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（Finnish Meteorological Institute）负责该SWE产品的开发与生成工作。
1979年至1987年5月期间，产品每两日更新一次；1987年10月至2018年5月期间，产品按日更新。产品仅针对北半球冬季生成，通常为每年10月初至次年5月中旬。6月与9月仅提供有限天数的SWE产品；7月和8月无可用产品，原因是地面天气站通常无积雪观测数据，而该数据是SWE反演所需的输入项。
由于高山地形存在已知局限性，本产品采用基于高分辨率数字高程模型（Digital Elevation Model, DEM）计算的亚网格高程变异性，生成复杂地形掩膜。同时，所有陆地冰体与大型湖泊均被掩膜，格陵兰岛沿海区域不生成反演结果。