CLARA-A2: CM SAF cLoud, Albedo and surface RAdiation dataset from AVHRR data - Edition 2

The CLARA-A2 record provides cloud properties, surface albedo and surface radiation parameters derived from the AVHRR sensor onboard polar orbiting NOAA and METOP satellites. This second edition is the improved and extended follow-up of the first version of the record (Karlsson et al., 2013; CLARA-A1 DOI: 10.5676/EUM_SAF_CM/CLARA_AVHRR/V001) which now covers a 34 year time period (1982-2015). Original visible radiances were inter-calibrated and homogenised, using MODIS data as a reference, before applying the various parameter retrievals. The inter-calibration was based on an original method introduced by Heidinger et al. (2010) which now has been updated (MODIS Collection 6) and extended (six years have been added). CLARA-A2 features a range of cloud products: cloud mask, cloud top temperature/pressure/height, cloud thermodynamic phase, and (for liquid and ice clouds separately) cloud optical thickness, particle effective radius and cloud water path. Cloud products are available as monthly and daily averages and also as daily resampled global products (Level 2b) for individual satellites. Cloud parameter results are also presented as multi-parameter distributions (i.e., joint frequency histograms of cloud optical thickness, cloud top pressure and cloud phase) for daytime conditions. Surface albedo is presented as monthly and pentad (5 day) averages and is derived using all available data during the studied period. Surface radiation products are provided as monthly averages for the downwelling shortwave (including also daily averages) and the down- and upwelling longwave components. The monthly and daily averages are available on a 0.25°x0.25° global grid. Surface albedo and cloud products are also provided in two equal area grids with a resolution of 25 km x 25 km covering the polar regions. Daily resampled cloud products (level 2b) are provided in a global grid with a resolution of 0.05°x0.05°. For the latter, also a probabilistic cloud mask is added as an experimental product. The advancements compared to CLARA-A1 can be summarized as follows: (1) Extended AVHRR measurement record (six years). (2) Upgraded inter-calibration (using MODIS Collection 6 as reference). (3) Significantly improved cloud screening (utilizing CALIPSO-CALIOP data during development). (4) Improved efficiency of cloud top height retrievals (i.e., reduced number of pixels with no retrieval solutions). (5) Improved cloud phase discrimination and ice cloud property retrieval. (6) Improved atmospheric correction using dynamic aerosol optical depths for the full surface albedo data record. (7) Extended product portfolio (i.e., adding daily resampled Level 2b cloud products including experimental cloud probabilities). A summary of the CLARA-A2 characteristics and a comprehensive evaluation of the results are provided by Karlsson et al., (2016). Along with the data, a comprehensive documentation including user guide, algorithm descriptions, reprocessing layout and extensive validation studies, is provided.

CLARA-A2数据集（CLARA-A2）提供了源自极轨NOAA与METOP卫星搭载的先进甚高分辨率辐射计（Advanced Very High Resolution Radiometer，AVHRR）的云属性、地表反照率与地表辐射参数。本第二版是初代CLARA数据集（Karlsson等，2013；CLARA-A1 DOI：10.5676/EUM_SAF_CM/CLARA_AVHRR/V001）的改进与扩展迭代版本，此次覆盖时长增至34年（1982年至2015年）。在开展各类参数反演前，研究团队以中分辨率成像光谱仪（Moderate Resolution Imaging Spectroradiometer，MODIS）数据为参考，对原始可见光辐射数据进行了交叉定标与均一化处理。此次交叉定标基于Heidinger等（2010）提出的原创方法，本次已完成更新（采用MODIS Collection 6标准）与扩展（新增6年观测数据）。CLARA-A2包含多类云产品：云掩膜、云顶温度/气压/高度、云热力相态，以及分别针对液态云与冰云的云光学厚度、粒子有效半径与云水路径。云产品可按月、日均值形式获取，同时也可为单颗卫星提供每日重采样的全球产品（Level 2b级）。针对日间工况，云参数结果还以多参数分布形式输出，即云光学厚度、云顶气压与云相态的联合频率直方图。地表反照率以月均与候均（5日平均）形式输出，其反演基于研究时段内的全部可用观测数据。地表辐射产品包含下行短波辐射（同时提供日均值）、下行与上行长波辐射的月均数据。月均与日均值数据采用0.25°×0.25°的全球网格格式发布。地表反照率与云产品同时提供覆盖极地地区的25 km×25 km等面积网格数据，共两种规格。每日重采样云产品（Level 2b级）采用0.05°×0.05°的全球网格格式发布，该版本额外附带了概率云掩膜这一实验产品。相较于CLARA-A1，CLARA-A2的改进点可归纳如下：1. 扩展了AVHRR观测数据集，新增6年观测时长；2. 升级了交叉定标流程，以MODIS Collection 6作为参考基准；3. 云筛选精度大幅提升，研发阶段引入了云气溶胶激光雷达与红外探路者卫星观测（Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations，CALIPSO）与正交偏振云气溶胶激光雷达（Cloud-Aerosol Lidar with Orthogonal Polarization，CALIOP）的观测数据；4. 优化了云顶高度反演效率，减少了无反演结果的像元数量；5. 改进了云相态判别与冰云属性反演算法；6. 针对完整地表反照率数据集，采用动态气溶胶光学厚度实现了更精准的大气校正；7. 扩展了产品类型，新增了每日重采样的Level 2b级云产品，其中包含实验性云概率产品。Karlsson等（2016）对CLARA-A2的数据集特性进行了总结，并对其反演结果开展了全面评估。本数据集附带完整的技术文档，包含用户指南、算法说明、重处理流程说明以及大量验证研究资料。