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数据集提供了源自极轨美国国家海洋和大气管理局（National Oceanic and Atmospheric Administration, NOAA）与METOP卫星搭载的先进甚高分辨率辐射计（Advanced Very High Resolution Radiometer, AVHRR）反演得到的云属性、地表反照率与地表辐射参数。
本数据集第二版是初代CLARA-A1数据集（Karlsson等，2013；CLARA-A1 DOI: 10.5676/EUM_SAF_CM/CLARA_AVHRR/V001）的改进与扩展后续版本，如今覆盖了1982-2015年共计34年的时间跨度。
原始可见辐射数据以中分辨率成像光谱仪（Moderate Resolution Imaging Spectroradiometer, MODIS）数据为参考进行了交叉定标与均一化处理，随后才开展各类参数反演。本次交叉定标基于Heidinger等（2010）提出的原创方法，如今该方法已完成升级（适配MODIS Collection 6）与扩展（新增6年观测时长）。
CLARA-A2数据集包含多类云产品：云掩膜、云顶温度/气压/高度、云热力学相态，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。云产品可获取为月均与日均数据，同时也包含针对单颗卫星的逐日重采样全球产品（Level 2b级）。针对日间工况，云参数结果还以多参数分布形式呈现（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（5日）均值形式呈现，其基于研究时段内所有可用数据反演得到。地表辐射产品以月均数据形式提供，包括下行短波辐射（同时提供日均数据）以及下行与上行长波辐射分量。月均与日均数据采用0.25°×0.25°的全球网格发布。
地表反照率与云产品还提供了两种等面积网格产品，分辨率为25 km×25 km，覆盖极地区域。逐日重采样云产品（Level 2b级）采用0.05°×0.05°的全球网格发布。针对此类产品，还附加了概率云掩膜作为实验性产品。
相较于CLARA-A1，CLARA-A2的改进可总结如下：(1) 扩展了AVHRR观测记录时长（新增6年数据）；(2) 升级了交叉定标方法（以MODIS Collection 6作为参考基准）；(3) 云筛选精度大幅提升（研发阶段利用了CALIPSO-CALIOP数据）；(4) 云顶高度反演效率得到优化（即无反演解的像素数量显著减少）；(5) 云相态判别与冰云属性反演精度提升；(6) 针对完整地表反照率数据集，采用动态气溶胶光学厚度开展大气校正，校正效果得到改善；(7) 扩展了产品体系（即新增逐日重采样Level 2b级云产品，包含实验性云概率产品）。
Karlsson等（2016）对CLARA-A2数据集的特征进行了总结，并对其反演结果开展了全面评估。随数据集一同发布的还包含一份详尽文档，其中涵盖用户指南、算法说明、再处理流程与大量验证研究内容。

搜集汇总

背景与挑战

背景概述

CLARA-A2数据集是一个基于AVHRR传感器数据的全球性云属性、地表反照率和地表辐射参数数据集，覆盖1982-2015年。该数据集提供了多种时间分辨率的产品，并在CLARA-A1基础上进行了多项技术改进，包括时间范围扩展、校准方法升级和算法优化。