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Benchmarking three low-cost, low-maintenance cloud height measurement systems and ECMWF cloud heights against a ceilometer
Solar Energy ( IF 6.7 ) Pub Date : 2018-07-01 , DOI: 10.1016/j.solener.2018.02.050
P. Kuhn , M. Wirtz , N. Killius , S. Wilbert , J.L. Bosch , N. Hanrieder , B. Nouri , J. Kleissl , L. Ramirez , M. Schroedter-Homscheidt , D. Heinemann , A. Kazantzidis , P. Blanc , R. Pitz-Paal

Abstract Cloud height information is crucial for various applications. This includes solar nowcasting systems. Multiple methods to obtain the altitudes of clouds are available. In this paper, cloud base heights derived from the European Centre for Medium-Range Weather Forecasts (ECMWF) and three low-cost and low-maintenance ground based systems are presented and compared against ceilometer measurements on 59 days with variable cloud conditions in southern Spain. All three ground based systems derive cloud speeds in absolute units of [m/s] from which cloud heights are determined using angular cloud speeds derived from an all-sky imager. The cloud speed in [m/s] is obtained from (1) a cloud shadow speed sensor (CSS), (2) a shadow camera (SC) or (3) derived from two all-sky imagers. Compared to 10-min median ceilometer measurements for cloud heights below 5000 m, the CSS-based system shows root-mean squared deviations (RMSD) of 996 m (45%), mean absolute deviations (MAD) of 626 m (29%) and a bias of −142 m (−6%). The SC-based system has an RMSD of 1193 m (54%), a MAD of 593 m (27%) and a bias of 238 m (11%). The two all-sky imagers based system show deviations of RMSD 826 m (38%), MAD of 432 m (20%) and a bias of 202 m (9%). The ECMWF derived cloud heights deviate from the ceilometer measurements with an RMSD 1206 m (55%), MAD of 814 m (37%) and a bias of −533 m (−24%). Due to the multi-layer nature of clouds and systematic differences between the considered approaches, benchmarking cloud heights is an extremely difficult task. The limitations of such comparisons are discussed. This study aims at determining the best approach to derive cloud heights for camera based solar nowcasting systems. The approach based on two all-sky imagers is found to be the most promising, having the overall best accuracy and the most obtained measurements.

中文翻译:

用云高仪对三个低成本、低维护的云高度测量系统和 ECMWF 云高度进行基准测试

摘要 云高信息对各种应用至关重要。这包括太阳能临近预报系统。有多种方法可以获得云的高度。在本文中,介绍了来自欧洲中期天气预报中心 (ECMWF) 和三个低成本和低维护地面系统的云底高度,并与西班牙南部 59 天云量变化情况下的云高仪测量结果进行了比较. 所有三个基于地面的系统都以 [m/s] 的绝对单位导出云速度,使用从全天成像仪导出的角云速度从中确定云高度。以 [m/s] 为单位的云速度来自 (1) 云阴影速度传感器 (CSS)、(2) 阴影相机 (SC) 或 (3) 来自两个全天成像仪。与 5000 m 以下云高的 10 分钟中值云高仪测量值相比,基于 CSS 的系统显示均方根偏差 (RMSD) 为 996 m (45%),平均绝对偏差 (MAD) 为 626 m (29%)和 -142 m (-6%) 的偏差。基于 SC 的系统具有 1193 m (54%) 的 RMSD、593 m (27%) 的 MAD 和 238 m (11%) 的偏差。两个基于全天空成像仪的系统显示出 RMSD 826 m (38%) 的偏差、432 m (20%) 的 MAD 和 202 m (9%) 的偏差。ECMWF 得出的云高度偏离云高仪测量值,RMSD 为 1206 m (55%),MAD 为 814 m (37%),偏差为 -533 m (-24%)。由于云的多层性质和所考虑方法之间的系统差异,对云高度进行基准测试是一项极其艰巨的任务。讨论了这种比较的局限性。本研究旨在确定为基于相机的太阳临近预报系统推导出云高度的最佳方法。发现基于两个全天空成像器的方法是最有前途的,具有整体最佳精度和最多获得的测量结果。
更新日期:2018-07-01
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