Abstract. The amount of solar radiation reaching the Earth surface (SSI) is critical for a variety of applications, ranging from surface-atmosphere interactions to solar energy. SSI is characterized by a large spatiotemporal variability, in particular in the presence of broken clouds. This results in complex spatial patterns of shadows and sunlight directly related to clouds' geometry and physical properties. Although key in many respects, the instantaneous spatial distribution of SSI remains largely unexplored. Here, we use unique observations from a dense network of pyranometers deployed during the HOPE field campaign to investigate the SSI spatial distribution. For cumulus scenes, bimodal distributions are found, with one mode corresponding to cloud shadows and the other to sunlit areas with enhanced SSI exceeding clear-sky values. Combining large-eddy simulations of cumulus clouds with Monte Carlo ray tracing, we demonstrate the capability of advanced numerical tools to reproduce the observed distributions and quantify the impact of cloud geometrical and physical properties on both modes. In particular, cloud cover strongly modulates their amplitudes, in addition to their location and width, which are also sensitive to cloud height, geometrical depth, and liquid water content. Combining observations and simulations, we propose sampling strategies to estimate the instantaneous spatial distribution of SSI with a limited number of sensors, highlighting that 10 pyranometers integrated over 10 min can capture most details of the full distribution. Such a strategy could be used for future campaigns to further investigate SSI distributions and their impact on land-atmosphere exchanges or PV farm management.

中文翻译：

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如何观测地表太阳辐照度的小尺度空间分布，以及积云对它的影响如何？

摘要。到达地球表面的太阳辐射量 (SSI) 对于从表面-大气相互作用到太阳能的各种应用至关重要。 SSI 的特点是时空变化较大，特别是在存在破碎云的情况下。这导致阴影和阳光的复杂空间模式与云的几何形状和物理特性直接相关。尽管 SSI 的瞬时空间分布在许多方面都很关键，但在很大程度上仍未得到探索。在这里，我们使用 HOPE 现场活动期间部署的密集日射强度计网络的独特观测来研究 SSI 空间分布。对于积云场景，发现了双峰分布，一种模式对应于云阴影，另一种模式对应于 SSI 增强超过晴空值的阳光照射区域。将积云的大涡模拟与蒙特卡罗射线追踪相结合，我们展示了先进数值工具重现观测到的分布并量化云几何和物理特性对两种模式的影响的能力。特别是，除了它们的位置和宽度之外，云量还强烈地调节它们的振幅，这对云高度、几何深度和液态水含量也很敏感。结合观测和模拟，我们提出了采样策略，用有限数量的传感器来估计 SSI 的瞬时空间分布，强调在 10 分钟内集成的 10 个日射强度计可以捕获完整分布的大部分细节。这样的策略可用于未来的活动，以进一步调查 SSI 分布及其对陆地-大气交换或光伏发电场管理的影响。