Evaluation of cloudless-sky periods detected by shortwave and longwave algorithms using lidar measurements

Identifying cloud-free periods is important as they are used as common references in cloud and aerosol radiative forcing studies. Their identification requires precise methods to distinguish condensed water from other aerosols (e.g. mineral or moist hydrophilic aerosols). In this study we combine analyses of wide field of view shortwave (SW) and longwave (LW) irradiances and Lidar backscatter measurements to explore situations that are considered neither completely clear nor cloudy. We find that situations classified as cloud-free by analysis of SW (LW) measurements are also classified as cloud free by the Lidar in more than 60% (50%) of situations. The remaining 40% (50%) situations are classified as cloudy by the Lidar, and are hence considered as hazy. These hazy situations are predominantly composed of high-altitude cirrus clouds, partitioned equally between subvisible and semi-transparent optical thickness classes. We find that, in hazy situations, the average cloud radiative forcing on surface SW irradiances ranges between -5 and -15 Wm(-2).