M (Multidisciplinary and Interdisciplinary) » M-SD Space Development & Earth Observation from Space
[M-SD46] Effects of lightning, severe weather and tropical storms
Clouds may significantly affect the radiation budget and water cycles of the Earth by altering their various properties. Besides the cloud microphysical properties, the cloud base height, which is one of the important cloud macrophysical structures, is of particular significance for the infrared radiation at the surface. The cloud base height is also extremely important for air safety, airborne military activities, and surveillance operations [Vislocky and Fritsch, 1997]. Thus, an accurate method for estimating cloud base height may further improve our understanding for the impact of clouds on the Earth's radiation budget and provide useful information for improvement of aviation safety. Ground-based instruments, such as radars and lidars, can help obtain relatively accurate the cloud base height locally [Takano and Takamura, 2014]. However, they cannot provide the cloud base height information in wide area. The advantage of satellites is that high-resolution, two-dimensional distributions of the microphysical and macrophysical properties of clouds may be retrieved on a global scale [Huang et al., 2006]. However, the retrieval of the cloud base height is considerably complex and not directly available from satellite observations. Wilheit and Hutchison  also proposed a method to retrieve the cloud base height by combining passive microwave brightness temperature and infrared cloud top temperature. However, these methods work mainly for special cloud types (e.g., relatively thin, stratiform, or convective clouds). In this study, we propose an estimation method of the cloud base height using a pair of cloud images observed by all-sky imagers at two sites. The advantage of all-sky imagers is to be cheaper than radars and lidars. The two all-sky imagers were installed on the roofs of Engineering Research Building 1 (35.6246N, 140.1037E) and 2 (35.6266N, 140.1040E) in Nishi-Chiba campus in Chiba University, Japan. The distance between the two all-sky imagers is 216 m. The all-sky imagers have equisolid angle projection method that cloud amount can be estimated with high accuracy. The estimation method of the cloud base height is described below. When the cloud base height is assumed in the range of 500-3000 m with step of 10 m, the two cloud images are projected to each map. Then we calculate cross-correlation between the two maps. When the cross-correlation coefficients take the maximum in the all cases, the cloud base height is determined. As a result, the cloud base heights were estimated to be 1330 m and 1370 m at 13:03:12 LT and 13:06:12 LT on 27 August, 2018, respectively. The cloud base height was observed to be 1830 m by a lidar installed by the National Institute for Environmental Studies (NIES) in the same campus in Chiba University at 12:45:00 LT. The difference of the cloud base height (about 500 m) could be caused by the time difference of observations (18-21 minutes). In addition, we estimated the cloud base height for two isolated clouds at 13:29:53 LT on 16 November, 2018. The cloud base heights were estimated to be 1950 m and 1850 m, respectively. The lidar measured a reflection altitude of 2160 m at 13:30:00 LT on the same day. There was a difference of about 200-300 m between them, because the cloud themselves observed by the all-sky imagers might be different from those observed by the lidar. In the session, we will compare between the cloud base height estimated by cloud images and that observed by the 95-GHz FMCW cloud radar FALCON-I that was originally developed by our group.