[A-AS01] Large-scale moisture and organized cloud systems

[AAS01-10] Estimation of cloud base height using cloud images captured by all-sky cameras

*Atsuya Nemoto¹, Hiroyo Ohya¹, Toshiaki Takano², Tamio Takamura², Alessandro Damiani², Hiroyuki Nakata¹ (1.Graduate School of Science and Engineering, Chiba University, 2.Center for Environmental Remote Sensing Chiba University)

Clouds have a great influence on the Earth's radiation balance and water cycle by changing their various properties. In addition to cloud microphysical properties, cloud base height, which is one of the macrophysical structures of clouds, has a particularly importance for infrared radiation on the surface of the Earth. Cloud base height is also extremely important in aviation safety, aerial military operations and surveillance [Vislocky and Fritsch, 1997]. Therefore, if there is a method to accurately estimate the cloud base height, it may be possible to better understand the effect of clouds on the radiation balance of the Earth and provide useful information for improving aviation safety. Radars and lidars can be used to locally obtain relatively accurate cloud base height [Takano and Takamura, 2014]. However, they detect only the overhead cloud base height. 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 acquisition of cloud base height is very complicated and cannot be obtained directly from satellite observations. Wilheit and Hutchison [2000] also proposed a method to retrieve the cloud base height by combining passive microwave brightness temperature and infrared cloud top temperature. In this study, we propose a method for estimating cloud base height using a pair of cloud images observed by all-sky cameras at two sites. The advantage of the all sky camera is that it is cheaper than radars and lidars. Cloud observations have been performed by the two all-sky cameras 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 cameras is 216 m. The all-sky cameras have equisolid angle projection method. The estimation method of the cloud base height is described below. When the cloud base height is assumed in the range of 500-2500 m with step of 50 m, the two cloud images are projected to each map. Then we calculate cross-correlation of RGB and binarized values between the two maps. The cloud base height is determined when the cross-correlation coefficients peak in all cases. We verified the estimation accuracy of the cloud base height by simulation. When we made pseudo-cloud images that the cloud base heights were set to be 500 – 3000 m with a step of 500 m, the errors height were 0 – 30 m. The errors are allowable range, because minimum spatial resolution for cloud height of a W-band cloud radar, FALCON-I (FMCW radar for cloud observations) is 48.8 m. FALCON-I is a cloud radar with high spatial and sampling resolution developed at Chiba University, Japan. As a estimation result of observed cloud images, the cloud base heights were estimated to be 1587 m and 1737 m in the cases of the RGB and binarized values at 11:45:13 LT on 16 March, 2020, respectively. The cloud base height was observed to be 1860 m by a lidar installed by the National Institute for Environmental Studies (NIES) in the same campus in Chiba University at 11:45:00 LT. In the session, we will show the results of verification for the accuracy of the estimation method in details.