Ground-based Dual-band Cloud Observing System and Its Comparative Experiments

Clouds affect the energy balance of the earth by means of absorbing and scattering radiation, and they have influences on global climate. Precipitation of clouds is the most familiar way for the earth water circulation. In the macroscopic parameters of clouds, cloud cover and cloud base height are the primary elements in the observation of clouds, where the cloud base height condition determines cloud classification and precipitation probability. However, artificial visual observation is the main method used in China at present, which are not objective and waste a mass of work force. Therefore, the research and development of an automatic measuring cloud device is extremely necessary.A ground-based cloud observing system consists of an infrared temperature measuring sensor and a double location digital ceilometer. The double location digital ceilometer calculates the cloud height of zenith by geometry method at regular time, and the infrared temperature measuring sensor acquires brightness temperature of atmospheric radiation in real time. Based on the different characteristics of visible and infrared cloud images, the height of clouds is derived and tested by the double location digital ceilometer, combined with real-time observation of zenith single-point infrared brightness temperature and ground environmental parameters. The influence of sub cloud air from ground to clouds bottom on infrared brightness temperature is analyzed, showing the feasibility of cloud base height remote sensing by ground-based sky infrared brightness temperature. The result shows that for low and middle clouds, the sky radiation brightness temperature of ground-based observation is relatively sensitive to the variation of cloud base height, therefore it can be used to estimate cloud base height. On the basis of decreasing principle of tropospheric vertical temperature, the temperature decreasing gradient parameter K from cloud base to the ground is defined and according to the decreasing gradient parameter inverse the cloud height of zenith direction. The algorithm does not depend on the air sounding data, and the formula of cloud base height is derived in practice. Double location digital ceilometer can measure a cloud height only when the bottom of the cloud has texture, and then K can be calculated, therefore the double location digital ceilometer is a calibration device in the ground-based measuring cloud system.Experimental observation has been performed using dual-band cloud observation system in the synthesize experiment base of CMA Meteorological Observation Center since July 2010, and cloud height data of the zenith is acquired each minute. Through the comparative data analysis with Vaisala CL31, ground-based dual-band cloud observation systems can provide results with high accuracy.