Cloud Phase Detection And Recognition By Raman-Polarization Lidar

Water is the only component in the atmosphere that has three phase transitions.Water vapor forms liquid water and ice water through condensation,which is the main reason for the formation of clouds and rain.The cloud is the most uncertain factor in atmospheric activity,because it is a a combination of water droplets and ice particles.About 60%of the earth's surface is covered by clouds.Clouds play a vital role in the energy transmission,water cycle and climate regulation of the earth-atmosphere.The formation of clouds and the precipitation of clouds have a profound impact on human life.Clouds with different weather conditions,different thicknesses,and different phase states have different degrees of influence on the earth-atmosphere radiation and water cycle.Therefore,the detection and analysis of cloud structural parameters and optical parameters has been a hot topic in atmospheric dynamics and thermodynamics research.Compared with other atmospheric detection methods,lidar has the advantages of high spatial,temporal resolution and low cost,which has been widely used in the measurement of atmospheric parameters.At present,the detection of cloud phase is mainly based on polarization lidar.A few units have conducted research on the phase water in clouds using Raman lidar.The thesis will carry out research on cloud phase detection and recognition technology based on Raman-Polarization lidar.The thesis firstly carried out the design and research of Raman-Polarization lidar system,then optimized and upgraded the existing set of atmospheric three-phase water Raman lidar system in Xi'an University of Technology.The new system adds a polarization detection lidar subsystem and uses a dual-telescope receiving system to achieve simultaneous detection of Raman lidar and polarization lidar,which is providing technical support for the next research of cloud phase states by Raman-Polarization lidar.We built a Raman-Polarization lidar system,and then carried out preliminary detection experiments and analysis.The polarization lidar subsystem can effectively detect clouds within 10 km.Using the parallel and vertical polarization channel echo signals,the main cloud structure parameters such as cloud base and cloud top,and cloud optical parameters such as cloud optical thickness,cloud transmittance,and depolarization ratio are obtained.The relationship between the layer-averaged attenuated backscatter and the layer-averaged attenuated depolarization ratio of cloud are discussed.Secondly,the Raman lidar subsystem is used to effectively detect clouds below 5 km,and the inversion algorithm of the linear equation system is used to separate and extract pure phase water from echo signals which is directly detected.It achieves a synchronous inversion of the profile of atmospheric water vapor,liquid water and ice water mixing ratio.And then integral liquid water content and integral ice water content of clouds are discussed emphatically.Finally,a preliminary identification study of cloud phase was carried out using Raman-Polarization lidar.The results of the simultaneous continuous observation experiments clearly show the characteristics of the spatiotemporal changes of the parameters such as liquid water,ice water and depolarization ratio in the cloud.We performed simultaneous inversion of the phase water mixing ratio profile and polarization characteristics of clouds at different heights under different weather conditions.We analyze the relationship of the integral liquid water content and integral ice water phase content of clouds,as well as the layer-averaged attenuated backscatter and the layer-averaged attenuated depolarization ratio to obtain a preliminary identification of the cloud phase.A preliminary statistical analysis of the main cloud parameters was carried out on the clouds detected by lidar.The results show that the clouds under 5 km currently detected by the system are mainly water clouds,and a small amount are mixed-phase clouds.In water clouds,There is a clear linear relationship between the integral liquid water content and integral ice water phase content of clouds,and the corresponding ratio is about 7.5:1.