Abstract: Significance　Clouds transfer heat to the atmosphere in the form of latent heat of condensation, so their distribution and variations are key to radiation budget and play a significant role in weather and climate change. Compared with traditional weather radars, millimeter-wave cloud radars have better detection capabilities for cloud droplets and can be used in the monitoring and research of cloud macro- and microphysical characteristics as well as dynamic properties. Specifically, they are helpful for analyzing the evolution of clouds and precipitation particles in the atmosphere, revealing the interaction mechanism between cloud systems and atmospheric circulation, and deepening our understanding of cloud responses to climate change. As an important active remote sensing tool, ground-based millimeter-wave cloud radars have received increasing attention in the meteorological field in recent years, and have shown great application potential in enhancing the cloud observation capability of the existing operational meteorological observation system, especially for the fine vertical structure of clouds. Since 2016, the China Meteorological Administration has carried out a series of megacity observation experiments, combining millimeter-wave cloud radars with microwave radiometers, wind profilers, aerosol lidars, and global navigation satellite system/meteorology (GNSS/MET) water vapor instruments to form a ground-based remote sensing vertical observation system. More than 50 such kind of systems have been deployed nationwide and officially put into operation in 2024. Therefore, this paper focuses on the application research of ground-based millimeter-wave cloud radars in meteorology in China. In this review, we summarize the advantages and development history of the detection technology of ground-based millimeter-wave cloud radars, as well as their application research results in cloud macro- and microphysical properties and dynamic characteristics, and in the monitoring and forecasting of different types of cloud and precipitation. Additionly, we also look forward to the future development direcitons. Progress　Compared with traditional meteorological radars, millimeter-wave cloud radars have shorter wavelengths and narrower beams, enabling them to describe the vertical structure and evolution of clouds in greater detail. With their high monitoring sensitivity, millimeter-wave cloud radars utilize the scattering characteristics of cloud particles on millimeter waves to achieve macro and micro monitoring of clouds, and invert and generate products including cloud base height, cloud top height, cloud thickness, liquid water content, raindrop size distribution, solid particle size variation, and particle falling velocity. On the other hand, their high temporal resolution enables millimeter-wave cloud radars to achieve minute-level allweather monitoring of cloud changes. Among these cloud radars, dual-polarization cloud radars, which are currently the most widely used in China, perform well in filtering out non-meteorological echoes such as abnormal echoes, ground clutter, and sea clutter, and can more accurately characterize the size, shape, type, and distribution of precipitation particles. By using ground-based millimeter-wave cloud radars, researchers can quantitatively monitor macro features such as cloud amount and cloud height, classify cloud types in different regions, and monitor the whole life cycle of clouds including their formation, development, and dissipation. This provides a foundation for studying cloud-precipitation mechanisms, parameterization schemes of cloud and precipitation physical processes, and the calibration of satellite retrieval results. In terms of studying the macroscopic characteristics of clouds, Chinese scientists have explored automatic cloud observation based on millimeterwave radars and realized automatic cloud classification using multi-parameter threshold discrimination methods. Millimeterwave cloud radars can alos be used to obtain cloud microphysical parameters, such as number concentration, size distribution, and liquid water content of cloud droplets and ice crystals, thereby detecting and retrieving cloud turbulence characteristics and revealing microphysical differences and evolution laws among different cloud systems. In the research of cloud microphysical and dynamic characteristics, the Ka/Ku dual-frequency cloud radar technology independently developed by China has significant advantages. Based on the observed echo intensity spectral density data and optimal estimation techniques, inversion methods for vertical air velocity, raindrop spectra, liquid water content, rainfall rate, and rainfall attenuation correction methods have improved the detection capability of cloud internal dynamic and microphysical parameters. For example, the bright bands on the reflectivity maps of Ka-band cloud radar, as well as the jumps in Doppler velocity and spectral width in temporal-spatial profiles, can be used to identify stratiform clouds. Whether convective clouds produce precipitation and its duration can also be determined from cloud radar reflectivity echo intensity images. The combination of dual-wavelength radar retrieval technology and multi-wavelength radars improves the retrieval accuracy of cloud and precipitation particles, providing more reliable data for snowfall estimation and microphysical characteristic research.Conclusions and Prospects　After the deployment of the ground-based remote sensing vertical observation system mainly composed of millimeter wave cloud radar, it is possible to establish relational expressions describing the microphysical characteristics of cloud and precipitation particles, refine the extraction of spectral shape features, and improve the simulation accuracy of numerical models by using long time-series observation data. Precipitation monitoring and forecasting, severe weather monitoring and warning, weather modification, as well as a deeper understanding of the impact of climate change on the Earth's ecosystem, will be the focus of applications for ground-based millimeter-wave cloud radar. With the continuous development of radar detection technology and signal processing algorithms, as well as improvements in the manufacturing processes of key components, the performance and accuracy of ground-based millimeter-wave cloud radars will be further enhanced. In the future, ground-based millimeter-wave cloud radars will develop toward technological innovation, multi-source data fusion, and intelligent applications, and their application in meteorological services will become increasingly widespread.

Key words: millimeter wave, cloud radar, cloud precipitation, microphysics