Millimeter-wavelength radars: New frontier in atmospheric cloud and precipitation research

During the past 20 yr there has been substantial progress on the development and application of millimeter-wavelength (3.2 and 8.6 mm, corresponding to frequencies of 94 and 35 GHz) radars in atmospheric cloud research, boosted by continuous advancements in radar technology and the need to better understand clouds and their role in the Earth's climate. Applications of millimeter-wavelength radars range from detailed cloud and precipitation process studies to long-term monitoring activities that strive to improve our understanding of cloud processes over a wide range of spatial and temporal scales. These activities are the result of a long period of successful research, starting from the 1980s, in which research tools and sophisticated retrieval techniques were developed, tested, and evaluated in field experiments. This paper presents a cohesive, chronological overview of millimeter-wavelength radar advancements during this period and describes the potential of new applications of millimeter-wavelength radars on sophisticated platforms and the benefits of both lower- and higher-frequency radars for cloud and precipitation research.