Over the last years our capabilities to remotely sense clouds, light rain, and snowfall have increased. Significant progress has been made in particular in two areas. Firstly, new in-situ and ground-based remote sensing techniques allow us to better specify particle size distribution, fall speed, particle shapes, and optical properties. Secondly, our theoretical understanding of basic interactions between non-spherical frozen particles and the radiation field has advanced considerably.
Despite this progress, passive microwave retrievals of clouds, light rain, and snowfall still pose are a significant challenge. I will review the state-of-the-art of passive microwave remote sensing with an emphasis on these categories. I will highlight recent advances in particular in three different areas: Firstly, I will show full Observation System Simulation Experiments for constellations of forthcoming and planned passive microwave and sub-millimeter instruments. Secondly, I will discuss new developments and ground-based approaches to understand the impact of ice particle scattering on passive microwave and sub-millimeter observations. Thirdly, I will discuss the potential of combining visible/near-infrared sensors with passive microwave observations for clouds and light rain.