Continuous Remote Sensing Observations of Hydrometeor Sedimentation Evolution during the Transition from Ice Cloud to Precipitating Mixed-Phase Cloud

Abstract. Hydrometeor growth and sedimentation in mixed-phase clouds are key processes linking cloud evolution to surface precipitation, yet continuous observations of when fast-sedimenting hydrometeors establish effective downward transport remain limited. We combine observations from a millimeter-wave cloud radar, multiwavelength polarization lidar, microwave radiometer, and radiosondes to investigate the evolution of a cloud system from a high-level ice cloud to a stratiform mixed-phase cloud and ultimately to short-lived light precipitation. High-level ice hydrometeors developed and began to sediment under ice-saturated or ice-supersaturated conditions. As they descended into warmer cloud layers containing liquid water, radar reflectivity and hydrometeor terminal fall velocity increased concurrently with Doppler spectral broadening, consistent with further hydrometeor growth and enhanced sedimentation. The mixed-phase cloud subsequently persisted for nearly 8 h, while liquid water and fast-sedimenting spectral components were sustained without continuous surface precipitation. We define the high-velocity spectral energy fraction (F_fast) and combine it with the linear radar reflectivity factor to construct a Hydrometeor Sedimentation Index (HSI). F_fast was present well before surface precipitation, whereas HSI increased markedly later and extended toward lower altitudes, showing a clearer temporal correspondence with short-lived light precipitation. The concurrent enhancement of fast-sedimenting spectral contributions and strong radar echo signal, together with their downward extension, was a key observational feature of the transition to precipitation. These results provide a process-oriented framework for using continuous Doppler spectral observations to characterize hydrometeor sedimentation evolution and the cloud-to-precipitation transition in mixed-phase clouds.