Detecting riming in stratiform clouds using EarthCARE Doppler observations

Abstract. The EarthCARE Cloud Profiling Radar (CPR), the first Doppler cloud radar in space, provides near-global Doppler velocity observations from which hydrometeor sedimentation velocity can be retrieved, creating a new opportunity to identify riming in clouds. Riming increases the density and fall speed of ice particles as supercooled liquid water (SLW) droplets freeze onto their surfaces, influencing cloud evolution and precipitation. Yet where riming occurs has remained largely beyond satellite observations. We introduce a method for detecting riming in stratiform clouds from sharp downward increases in hydrometeor sedimentation velocity. The detection threshold is derived from ice and snow gates where the Atmospheric Lidar (ATLID) signal remains unextinguished and no SLW is detected, and marks an acceleration rarely produced by vapor deposition and aggregation alone. The criterion requires a downward gradient of at least 0.4 m s⁻¹ km⁻¹ sustained over a minimum vertical distance of 300 m. In shallow mixed-phase clouds over ocean, detections concentrate within the independently estimated SLW layer and become more frequent as the SLW amount increases. Riming detection frequency often exceeds 50 % within the layer once SLW path surpasses about 200 g m⁻². During a coordinated aircraft underflight, rimed particles were visually identified within and below detected riming regions, while a region dominated by unrimed crystals and aggregates showed little detected riming. These aircraft observations consistently support the interpretation that the detected signatures correspond to riming. The resulting observations provide a new satellite-based constraint on the occurrence of riming and its representation in weather and climate models.