Visualizing the Spatial Structure of Strong Riming Events using Scanning Cloud Radars

The coexistence of liquid water and ice crystals in mixed phase clouds allows for collisions and subsequent freezing of droplets on ice crystals and aggregates, which is called riming. Because riming fills air cavities in aggregates and makes particles more round, rimed snow has a higher fall speed compared to unrimed crystals and aggregates. Therefore, a reliable method to detect riming are fall speed measurements based on the Doppler shift of cloud radar echos. For this, the cloud radar has to be oriented vertically, otherwise the Doppler shift is dominated by the horizontal wind advection of the hydrometeors. Limited to vertical observations, one of the key strengths of atmospheric radar is missing: To probe the atmosphere in 3D. In this contribution, we present the results from a winter measurement campaign, where we performed elevation scans through strong riming events. Assuming a model for the horizontal wind speed, we can remove the horizontal wind contribution from the Doppler signal. This reveals the underlying riming signatures from the measurements, allowing us to create snapshots in time of the actual spatial organization of strongly rimed particles in mixed phase clouds. By choosing the scanning plane into the main wind direction, we are able to track spatial features over multiple snapshots. A better characterization of the spatial picture can enhance our conceptual understanding of the structure and organization of strong riming in mixed phase clouds. Since supercooled liquid water is a precondition for riming and aircraft icing alike, our results could also proof helpful to design aircraft icing hazard warning products.