Spatial and Temporal Scales of Riming Events in Nonconvective Clouds Derived From Long-Term Cloud Radar Observations in Germany

Abstract

Despite the relevancy of riming for precipitation formation, our observational knowledge of spatiotemporal scales of riming in clouds is poor. We use long-term cloud radar observations to statistically investigate the horizontal and vertical dimensions as well as the typical duration of riming events. We extend a recent retrieval for rime mass fraction into an algorithm that can separate the data into individual riming events and estimate the spatial dimensions using horizontal wind profiles. For 2,500 riming events, we find an average horizontal extent of the riming regions of 13 km and a duration of 18 min. Vertical profiles indicate that the majority of rime mass is built within the uppermost 250 m of the region where the radar can detect riming. Similar to previous studies, the riming events are almost exclusively detected between 0°C and −15°C. To further examine the correlation between riming and thermodynamic profiles, we derived liquid water content from radiosonde data. We find that strong riming usually starts close to the level where the liquid water path exceeds 0.2 kg m⁻². By defining a control group of nonriming events, we also find significantly enhanced liquid water below the −15°C isotherm for the riming cases. However, the existence of the 0.2 kg m⁻² level in ice clouds alone is not indicative of strong riming. We find this level to be four times more likely than strong riming events. We expect our multiyear statistical riming characteristics to be valuable for the future development of riming retrievals and model validation.