Treffer: Spatial and Temporal Scales of Riming Events in Nonconvective Clouds Derived From Long‐Term Cloud Radar Observations in Germany.

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. Plain Language Summary: In clouds, ice crystals can collide with liquid water droplets. Upon contact, the droplets freeze on the crystals, which is called riming. Riming is an important process to understand the formation of rain and snowfall at the ground. Because the crystals get heavier and denser, they fall down faster. Looking with radars into the clouds from below, we can measure this acceleration. We wrote a program to automatically detect regions with riming in clouds. This allows us to calculate the duration (18 min), size (13 km), and temperature range (from 0° ${}^{\circ}$ to −15° ${}^{\circ}$) of those regions. From vertical profiles through those regions, we conclude that the particles grow faster at the top of the regions. When we compare weather balloon measurements in clouds with riming to measurements in similar ice clouds without riming, we see that the clouds with riming have more liquid water inside. Similarly, surface‐based instruments show that the precipitation rates below riming clouds are slightly increased. Our results are useful to validate computer models of clouds and precipitation and create new methods to detect riming. Key Points: Using vertically pointing radar, we derive typical temporal and spatial scales of strong riming events to be 18 min and 13 km, respectivelyVertical profiles reveal that the majority of additional rime mass is deposited within 250 m after a rime mass fraction of 0.6 is exceededClouds with riming show on average significantly higher liquid water content than similar ice clouds without riming [ABSTRACT FROM AUTHOR]

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