Implementation of predicted rime mass in the bin microphysics scheme DESCAM 3D: evaluation for an idealized squall line system and a heavy snowfall event during ICE-POP 2018

Abstract. Due to their wide variety of properties, the representation of ice particles in cold and mixed-phase clouds are challenging to represent for microphysical schemes. To improve their representation, this study evaluates the implementation of predicted rime mass distribution in the bin microphysics scheme DESCAM. Based on the “fill-in” concept, the model allows a smooth transition in ice particle properties between unrimed and graupel particles. Consequently, the terminal velocity and collision kernels of ice particles were updated as a function of rime fraction. First, this study investigates the impact of the new implementation on cloud microphysics for an idealized squall-line system. The results show that large ice particles acquire substantial rime mass, enhancing their sedimentation and leading to earlier and more intense precipitation (+15 %), while weakening the cold pool, reducing buoyancy, and slowing squall-line propagation. The second part of this study is dedicated to evaluating the new version of DESCAM against field observations from the ICE-POP 2018 campaign, focusing on a heavy snowfall event that occurred from 7–9 March. For this case, we found that the simulated rime mass fraction at ground evolves similarly to the rime index measured by the MASC instrument. The new DESCAM version also produces significant changes in the amount and spatial distribution of precipitation, with strong local variations exceeding 10 mm and a 7.2 % increase in total accumulation, which leads to a better agreement with field observations. Overall, accounting for predicted rime mass improves consistency between the model and ground-based observations from ICE-POP 2018.