Effects of Riming on Ice‐Phase Precipitation Growth and Transport Over an Orographic Barrier.

The evolution of ice‐phase particles within precipitating clouds depends on the environmental properties of the cloud and on physical characteristics of the particles themselves, which can be modified by airflow over steep terrain. Through employing a unique Lagrangian particle‐based precipitation model, this study investigates the sensitivities in ice‐phase particle growth and transport due to variabilities in riming processes over an orographic barrier. This analysis is applied to two wintertime stratiform cyclones sampled by in situ aircraft over windward slopes during the Olympic Mountains Experiment. For both events, we simulate the ice‐phase particle evolution and trajectory within a two‐dimensional prescribed state representative of median observed cloud properties. Sensitivity simulations were constructed based on observed variabilities in supercooled liquid water (SLW) properties and its vertical extent above the melting level. Perturbations of SLW concentration equivalent to the 85th and 15th percentiles of observed values, which typically amounted to a change of less than 0.05 g m−3, resulted in respective increases or decreases in the ice‐phase contribution to surface precipitation mass by as much as 50% and horizontal particle trajectories differences exceeding 10 km. Similar sensitivities were found in response to varying the vertical extent of SLW above the melting level and to adjustments in mean SLW droplet size. The significant precipitation response to small variations in cloud properties principally arises from changing rates of rime mass accumulation and correspondingly, increases in particle fall speed. Considerations for the numerical representation of the riming process and its complex effects on precipitation are discussed. Plain Language Summary: Precipitating clouds produce rain and snow at varying rates depending on the properties of the cloud. At below freezing temperatures in clouds, it is common to have a mixed‐phase layer where ice crystals and liquid droplets, in a supercooled state, coexist. Collection of the supercooled liquid droplets on the ice crystals greatly affects the mass and fall speed of the ice crystals, thereby affecting where, and how much, precipitation reaches the surface. In this study, we use a numerical model to simulate the evolution of individual precipitating ice crystals and evaluate sensitivities to the concentration and distribution of supercooled liquid droplets in the cloud. Based on variabilities measured in clouds upstream of the Olympic Mountains, small changes in the concentration of supercooled liquid droplets, the size of the liquid droplets, or the depth of the mixed‐phase layer produces changes in rain rates of up to 50% and increases or decreases in the distance a particle travels by as much as 10 km. We discuss the implications of these significant sensitivities to the precipitation rate and fall characteristics. Key Points: Particle‐based models provide frameworks for simulating the evolution and trajectory of precipitating particles in stratiform winter stormsFor sufficiently deep mixed‐phase clouds, small variations in supercooled liquid water mass may alter precipitation rates by as much as 50%Particle trajectory is significantly modulated by mixed‐phase layer depth through cumulative adjustments in fall velocity during riming [ABSTRACT FROM AUTHOR]

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