Thin Clouds Producing Graupel in the Arctic

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Description

The riming process is a critical component of any bulk microphysical parameterization for mixed-phase clouds. It is typically represented using a continuous collection kernel approach, where relatively massive ice crystals fall through homogeneous, suspended clouds of supercooled liquid water droplets in still air. In reality, these cloud particles are exposed to turbulent motions that enhance the role of inertia (Naso et al., 2018) and lead to changes in localized precipitation intensities (Lee et al., 2014; Lee and Baik, 2016). Here we present recent theoretical and observational work providing a better understanding of the problem. Talaei et al. (in prep.), demonstrate the effect of turbulent flow on the settling velocities of spherical particles. Simulations using derived equations showed a settling velocity reduction of approximately 50% for an ice particle with a diameter of less than 0.1 mm falling in turbulent flow. This decrease lengthens the in-cloud residence time, thereby increasing the number of potential collisions with droplets that are subject to random motions. Additionally, Garrett (2019) derived analytical solutions for steady-state size distributions of rain and snow particles assuming growth by collection of smaller, suspended cloud droplets. One expression relating the cloud liquid water path (LWP) to the slope of the size distribution was compared to observations of Arctic graupel, revealing that thin clouds were frequently producing disproportionately large graupel relative to measured cloud LWP values (Fitch et al., in prep.). In fact, for over 13,000 of the most heavily rimed particles measured at this location, the mean LWP was a mere 30 g/m2. Analysis of a week-long thin cloud event revealed that relatively strong sensible heat fluxes, relaxed mixed-layer capping inversions, and cloud layer thinning preceded the two periods of graupel occurrence, implying that cloud top turbulence was involved in the enhancement of growth by riming. This decrease of hydrometeor settling speed by turbulent flow appears to lengthen the riming path, leading to surprisingly large graupel. Comparison of observations to traditional riming growth calculations indicates a riming path enhancement factor of approximately 6. Therefore, this result should help to explain the frequent occurrence of graupel precipitating from thin clouds in the Arctic, where strong cloud-top radiative cooling regularly generates turbulent mixing in the boundary layer.