119 Effect of Turbulence on Collisional Growth of Cloud Droplets

Monday, 9 July 2018
Regency A/B/C (Hyatt Regency Vancouver)
XIANGYU Li Sr., Stockholm University, Boulder, CO

We investigate the effect of turbulence on the collisional growth of μm-sized droplets by high-resolution
numerical simulations with well resolved Kolmogorov scales, assuming a collision and coalescence efficiency
of unity. The droplet dynamics and collisions are approximated using a superparticle approach. We show that
the time evolution of the shape of the droplet-size distribution due to turbulence-induced collision depends
strongly on the turbulent energy-dissipation rate, but only weakly on the Reynolds number. The size dis-
tribution exhibits power law behavior with a slope of −3.7 in the size range of about 10 ∼ 40 μm, which
is close to the power law size distribution found for interstellar dust grains. When gravity is invoked, the
strong dependency becomes weakened. Turbulence is found to dominate the time evolution of an initially
monodisperse droplet distribution at early times. At later times, however, gravity takes over and dominates
the collisional growth. With combined turbulence and gravity, the time scale to reach drizzle sized droplets is
about 900 s, which is close to the time scale of rapid warm rain formation. The collision rate grows exponen-
tially, which is consistent with the theoretical prediction of the continuous collisional growth even when the
turbulence-generated collision is invoked.
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