Seminarium fizyki atmosfery

Condensational and collisional growth of cloud droplets in a turbulent environment

dr Xiang-Yu Li

NORDITA, KTH & Stockholm University, Department of Meteorology, Stockholm University


25 stycznia 2019 13:15, ul. Pasteura 5, B0.14

The effect of turbulence on combined condensational and collisional growth of cloud droplets is investi-
gated using high-resolution direct numerical simulations. The motion of droplets is subjected to both turbu-
lence and gravity. We solve the thermodynamic equations that govern the supersaturation field together with
the hydrodynamic equations describing the turbulence. The collision-coalescence process is approximated by
a superparticle approach assuming unit collision and coalescence efficiency, i.e., droplet coalesce upon colli-
sion. Condensational growth of cloud droplets due to supersaturation fluctuations depends on the Reynolds
number, while the collisional growth was previously found to depend on the mean energy dissipation rate.
Here we show that the combined processes depend on both Reynolds number and the mean energy dissipation
rate. Droplet size distributions broaden either with increasing Reynolds number or mean energy dissipation
rate in the range explored here. Even though collisional growth alone is insensitive to Reynolds number, it
is indirectly affected by the large scales of turbulence through condensation. This is argued to be due to the
fact that condensational growth results in wider droplet-size distributions, which triggers collisional growth.
Since turbulence in warm clouds has a relatively small mean energy dissipation rate, but a large Reynolds
number, turbulence mainly affects the condensational growth and thus influences the collisional growth indi-
rectly through condensation. Thus, the combined condensational and collisional growth of cloud droplets is
mostly dominated by Reynolds number. This work, for the first time, numerically demonstrates that supersat-
uration fluctuations enhance the collisional growth. It supports the findings from laboratory experiments and
the observations that supersaturation fluctuations are important for precipitation.

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