< strong>Droplet growth i n atmospheric turbulence

by Xiang-Yu Li **Department of Meteorology – MISU, Stcokholm University, Sweden**

~~Time: September 21 2018, 10h00~~

**Place:** Högbomsalen, Geoscience Building, sectio
n Y1, ground floor

**Abstract<
/strong>This Ph.D. thesis examines the challenging problem of how tur
bulence affects the growth of cloud droplets in warm clouds. Droplets grow
by either condensation or collision. Without turbulence, the condensation p
rocess driven by a uniform supersaturation field is only efficient when dro
plets are smaller than 15 μm (radius). Gravitational collision becomes effe
ctive when the radius of droplets is larger than 50 μm. The size gap of 15–
50 μm, in which neither condensation nor collision processes dominate dropl
et growth, has puzzled the cloud microphysics community for around 70 years
. It is the key to explaining the rapid warm rain formation with a timescal
e of about 20 minutes. Turbulence has been proposed to bridge this size gap
by enhancing droplet growth processes, and thereby, to explain rapid warm
rain formation. Since cloud–climate interaction is one of the greatest unce
rtainties for climate models, the fundamental understanding of rapid warm r
ain formation may help improve climate models. The condensatio
nal and collisional growth of cloud droplets in atmospheric turbulence is e
ssentially the problem of turbulence-droplet interaction. However, turbulen
ce alone is one of the unresolved and most challenging problems in classica
l physics. The turbulence–droplet interaction is even more difficult due to
its strong nonlinearity and multi-scale properties in both time and space.
In this thesis, Eulerian and Lagrangian models are developed and compared
to tackle turbulence–droplet interactions. It is found that the Lagrangian
superparticle model is computationally less demanding than the Eulerian Smo
luchowski model. The condensation process is then investigated
by solving the hydrodynamic and thermodynamic equations using direct numer
ical simulations with droplets modeled as Lagrangian particles. With turbul
ence included, the droplet size distribution is found to broaden, which is
contrary to the classical theory without supersaturation fluctuations, wher
e condensational growth leads to progressively narrower droplet size distri
butions. Furthermore, the time evolution of droplet size distributions is o
bserved to strongly depend on the Reynolds number and only weakly on the me
an energy dissipation rate. Subsequently, the effect of turbulence on the c
ollision process driven by both turbulence and gravity is explored. It is f
ound that the droplet size distribution depends moderately on the mean ener
gy dissipation rate, but is insensitive to the Reynolds number. Finally, th
e effect of turbulence on the combined condensational and collisional growt
h is investigated. In this case, the droplet size distribution is found to
depend on both the Reynolds number and the mean energy dissipation rate. Co
nsidering small values of the mean energy dissipation rate and high Reynold
s numbers in warm clouds, it is concluded that turbulence enhances the cond
ensational growth with increasing Reynolds number, while the collision proc
ess is indirectly affected by turbulence through the condensation process.
Therefore, turbulence facilitates warm rain formation by enhancing the cond
ensation process, which predominantly depends on the Reynolds number.**

**Welcome!**

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