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CATEGORIES:Defense
SUMMARY:MISU PhD defense | Xiang-Yu Li
DESCRIPTION;ENCODING=QUOTED-PRINTABLE:\n\nDroplet growth in atmospheric turbulence\n\n\nby Xiang-Yu Li \nDepartme
nt of Meteorology – MISU, Stcokholm University, Sweden\n\n\nTime: September
21 2018, 10h00\n\n\nPlace: Högbomsalen, Geoscience Building, section Y1, g
round floor\n\n\n\n\n\nAbstract\nThis Ph.D. thesis examines the challenging
problem of how turbulence affects the growth of cloud droplets in warm clo
uds. Droplets grow by either condensation or collision. Without turbulence,
the condensation process driven by a uniform supersaturation field is only
efficient when droplets are smaller than 15 μm (radius). Gravitational col
lision becomes effective when the radius of droplets is larger than 50 μm.
The size gap of 15–50 μm, in which neither condensation nor collision proce
sses dominate droplet growth, has puzzled the cloud microphysics community
for around 70 years. It is the key to explaining the rapid warm rain format
ion with a timescale of about 20 minutes. Turbulence has been proposed to b
ridge this size gap by enhancing droplet growth processes, and thereby, to
explain rapid warm rain formation. Since cloud–climate interaction is one o
f the greatest uncertainties for climate models, the fundamental understand
ing of rapid warm rain formation may help improve climate models. \n\n The
condensational and collisional growth of cloud droplets in atmospheric turb
ulence is essentially the problem of turbulence-droplet interaction. Howeve
r, turbulence alone is one of the unresolved and most challenging problems
in classical physics. The turbulence–droplet interaction is even more diffi
cult due to its strong nonlinearity and multi-scale properties in both time
and space. In this thesis, Eulerian and Lagrangian models are developed an
d compared to tackle turbulence–droplet interactions. It is found that the
Lagrangian superparticle model is computationally less demanding than the E
ulerian Smoluchowski model. \n\n The condensation process is then investiga
ted by solving the hydrodynamic and thermodynamic equations using direct nu
merical simulations with droplets modeled as Lagrangian particles. With tur
bulence included, the droplet size distribution is found to broaden, which
is contrary to the classical theory without supersaturation fluctuations, w
here condensational growth leads to progressively narrower droplet size dis
tributions. Furthermore, the time evolution of droplet size distributions i
s observed to strongly depend on the Reynolds number and only weakly on the
mean energy dissipation rate. Subsequently, the effect of turbulence on th
e collision process driven by both turbulence and gravity is explored. It i
s found that the droplet size distribution depends moderately on the mean e
nergy dissipation rate, but is insensitive to the Reynolds number. Finally,
the effect of turbulence on the combined condensational and collisional gr
owth is investigated. In this case, the droplet size distribution is found
to depend on both the Reynolds number and the mean energy dissipation rate.
Considering small values of the mean energy dissipation rate and high Reyn
olds numbers in warm clouds, it is concluded that turbulence enhances the c
ondensational growth with increasing Reynolds number, while the collision p
rocess is indirectly affected by turbulence through the condensation proces
s. Therefore, turbulence facilitates warm rain formation by enhancing the c
ondensation process, which predominantly depends on the Reynolds number.\n\
n\nWelcome!\n\n\n \n\n\n \n
DTSTAMP:20190922T075742Z
DTSTART:20180921T100000Z
DTEND:20180921T130000Z
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