Geoscience Reference
In-Depth Information
(a)
(b)
Complicating process:
R
Small droplet caught in air
flow around large droplet
Wake of large drop allows
small droplet to coalescence
Figure 11.1
(a) Collision of
a large cloud droplet with a
small cloud droplet (b)
complicating process that
means the impact factor,
y
,
is not a perfect diagnostic
of collision likelihood.
y
Thin air layer keeps droplets
separate
r
1.0
Large collector
drop (e.g. 30
μ
m)
Medium collector
drop (e.g. 20
μ
m)
0.1
0.01
Small collector
drop (e.g. 10
Figure 11.2
Typical profiles
of collision efficiency for two
falling droplets with different
radii. Note the low magnitude
of the collision efficiency for
a small collector drop.
μ
m)
0.001
0
0.25
0.5
0.75
1.00
Ratio of radius of smaller drop to larger drop
(
)
vv E
−
dR
dt
R
r
c
=
(11.1)
4
r
w
where
v
R
and
v
r
are the terminal velocities of the droplets with radii
R
and
r
, respec-
tively,
w
is the liquid water content of the cloud per unit volume,
r
w
is the density
of liquid water, and
E
c
is the collection efficiency for droplets with radii
R
and
r
.
