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and other unknown particles. Dust is a kind of aerosol that only occurs
occasionally in the spring time. So, they should be treated as separate
cases in addition to BGA.
For clouds definition we have to consider two points: First, the
backscattering coecient should be five times greater than that scattered
by air molecules. Such a higher backscattering coecient usually occurs
when the scattering is by larger particles (such as water drop), and it is
independent of the measured wavelengths of the laser. Second, the local
RH
should be higher than 70% at the cloud height. Usually, the clouds observed
in the regions (0.7-3 km) have temperature higher than 0
◦
Cwith
close
to 0, because they consist of water drops (refer to Fig. 5), except for some
special events such as the occurrence of dust storm from Northern China
(Fig. 9).
DR
3. Results and Discussion
3.1
. Optical properties of BGA and water clouds
Figure 2 shows the results of
BR
,
DR
,and
RH
for one of the BGA cases
on 7 March 2002. The mean height profile of
for39casesofBGAin
Spring during the period 2002-2004 is shown in Fig. 3. Each case used in
the analysis has been integrated over an hour from 20.00 to 21.00 local
time, and this time is very close to the time of launching of the radiosonde,
and thus both can be compared. We found that most BGA are distributed
below 2 km over our location. Figure 4 shows the relationship between the
aerosol
BR
for all the BGA cases in the altitude range of 0.7-2 km
in Spring. On average, the aerosol
DR
and
RH
DR
of BGA is smaller than 0.06. The
mean aerosol
DR
(solid line) shows the decreasing trend with increasing
RH
is expected as the particles are more
nonspherical in the dry air. The increased
. The effect of
RH
on aerosol
DR
.Thisis
due to the fact that increased humidity will cause hygroscopic aerosols to
grow and its shapes will become more symmetrical like water drops, which
will ultimately reduce the
RH
will reduce the
DR
.
Figure 5 shows a water-cloud case on 21 March 2002. The water cloud
layers were observed in the height range of 1-2 km. The
DR
DR
of this water
cloud shows similar tendency with
BR
in cloud layers. Figure 6 shows the
relationship between
for 107 water-cloud cases at an altitude
range of 0.7-3 km, where no correlation can be identified. The mean
DR
and
RH
DR
is
around 0.015.
