Table 1 Scattered intensity measurements for different combinations of laser and analyzer positions
Laser position Analyzer position
Measured intensity Parallel
Parallel M1
Parallel Perpendicular
M2 Perpendicular
Parallel M3
Perpendicular Perpendicular
M4
first carried out for a 458 angle and then later repeated at 1358 and 1578 angles to the forward direction. There is a light trap fixed on the port opposite the laser to prevent any
spurious backscattering signal. The plane of polarization of the incident and scattered beam is switched between parallel and perpendicular positions by alternately rotating the
laser and the analyzer. The scattered intensity measurements for different combinations of laser and analyzer positions are given in Table 1. The LDR is determined from these
Ž . measured intensities. For vertically polarized incident energy, LDR V is the ratio of
Ž . M2rM1 and for horizontally polarized incident energy, LDR H is the ratio of M3rM4.
3. Results
The LDR for sulfuric acid clouds of different concentrations is measured at angles of 458, 1358 and 1578 to the forward direction. The LDR values plotted in Fig. 2 are those
observed during the first 3 min after seeding the cloud. During these 3 min, the cloud is almost in a glaciated state. Each data point on the graph in Fig. 2 represents an average
Fig. 2. Average LDR values for the scattered intensities measured at 458, 1358 and 1578 plotted against percentage of sulfuric acid in water by weight. Each data point on the graph represents an average of 25
experiments performed at that particular concentration. Error bars represent standard error.
Table 2 Number density, size and LDRs observed at 1358 for different cloud runs
Ž . Ž .
Percentage of Number density
Size LDR V
LDR H
2
Ž . Ž
. Ž
. sulfuric acid
crystalsrmm rs mm
13 30
0.304 0.437
10 17
20 0.307
0.450 50
15 30
0.299 0.411
70 13
20 0.261
0.317
of 25 experiments performed at that particular concentration and error bars represent the standard error. It is seen that the magnitude of LDR increases with the scattering angle.
The magnitude of LDR observed at angles closer to backscattering is higher and agrees Ž
. with the angular scattering pattern for mixed phase clouds Sassen and Liou, 1979 . It is
seen that average values of LDR at 458 and 1358 does not change much as the strength Ž .
of sulfuric acid increases. However, for 1578, the change is large, LDR V for water is Ž .
0.69, and it increases to 0.85 for 50 sulfuric acid. Similarly, LDR H for water is 0.73, and it increases to 0.92 for 50 sulfuric acid. Scattering experiments at 1578 were done
only for pure water and 50 sulfuric acid. Since the slide collection port was not at a convenient location while the system was set for observations at 1578, some of the slides
are over exposed and the microphysical data associated with theses runs was not good. Due to this reason, we did not carry out experiments for 70 sulfuric acid at this angle.
It is seen that the behavior of LDR at 1578 agrees with the polarization lidar studies Ž
. of corona producing cirrus clouds. Sassen et al. 1995, 1998 have found that these high
Ž .
clouds produce relatively strong laser depolarization f 0.5 to 0.8 , indicative of
Ž .
complex shaped ice crystals. Sassen et al. 1995 presumed that these are radial particles that have a polycrystalline structure due to the effects of a liquid sulfuric acid coat,
which is driven to the surface of the ice germ during droplet freezing under the observed Ž
. environmental conditions. Sassen and Liou 1979 have observed that the depolarized
component is not a simple function of scattering angle. Polarization measurements by Ž
. Sassen and Liou 1979 demonstrate that the dominant depolarizing process is a function
of cloud particle shape. Table 2 gives the parameters measured for cloud runs with various concentrations of
sulfuric acid with the PMT mounted at 1358. If we consider the cases for pure water and Ž
. 50 sulfuric acid, with crystals having the same size 30 mm , it is seen that LDR is
higher in the case of pure water as compared to the 50 sulfuric acid cloud. If we consider the case for pure water and 70 sulfuric acid, having similar crystal number
Table 3 Number density, size and LDRs observed at 1358 for water and 70 sulfuric acid cloud 4 min after seeding
Ž . Ž .
Concentration Time after seeding
Number density Size
LDR V LDR H
2
Ž .
per mm rs mm
Water fourth minute
11 25
0.296 0.412
70 H SO fourth minute
11 20
0.211 0.298
2 4
Table 4 LDR at the first and fourth minute after seeding for two different runs having 70 sulfuric acid concentration
Ž . Ž .
Percentage of Time after seeding
Crystal number density Size
LDR V LDR H
Run
2
Ž . Ž
. H SO
per mm rsec mm
2 4
70 first minute
22 15
0.345 0.319
Run A 70
fourth minute 22
20 0.306
0.310 Run B
Ž
2
. density 13 crystalsrmm rs , it is seen that LDR for pure water is higher than that of
the 70 sulfuric acid cloud. The LDR for 70 and for 50 acid concentrations are lower than pure water, which could be due to either the size factor or an acid coat on the
crystal or could be due to both effects.
It is difficult to explain the tendency for the depolarization ratio to decrease with the increasing concentration of sulfuric acid at 1358 scattering angle. It may be the case that
LDR responds to increasingly mixed phase cloud conditions because the more concen- trated acid drops will shrink into haze particles, but would not disappear during seeding
like pure water as the humidity drops. It is also possible that the acid coat could be
Ž .
changing the refractive index of the crystals which is influencing LDR. Kerker 1969 has studied the influence of refractive index on polarization ratio and found that for the
same refractive index the behaviour of the polarization ratio is different at different angles.
Although it is possible to monitor the variation of LDR with time during a given cloud cycle, it is difficult to attribute the magnitude of variation in LDR to a particular
factor. The number density of crystals is falling with time in a given cloud cycle, which causes LDR to reduce as the ratio of crystal to droplets changes. LDR variation in a
cloud cycle, observed at a particular angle, will be a function of not just the concentra-
Fig. 3. Typical photomicrograph of crystalline ice particles collected from a cloud formed from pure water seeded with liquid nitrogen. Dominant crystal habit is plate and average size is ;15 mm. 1 scale divisions10
mm.
tion of sulfuric acid but will also depend on size, shape and number density of the Ž
. crystals effectively the ratio of crystals to drops at that instant.
To study the effect of sulfuric acid concentration on LDR, we tried to isolate some of the runs having identical number densities and comparable sizes observed at the same
time after seeding. LDR values for identical runs at 1358 for water and 70 sulfuric acid are tabulated in Table 3. LDR values are smaller for 70 H SO crystals, which could
2 4
be either due to the size factor or due to an acid coat. However, the size is not significantly different, and, therefore, it is possible that the effect due to an acid coat is
dominant here. Table 4 shows the variation of LDR during the first and fourth minute after seeding for 70 sulfuric acid at 1358 scattering angles and gives values for two
separate runs referred as Run A and Run B obtained under similar conditions of
Ž . Fig. 4. a Photomicrograph of ice crystals collected from a cloud formed from 50 sulfuric acid by weight in
Ž . water. Dominant crystal habit is plate and average size is ;12 mm. 1 scale divisions10 mm. b
Photomicrograph of ice crystals collected from a cloud formed from 50 sulfuric acid by weight in water seen at higher magnification. Size of the crystal is ;80 mm. 1 scale divisions2.5 mm.
Fig. 5. Photomicrograph of an individual representative ice crystal collected from a cloud formed from 70 sulfuric acid by weight in water. Size is ;55 mm. 1 scale divisions2.5 mm.
concentration, temperature, number density and size. It is clearly seen that LDR for Run B is less than that of Run A. This is because the parameters for Run A are for the first
minute when the crystals have sharp edges and that of Run B are for the fourth minute when the crystals have acquired a coat of acid film and the edges are rounded.
Fig. 3–5 shows photomicrographs of crystalline ice particles collected from a cloud formed from pure water, 50 and 70 sulfuric acid, respectively. For crystals formed
from pure water, the crystal habit is well defined and the crystal boundaries are clear and sharp. For acid crystals, it is found that each crystal is surrounded by a halo of
evaporated acid solution products, which resemble fuzzy dark spots and the crystal habit cannot be recognized. These evaporation products must be responsible for changing the
refractive index and shape of the crystal, and thereby the scattering properties. When the cloud is seeded with liquid nitrogen, homogeneous nucleation is initiated by adiabatic
cooling thereby causing water and acid to freeze together. As the cloud becomes warmer, the frozen particles start to evaporate and the sulfuric acid leaves the bulk ice
Ž .
and reaches the surface to form droplets, or a film of sulfuric acid. Sassen et al. 1989 have observed such spots while studying backscatter laser depolarizing properties during
evaporation of clouds composed of sulfuric acid solution droplets, some treated with ammonia gas. It is also possible that the acid does not freeze, even when near the LN
2
rod, only the water part freezes. When the vapour supply is turned off, the ice particle growth will tend to reduce the ice super-saturation, and it will take a while for
evaporation to begin to occur. The end result, however, would be the same, an acid coated ice particle.
4. Discussion
