Ž .
MPIK-Heidelberg Mohler and Arnold, 1992 indicate that the solid particles are coated
¨
by a supercooled liquid solution, which is formed by heterogeneous H SO rH O
2 4
2
nucleation and condensation since gaseous sulfuric acid is continuously formed during daytime and highly supersaturated with respect to supercooled liquid H SO rH O. It is
2 4
2
conceivable that the coating may also proceed by coagulation of the very small but numerous H SO rH O droplets with solid aerosol particles.
2 4
2
Ž .
Takano and Liou 1995 observe that fundamental scattering, absorption and polariza- tion data on the types of nonspherical ice crystals occurring in cirrus clouds are required
for reliable modelling of the cloud radiative properties; for interpretation of the observed fluxes and heating rates; for incorporation in GCMs and mesoscale cloud models; and
for development of remote sensing techniques to infer cloud optical depth, temperature and ice crystal particle size. Retrieval of cloud parameters from remote sensing devices
and accurate calculations of precipitation growth also require precise knowledge of the ice particle habits. The use of wrong particle shapes in satellite retrievals of cloud
optical thickness may result in an underestimation or overestimation of the optical
Ž .
thickness of clouds by a factor that can exceed three Mishchenko et al., 1996 . In calculating the mass of precipitation, the wrong use of ice particle shape may give an
Ž .
error in particle mass by a factor of 15 Mason, 1994 . The focus of the present experiment was to look into the scattering and polarizing
properties of water clouds and that of sulfuric acid clouds in the forward and backscatter Ž
. region. As discussed by Sassen and Liou 1979 , there is a lack of rigorous theoretical
solution to the scattering of light by particles with arbitrary geometry such as ice crystals. Mie scattering theory cannot be applied to the various crystal habits with
Ž .
favoured free fall orientations. Liou 1972 has proposed a theory for cylinders oriented randomly in a horizontal plane, which could be applied to the scattering behaviour of
Ž .
needles and similar habits. Takano and Liou 1995 have developed a new Monte Carlo geometric ray-tracing method for the computation of the scattering, absorption, and
polarization properties of ice crystals with various irregular structure, including hollow columns, bullet rosettes, dendrites, and capped columns. The numerical methods for
determining cloud composition are limited by the fact that the results therein are based on the parameterized optical properties of cirrus clouds.
Experimental findings must be relied on for the present to characterize the scattering properties of mixed phase clouds and assess the validity of approximate theoretical
approaches to this problem. Laboratory experiments performed under defined controlled conditions can help in gaining insight into the depolarization processes that brings about
Ž .
variation in the linear depolarization ratio LDR at various scattering angles with changing acid concentrations.
2. Experimental arrangement
Experiments were carried out in an experimental chamber kept inside a walk-in cold room, which has a lowest attainable temperature of y308C. The experimental chamber
consists of a spherical glass flask with extended cylindrical limbs on top and bottom. View ports in the form of protracted glass tubes are provided along the greater
circumference of the flask at regular intervals of 458. In addition, there is a port at 1578 to facilitate observations closer to backscattering. Fig. 1 shows the schematic representa-
tion of the experimental arrangement. Ž
A solution of sulfuric acid and water in definite proportion 0, 10, 50 and 70 .
sulfuric acid by weight in water is heated at a controlled rate and the vapour is introduced into the chamber to form a cloud of supercooled droplets and vapour. The
total system is closed with no leakage of air in or out. A PT-1000 sensor records the temperature of the cloud and the data is recorded in real time on a computer-based data
acquisition system. When a steady-state temperature is reached, the cloud is seeded with a rod dipped in liquid nitrogen. This initiates ice crystal formation by homogeneous
nucleation followed by other ice multiplication processes. The cloud temperatures at the time of seeding varied from y158C to y178C. As the particles grow and dissipate, the
falling ice crystals are collected on a formvar-coated microscope slide and are analyzed for crystal shape, size, number density and growth pattern. Each slide is inserted into the
cloud for about 10 s, but at times, the slide remained inside the cloud for about 11 to 12 s. In order to remove this error of extra exposure time, the number density of ice crystals
has been expressed as number of crystalsrmm
2
rs. Each cloud cycle lasts for about 5 min duration and four slides are collected per cloud cycle. It should be noted that the
quoted solution strengths are not the actual strengths of the droplets andror crystals forming the cloud in the chamber. In order to avoid contamination of the chamber, we
carried out pure water runs first followed by increasing acid strength of the solution. For every acid strength, a fresh solution was prepared and the entire chamber was cleaned.
Ž .
A laser beam from a 2-mW polarized He:Ne laser manufactured by Melles Griot is Ž
. directed into the cloud and a Photo Multiplier Tube manufactured by Thorn EMI ,
which has an analyzer on its front, measures the scattered intensity. Experiments were
Fig. 1. Schematic representation of the experimental arrangement.
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
