Optical parameters when DMSO is seeded

In clear conditions the intensity of direct beam is always 13.4 J/m only. However, after the introduction of steam, the same intensity drops to a value about 6 J/m only. In case of seeding, the scattered beams are produced at the expense of direct intensity. The forward-scattered intensities at 30°

Table 2. Crystal habits when DMSO-coated Ammonium Sulfate is seeded. Temperature Crystal habit

— 22°C to —17°C Dendrite formations are highest. Rod and rectangular shapes are also significant. [Dendrite (43%): 12.5-17.5 jm; Rod (32%): 12.5-17.5 um; Rectangular (25%): 10-15 jm]

— 17° C to —8° C Rectangular formations are dominant. Rod, dendrite, hexagonal, and cubic forms are also present. [Rectangular (61%): 5-7 jm; Rod (19%): 15-50 jm; Dendrite (10%): 8-12 jm; Hexagonal (6%): 5-12 jm; Cubic (4%): 7-15 jm] —8° C to 0°C Rod shapes are dominant. Cubic, rectangular, and hexagonal are other formations. [Rod (53%): 5-12 jm; Cubic (19%): 7-10 jm; Rectangular (18%): 5-12 jm; Hexagonal (10%): 10-20 jm]

Fig. 5. Variation of scattering intensity at different angles against seeding temperature.

and 36° are of the order of mJ/m, whereas, in general, the corresponding intensities at 144° and 150° are one or two orders less. The temperature variation of maximum scattered intensity at different seeding angles is presented in Fig. 5. All these values are the mean of four observations. The direct intensity sharply falls as the seeding temperature increases from —22°C to —21°C, and then it shows a slow rise as the temperature rises. This is understandable, as below —21°C, the crystals are quite insignificant in number as well as size. As the crystal number and modal dimension show an overall gradual fall with a rise in temperature beyond — 21°C, the direct intensity also slowly rises.

In case of scattered intensity, there are three distinct peaks in forward-scattering intensity (at 30° and 36°) at —21°C, close to —14°C, and —8°C. The peaks at —21°C and close to — 14°C are understandable, as both the crystal number and modal dimension show peak at those temperatures. The highest peak naturally occurs at —21°C. But, the justification for the peak in forward-scattering intensity at —8°C is unclear.

In case of backward-scattering intensity (at 144° and 150°), the highest peak occurs at —13° C. The backward-scattering at —13° C is even greater than the forward-scattering intensity at the corresponding temperature. Nonspherical ice crystals are known to produce various features like corona and halo.30-32 It is reported that hexagonal crystals can produce halo when back-scattering intensity significantly increases. This happens mainly due to successive reflection and refraction in a hexagonal crystal.33 Since at the seeding temperature of —13° C, the crystals are dominantly hexagonal in shape and also there are some cubic and prism-shaped crystals, the dominant back-scattering intensity at —13°C seems to be due to halo effect. In fact, strong back-scattering effect is generally observed in cirrus cloud top region.31

The optical parameters like scattering coefficient, extinction coefficient, and optical thickness are three important optical parameters, characterizing a crystal cloud. The variation of scattering coefficient with seeding temperature is presented in Fig. 6, for different angles of scattering. The scattering coefficient is found to attain the peak value at —21°C for all angles of scattering. It shows a second peak close to —13°C. The back-scattering coefficient at 144° is found to be greater than the forward-scattering coefficient at 36° at that temperature. Around —18°C to —16°C, the back-scattering coefficient at 144° is found to have the maximum value, as cubic crystals are dominant. However, the absolute value is quite less there. A representative diagram for the variation of extinction coefficient

Fig. 6. Variation of scattering coefficient with seeding temperature.

temperaturefc)

Fig. 6. Variation of scattering coefficient with seeding temperature.

Fig. 7. Variation of extinction coefficient and optical thickness with seeding temperature.

and optical thickness with seeding temperature is presented in Fig. 7. These curves are prepared from the scattering intensities at 30°. As expected, the curves for both the parameters have three peaks at — 21°C, close to —14°C, and —8°C. A plot for extinction coefficient and the ratio of scattering intensity at 30° to extinction coefficient against time counted from the instant of seeding is presented in Fig. 8. Time variation of extinction coefficient from the instant of seeding is also presented in the same figure. The curve becomes almost flat only 15 s after seeding. During the later period, the actual intensity shows some fluctuations though the above-mentioned ratio remains more flat. This result confirms an earlier observation made by Saunders et al.34

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