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 BR for 39 cases of BGA in 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 DR and RH for all the BGA cases in the altitude range of 0.7-2 km in Spring. On average, the aerosol DR of BGA is smaller than 0.06. The mean aerosol DR (solid line) shows the decreasing trend with increasing RH. The effect of RH on aerosol DR is expected as the particles are more nonspherical in the dry air. The increased RH will reduce the DR. This is 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 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 of this water cloud shows similar tendency with BR in cloud layers. Figure 6 shows the relationship between DR and RH for 107 water-cloud cases at an altitude range of 0.7-3 km, where no correlation can be identified. The mean DR is around 0.015.

Backscattering Ratio

Fig. 3. Backscattering ratio profile of mean background aerosols in spring (March-May) during 2002-2004. The horizontal bars represent the standard deviation from the mean.

Fig. 3. Backscattering ratio profile of mean background aerosols in spring (March-May) during 2002-2004. The horizontal bars represent the standard deviation from the mean.

0.00 0.01 0.02 0.03 0.04 0.05 0.06 Depolarization Ratio (8 a)

Fig. 4. Depolarization ratio of aerosols (cross symbol) at 0.7-2 km vs RH for all the cases in Spring (March-May) from 2002 to 2004. The solid line shows the mean profile of the DR as a function of RH, obtained by calculating the average (cross symbol) values with interval ranges of 5%. The error bars represent the variability within these intervals of RH ranges.

Depolarization Ratio (8 a) 0.00 0.02 0.04 0.06

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Backscattering Ratio

Backscattering Ratio

Fig. 5. The height distribution of backscattering ratio (—), depolarization ratio (—), and relative humidity (Q) on 21 March 2002.

Fig. 6. Depolarization ratio measured for water cloud layers (square symbol) at 0.73 km as a function of RH for all the cases in Spring (March-May) from 2002 to 2004. The solid line shows the mean profile of the DR as a function of RH, obtained by calculating the average (square symbol) values with interval ranges of 5%. The error bars represent the variability within these intervals of RH ranges.

Fig. 6. Depolarization ratio measured for water cloud layers (square symbol) at 0.73 km as a function of RH for all the cases in Spring (March-May) from 2002 to 2004. The solid line shows the mean profile of the DR as a function of RH, obtained by calculating the average (square symbol) values with interval ranges of 5%. The error bars represent the variability within these intervals of RH ranges.

Fig. 7. The occurrence probability of backscattering ratio with the value higher than 6 vs relative humidity.

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