The Taklimakan Dust acting as Ice nuclei observed by dual-wavelength and polarization lidar

Aerosol particles affect on the climate system through acting as cloud condensation nuclei (CCN) and ice nuclei (IN) as well as scattering and absorbing solar radiation. The CCN from aerosol increases cloud droplet number and cloud albedo, and reduces precipitation efficiency. Therefore CCN can be alternate cloud microphysics. While the ice formation is one of the important processes initiating precipitation and can alternate cloud lifetime. However, the influence of aerosol particles on changing the properties of ice formation is not yet understood. Mineral dust is the most abundant aerosol in the atmosphere in terms of mass and is considered the most important aerosol acting as ice nuclei. Taklimakan Desert is one of the most significant source of Asian dust. It is located in the northwest of China and surrounded by high mountains that exceeding an altitude of 4000 m. The Taklimakan dust is lifted up to about 5 km by the local circulation and it can play a role of the formation of the ice particles rather than other dust sources. Therefore Taklimakan dust is an important part of the climate system. However, the observational research that investigate the dust aerosol acting as IN is insufficient. Investigating the global distribution of clouds and aerosols as well as the vertical distribution of them can be possible by the space-borne lidar Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) which was launched in April 2006 onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite. However, the properties of the dust aerosols are to be difficult to assess due to attenuation of the laser because the clouds overlying the dust aerosols over the Taklimakan Desert often exist in spring. Consequently, in the present study, the ground-based lidar observation was carried out at Aksu, Xinjiang province to investigate the properties of the dust aerosol acting as IN over the Taklimakan Desert in March 2009. The avalanche photodiode unit was installed in the Aksu lidar at that time. Therefore the Aksu lidar was able to detect the 1,064 nm wavelength signal and to be examined the particle size of aerosols in addition to depolarization ratio of them. The results indicate that the Taklimakan dust was lifted up to 7 km by the local circulation and has a possibility of acting as ice nuclei. The total depolarization ratio of the dust aerosols was `0.3 and the cirrus clouds was `0.5, respectively. It was observed larger value (`0.5) of the total depolarization ratio at the cloud base height where might be coexisting with the dust than the value at above height. As a result, it was indicated that the Taklimakan dust act as ice nuclei when it was lifted up to high altitude. The attenuated backscatter color ratios (i.e. the ratio of attenuated backscatters at 1064 and 532 nm) of the aerosols and clouds were also examined. We determined the attenuated backscatter color ratio of the Taklimakan dust was less than unit. While the attenuated color ratio of cirrus clouds was larger than unit although it was to be difficult to assess because the backscatter was attenuated by the dust layer. The calibration method of 1,064 nm wavelength signal for existing the dust at lower layer that has large optical thickness is also described in this paper.