Cirrus clouds play an essential role in the radiative balance of the system Earth-atmosphere. At present, they are one of the main sources of uncertainties appearing in the up-to-date numerical models of the Earth’s circulation and global climate change. Indeed, though cirrus clouds are often optically thin, they cover in average 30% of the Earth surface, the coverage being 60%-70% in tropics.

Therefore, the microphysical characteristics of cirrus clouds (sizes, shapes, spatial orientation, number density and their vertical profiles) are very important and need to be further studied in terms of experiment and theory.

A recent study jointly conducted by Dr. WANG Zhenzhu with his colleagues in Anhui Institute of Optics and Fine Mechanics (AIOFM) and Prof. Anatoli Borovoi with his group in Institute of Atmospheric Optics (IAO) of the Chinese Academy of Sciences has made new improvement on theory of lidar exploring of cirrus under international co-operation.

In their study, it was the first time to calculate the backscattering Mueller matrix for the hexagonal ice columns and plates with both zenith and azimuth preferential orientations. Besides, the possibility of a vertically pointing polarization lidar measuring the full Mueller matrix for retrieving the orientation distributions of the crystals is considered.

Through their study, It is shown that the element m₄₄ or, equivalently, the circular depolarization ratio distinguishes between the low and high zenith tilts of the crystals. Then, at their low or high zenith tilts, either the element m₂₂ or m₃₄, respectively, should be measured to retrieve the azimuth tilts.

The study was pulibshed in OPTICS EXPRESS with title Backscatter by azimuthally oriented ice crystals of cirrus clouds.

This work is funded by National Natural Science Foundation of China (NSFC) and Chinese Academy of Sciences President’s International Fellowship Initiative (PIFI).

The element m₃₄ versus the azimuth A and zenith B tilts (Image by WANG Zhenzhu)