Report of Group 5: Claudia Acquistapace, Muhammad Bilal, Andreas Foth, Pilar Gumà-Claramunt, Edouard Martins

LIDAR is an active remote sensing technique that measures the properties of atmospheric features by sending linearly-polarized electromagnetic pulses in the ultra-violet, visible and near-infrared ranges and measuring the backscattered radiation. In particular, the depolarization lidar is able to measure the backscattered light, which gives information about the degree of depolarization of the scatterers by calculating the linear depolarization ratio, that is the ratio between the parallel and perpendicular components of the backscattered light at 532 nm: the lower this ratio, the more spherical are the particles. Therefore, by knowing the linear depolarization ratio of the different targets, and combining it with other information provided by the lidar, discrimination between different types of targets can be done.

Sun photometer is a multi-channel photometer tracking the sun that measures sun and sky radiances at different wavelenghts (between 340 nm and 1020 nm) and provides column-integrated measures, such as aerosol optical depth (AOD) and Ångström exponent (AE), that is a parameter that provides information on the size of the aerosols because of the wavelength dependence of aerosol optical depth: the larger the exponent, the smaller the particles. Sun photometer is very sensitive to cirrus contamination, while Lidar has no observation below its full-overlap height. During the measurement campaign, the absence of cirrus-free conditions made the direct comparison between measured column-integrated profiles from the sun photometer and the lidar not feasible. Nonetheless, a synergy between these two instruments can provide further information concerning the entire atmospheric column.

The case study analyzed is 26th September from 11.00 to 12.30 UTC. Figure 1 shows the Range Corrected Signal (RCS) for 1064 nm channel and the linear depolarization ratio. The 1064 nm RCS evidently shows the presence of three different layers, indicated in figure as layer 1, 2 and 3. Layer 1 can be identified as the Planetary Boundary Layer (PBL) by the presence of periodic thermal turbulent structures. Typical values of the Ångström exponent are shown in figure 2, while the Ångström exponent of layer 1 is 0.8 which indicates medium size particles. The depolarization ratio for this layer is 4.17%, showing that particles are low depolarizing thus not spherical. These two values indicate that the PBL likely contains a mixture of different aerosol types, basically spherical, but with some non-spherical particles, like dust for example, that can increase the depolarization ratio across this layer due to turbulent mixing. The layer 2 is extending from 1.25 to 2 km high and has an Ångström exponent of 0.53, while layer 3, from 2 to 2.25 km high, exhibits a lower Ångström exponent, equal to 0.18. The depolarization ratio value is close to 2%, which indicates that the aerosol particles in these layers are less depolarizing than the ones in the PBL. From this data, we can deduce that layers 2 and 3 can contain smoke. Additionally, from the Ångström exponent it can be assumed that layer 3 is made of larger particles, probably aged smoke. Since these aerosols resided in the atmosphere for longer time, water vapour had more time to condensate on them and to create a bigger spherical aggregate compared to the layers below. To support this hypothesis, backtrajectories of the corresponding air masses have been calculated with HYSPLIT model and compared with fire maps derived from MODIS images over the region: a good agreement has been found, with the air flow passing over some fires in Belgrad area.

Finally, we tried to investigate the presence of some oscillating structures, clearly evident in the 1064 nm channel between 2 and 2.5 km high, 12.00 to 12.30 UTC. Observations reveal the presence of a periodical oscillation, with a spatial period on the scale of 100 m over 30 minutes. The length and time scale of this phenomenon did not give us the opportunity to find a definite answer concerning what is observed. In fact it can be due to boundary layer thermals inducing oscillations in the upper layer or to atmospheric waves due to instabilities of the upper layers themselves. Additional measurements are needed to understand and confirm one of the two hypotheses we made concerning this feature.