Changes in low clouds optical characteristics in the atmosphere of two different urban sites in Romania

The aerosol particles that become condensation nuclei affect the formation and evolution of clouds, their cycle lifetime, and their optical properties, having thus a major impact on the atmosphere, the radiative balance and leading therefore to climate changes. The aim of the study is to investigate the second indirect aerosol effect and the relationship between cloud droplet effective radius and cloud albedo for cleaner and polluted clouds over two sites in Romania (Bucharest and Cluj-Napoca), using satellite data collected from March 2000 to March 2022. Present study is the first one over sites in Romania. A series of physical parameters (albedo, cloud cover fraction, cloud optical depth, liquid water path and cloud water radius) were extracted from the Clouds and the Earth's Radiant Energy System (CERES) database for Bucharest, as a high polluted city, and for Cluj - Napoca, as a cleaner city in Romania. The time series for albedo and low cloud characteristics contained 193.584 hourly profiles. In addition, based on the optical depth from CERES, we calculated low cloud albedo using a parameterization currently used in the climate models. The study was also focused on the life cycle of the low clouds and on the cloud cover fraction over the two sites. The annual and seasonal variations of the cloud physical parameters were investigated and compared for both sites. We highlight how the cloud droplet effective radius modifies differently the cloud albedo for polluted clouds over Bucharest (presence of more and smaller cloud droplets and thus, a higher cloud albedo and less drizzle size drops) than for the cleaner clouds over Cluj-Napoca (presence of fewer and larger cloud droplets, and therefore a lower cloud albedo). The results also shows comparatively the frecqueny of occurence of this type of clouds over both sites and the temporal trend of analyzed physical characteristics. Modifications and variations of cloud characteristics at a city scale help us to better understand the second indirect aerosol effect, life cycle and the climatology of low clouds.

Acknowledgment

GLSG, SS and GI acknowledge the support from NO Grants 2014-2021, under contract no 31/01.09.2020, Project EEA-RO-NO-2019-0423. GLSG work was also supported by the University of Bucharest, PhD research grant and by the Romanian Nucleu Programme.