Urban effects on cloud base height and cloud persistence over Sofia, Bulgaria

Cities may have local weather and climate that are significantly different from their surrounding rural areas due to different physical characteristics of urban surfaces and emissions of substances, the latter being modulated by the rhythm of the urban ecosystem. Although the urban heat island is the most recognizable phenomenon caused by these features, the altered radiative, thermal, moisture, and aerodynamic properties of the urban surface relative to the rural one also influence clouds formation as well as their characteristics. The study of the latter is of considerable interest as they are an important element of the energy and water budget of the surface. Regardless of the cloud’s importance, establishing and quantifying urban effects on cloudiness is a significant challenge, as their formation is not limited to processes confined within the urban area, but depends on processes spanning different spatial scales. The inhomogeneous surface of the rural surrounding and the complex landscape are common to many cities and that can further hinder the detection of the urban signal in the cloud features. Also, clouds can be blown by the prevailing wind, so those over the city may originate from quite a distant area. By using in-situ measurements as well as data from remote sensing instruments (ceilometers) located in the city centre and its outskirts, we attempt to detect and quantify the urban impact on cloudiness over the city of Sofia in this work. To achieve this, we analysed 10 years of data from automatic weather stations and ceilometers. The former allows us to calculate lifting condensation level, which has been used as a proxy for the cloud base height. The latter parameter is routinely determined by the ceilometers, which also allow us to evaluate cloud persistence. Urban effects were sought in both cloud parameters.