Lidar observation of the nocturnal boundary layer formation over Sofia, Bulgaria

A study of the formation of stable stratification after sunset is presented. The dynamics of atmospheric parameters (e.g., the wind velocity and temperature profiles, height of various layers, etc.) associated with the interaction processes of mountain–valley circulation and urban heat island is followed. The observations are carried out using lidars (aerosol and Raman), kytoon (tethered balloon) and pilot balloons over the city of Sofia (42°39′N, 23°23′ E, 591 m above MSL) and cover heights from 70 to 900 m. The presence of neighboring mountains and the urban heat island effect lead to the formation of three temperature inversions (ground, elevated and capping) and specific vertical profiles of the wind velocity. The formation mechanism is considered of two-layer aerosol structure in the atmosphere over the urban area and its dynamics observed by the lidar after sunset. The results obtained are compared with a model developed on the basis of common models: one describing nocturnal boundary layer development within a mountain valley and the other dealing with stable stratification formation in the case of urban area situated on plain terrain.