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 tempe rature profiles, height of various layers, etc.) associated with the intera ction processes of mountain-valley circulation and urban heat island is fol lowed. The observations are carried out using lidars (aerosol and Raman), k ytoon (tethered balloon) and pilot balloons over the city of Sofia (42 degr ees 39' N, 23 degrees 23' E, 591 m above MSL) and cover heights from 70 to 900 m. The presence of neighboring mountains and the urban heat island effe ct lead to the formation of three temperature inversions (ground, elevated and capping) and specific vertical profiles of the wind velocity. The forma tion mechanism is considered of two-layer aerosol structure in the atmosphe re 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 c ommon 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. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.