Lidar observation of breeze stucture during the transition periods at the Southern Bulgarian Black Sea coast

The paper presents a study of the sea-breeze transition periods performed using the relative volume extinction coefficient of the aerosol. The study is based on data acquired by a ground-based aerosol lidar and conventional meteorological instruments. The observations took place near Akhtopol in the southeastern corner of Bulgaria in the period 18–25 June 1993. The attention is focused on aerosol structure, and in particular on the distribution of aerosol extinction, in the lowest 700 m across shoreline during the transition periods and development of the breeze front. The lidar cross-sections from the transition periods reveal existence of three differing zones in the relative extinction distribution: a zone over the surface, which possesses the optical properties of the entire layer which develops subsequently; a second zone, which possesses the optical characteristics typical for the circulation before the transition; and a third zone, characterized by the lowest relative extinction and placed between the other two, which dissipates by the end of the transition. The estimated values of the extinction coefficient are significant for comparison only, i.e. relatively, rather than as absolute magnitudes, since no calibration using an independent mean was performed. The paper also discusses some peculiarities of the transition regime caused by the orography of the site.