Internal boundary layer characteristics at the southern Bulgarian Black Sea coast

The marine Atmospheric Boundary Layer (ABL) over the southern Bulgarian Black Sea coast is studied based on remote sensing measurements with a monostatic Doppler sodar system located about 400 m inland. The accumulated long-term acoustic data (August 2008 – October 2016) with high spatial (10 m) and temporal (20 minutes running averages at every 10 min) resolution was used by applying statistical approaches to the wind and turbulent profiles to reveal the complex vertical structure of the coastal boundary layer when marine airflow comes over the land. Тhe processes observed in air masses transformation due to the sharp change in physical characteristics of the surface are related to Internal Boundary Layer (IBL) formation. Its spatial scales as a sublayer of the coastal ABL are studied by a height dependent on the distance from the shore. In the absence of temperature and humidity profiles measurements, an approach through the turbulent profiles’ characteristics representative for different lengths of the marine air masses run (400 to 2500 m) was used to examine whether the formation of IBL over land can be observed in sodar data. The different length distances passed by the marine airflow are fixed by a selection of intervals of geographical directions of the wind in the study region. Analysis of the observed changes in the values of the averaged profiles and their dispersions reveals IBL heights up to 150 m depending on the lengths of the marine air masses run. 
This study of long-term remote sensing data is a pioneer for the Black Sea basin region and was conducted in a coastal area with modest observation networks in Bulgaria. The work is inspired by the activities of its authors within COST Action CA18235 PROBE (PROfiling the atmospheric Boundary layer at European scale), supported by COST (European Cooperation in Science and Technology) and the National Science Fund of Bulgaria, Contract KP-06-N34/1 "Natural and anthropogenic factors of climate change – analyzes of global and local periodical components and long-term forecasts".