Experimental study of the evolution of the inland vertical structure of a coastal Atmospheric Boundary Layer in the Central Mediterranean using surface and ground-based remote sensing

The understanding of the coastal atmospheric processes requires the availability of complete datasets spanning from the surface to the top of the Atmospheric Boundary Layer (ABL) and high resolution modelling to resolve the coastal discontinuity. To study the development of the vertical structure of the coastal flow under different meteorological situations, we carried out an intensive experimental campaign at a site located 600 m inland from the shoreline in the Central Mediterranean area during July 2009, integrating optical and acoustic ground-based remote sensing information and surface standard measurements. In this area, the sea breeze always develops during the summer but sometimes it is overdriven by the synoptic flow that blows from the same direction. A Leosphere WLS7 Windcube Doppler LIDAR (LIght Detection And Ranging) and a DSDPA.90-24-METEK SODAR (SOnic Detection And Ranging) were used to derive the vertical profiles of wind speed and direction and of some turbulence characteristics. Furthermore, the vertical profile of the backscatter intensity of a CL31, Vaisala ceilometer (LIDAR) was used to detect the height of the boundary layer with respect to the aerosol concentration. We observed that when synoptic conditions are favourable to the sea breeze development, the air masses with marine aerosols are advected over land in the early morning interacting with the nighttime boundary layer. After the onset of the sea breeze an internal boundary layer develops from the coastal discontinuity, the height of the boundary layer detected by the ceilometer decreases, likely due to the advection of the marine aerosols above the IBL creating a discontinuity in the aerosol concentration and size distribution. Later in the morning, when the breeze is well developed, convection takes over and mixes marine and continental aerosols creating a homogeneous content of aerosols filling the convective layer. During stationary synoptic flow with wind speed typically larger than 4 m/s, marine aerosols are mixed with continental aerosols and the height of the boundary layer detected by the ceilometer does not varies. If we focus on the Doppler LIDAR performance. during nighttime and stable conditions, the LIDAR signal reached the maximum set height i.e. 250 m, often detecting a low level jet confirmed by the SODAR measurements. Also during daytime and stationary westerly synoptic winds the LIDAR signal reached the maximum measurement height; on the other hand, during sea breeze conditions, after the onset of the breeze, the Doppler LIDAR vertical wind profile rarely reached higher than 180 m. We believe that the sea breeze advection of marine aerosols causes a non homogeneous columnar distribution inducing a low LIDAR signal-to-noise ratio above the internal boundary layer.