Turbulent Flow Regimes of a Coastal Boundary Layer, Observations and Models.

The concept of Coastal Boundary Layer (CBL) was introduced for the first time by Csanady in 1972 as a feature observed for the hydrodynamics of the Great Lakes. It is a little studied area, being in the middle between the scale of Coastal Oceanography and that of the nearshore. Recent observation tools, and in particular X-band coastal radars, nowadays can collect fundamental data on hydrodynamic processes that arise from the interaction between a prevailing ocean current, the coastal bathymetry and the coastline. These processes are similar to the classical “wall turbulence” phenomena studied in hydrodynamics, such as logarithmic attenuation of the velocity profile in approaching to the coast, flow separation in proximity of headlands and capes, evolution and development of a turbulent boundary layer with detachment of small eddies.

In this paper we show new observations obtained by an X-band radar installed at the Isola del Giglio , from which many remote data up to 2-3 km from the coast were derived. Such data consist in waves and surface currents at high resolution (around 100 m) obtained through an algorithm of new generation (Ludeno et al. 2014). The observed phenomena, with formation of eddies of size up to 1 km, have been validated through Lagrangian drifters, and finally reproduced with a high resolution ROMS model (up to 50 m) for the area. Using this methodology, results have been extended to a much longer period than the available observations, and beyond the limits of the X-band radar, related to the need for a sufficient sea surface roughness to obtain significative measurements. In such a way, CBLs in different regimes of turbulence can be characterized, caused by different ocean currents, winds, and surface waves, and in relation to the coastline shape. The model also provides a credible explanation about coastal eddies phenomena, that appear related to turbulent interaction mechanisms of ocean currents with the coastline rather than caused by the importation of vorticity from the outer ocean.