HF radar spectrum comparison method for current maps analysis

International audience; Oceanographic HF radar allows todescribe the hydrodynamics of a coastal area by providing a synoptic surface current map at high frequency and regular time steps (10mn). Its interest lies in its wide cover,the high frequency of acquisition and the regularity of the measurements (regardless of weather conditions). Nevertheless, the estimation of surface currents is not straightforward and requires processing of the data acquired by radarin order to extractsurface radial current. The methods involving a beamforming or methods known as direction finding are the most used in this scope. Unfortunately, these methods do not provide exactly unambiguous estimations. In addition, HF radar can be associate to others sensors (ADCP, satellites, etc.) or methods (numerical models) that also provide relevant information about the area of interest. Thus, the analysis of hydrodynamics behavior at various time scales (from tidal to pluriannual time scale) requires the analysis of all these sources of information in order to better catch and possibly improve the knowledge of this key area in between the bay of Biscay and the Celtic Sea.In this presentation, a method which intends to compare a current map to HF radar datawill be introduced. The surface current could come from any sensors that provides surface current map or from numerical simulations. This comparison is directly performed with the low level data acquired by the radars ; we would rather avoid the comparison to geophysical data (current themselves) in order to prevent from information loss due to the estimation process or due to geometrical configurations of the radars (geometrical dilution of precision). Indeed, this comparison is performed thanks to an analysis of the radar Doppler spectrum. Hence the comparison is done by radar and therefore only the radial component is studied at the same time. This presentation will focus on the impact of the met-oceanic (wind, waves, tides) conditions and will highlight the main differences observed from current map estimating from HF radar itself and from a high resolution numerical simulation. The area of interest for this study is the Iroise Sea on the west of Brittany in France where data have been acquiring since 2006. This large amount of data covering a wide span of environmental conditions has been fundamental to investigatethis area in a wide variety of hydrodynamical situations.