Characteristics of the Low-Level Jets Observed over Dunkerque (North Sea French coast) using 4 years of wind lidar data

Previous observational studies about Low-Level Jet (LLJs) in the North Sea area provided insight about LLJs occurring above offshore and inland sites. The present work completes this corpus by adding a statistical study performed above a coastal site, relying on four years of wind profiles recorded by a small Doppler lidar installed in Dunkerque port (northern coast of France, southernmost North Sea). The instrument, a Leosphere Windcube v2, had a range of 280 m allowing the detection of LLJs up to 244 m above mean sea-level. LLJs were found on 4.74 % (9,649) of the 10-minute average wind profiles, laying the basis for a robust statistical analysis of the jet core height, speed and direction, and for the study of the seasonal, annual and daily cycles of the LLJs occurrence. The altitude of the jet core (wind peak) was typically 114 m above mean sea-level, with a core speed around 7-8 m.s^-1. The core wind direction exhibited a dominant mode in the NE direction (onshore coastwise) that occurred mostly during the afternoon (11:00 to 18:00 UTC), and a secondary mode in the ESE direction (offshore cross-coast) that occurred mostly during the late night (00:00 to 07:00 UTC). The months with the highest LLJ frequency were April, May and August, following the annual cycle of the dominant NE mode. The conditions of occurrence and possible formation mechanisms of the LLJs over Dunkerque were investigated using three parameters computed from the ERA5 reanalyzes data: the land-sea thermal contrast, the bulk Richardson number and the geostrophic flow. The dominant NE LLJ mode was associated with anticyclonic conditions (easterly geostrophic flow, warmer land, unstable atmosphere), this mode likely gathering local sea breeze events and regional circulations generated by the wind channeling at the entrance of the English Channel.