Low-Level Jets at Utö, Finland Based on Doppler Lidar Observations

A low-level jet is a localized maximum in the vertical profile of wind speed that typically occurs in the lowest 1000 m above the surface. Low-level jets are important for aviation, transportation of moisture and pollutants, and wind energy applications. The strong wind shear and the turbulence associated with low-level jets can be harmful for wind turbines but enhanced wind speeds related to low-level jet maxima can possibly be utilized for wind power production. Therefore, it is important to know where low-level jets occur and to obtain detailed information about their characteristics as well as their forcing mechanisms.

Using over two years (from 1 January 2013 to 4 May 2015) of Doppler lidar observations from Utö (59.78 N, 21.37 E), a small island in the south-western edge of the Finnish archipelago in the Baltic Sea, low-level jet occurrence, characteristics and potential forcing mechanisms are determined. An objective low-level jet detection algorithm suitable for high resolution Doppler lidar data was created and applied to merged wind data which combines data from two different scanning schemes, Velocity-Azimuth Display and Doppler Beam Swinging, and sonic anemometer measurements from two meters above ground. Additionally, sea level pressure, air temperature, and attenuated backscatter from Doppler lidar were used to identify the potential forcing mechanisms of low-level jets.

Low-level jet frequency of occurrence at Utö is 14 %. Mean height of low-level jet is 180 m above ground and mean wind speed is 12 m/s. Different forcing mechanisms of low-level jets were found: large scale baroclinicity, inertial oscillation in space and in time, and local circulation. Low-level jets favor south-westerly (forced by large scale baroclinicity) and easterly directions (forced by inertial oscillation in space). It was found that low-level jets rarely produce extremely high speed or directional (bulk) shear values. However, persistent low-level jets lasting up to several days were observed.