Scaling of Streaky Structures Observed by a Doppler Lidar

The characteristics of streaky structures in the surface layer were examined. The study focused on the following two points; (1) the frequency of occurrence and conditions required for the presence of streaky structures, and (2) the universal scaling of the spacing of streaky structures. We analysed observation data obtained by a three-dimensional scanning Doppler lidar (DL) together with reference to the previous large-eddy simulation and wind-tunnel studies. The DL measured the spatial distribution of radial velocity within 4.3 km in radius for three months in an urbanized area of Tokyo. The horizontal snapshots of the radial velocity were classified into six groups; Streak, Mixed, Fishnet, No streak, Front and Others. Streak accounted for more than 50 % of all possible flows and occurred when the horizontal wind speed was strong and conditions were near neutral. The spacing of streaky structures was quantitatively estimated from the spatial pattern of radial velocity. The spacing of streaky structures became smaller with an increase in the velocity gradient. The relationship between the spacing of streaky structures and the local velocity gradient was further investigated in non-dimensional forms. The spacing of streaky structures normalized by the boundary layer height correlated with the normalized velocity gradient which can be formed as the product of the height normalized by the boundary layer height and the wind shear normalized by the friction velocity and height. The normalized wind shear which is the same form as the universal function of Monin-Obhukov similarity is affected by the stability and the horizontal wind velocity. It is suggested that the stability and the horizontal velocity implicitly affect the spacing of streaky structures normalized by the boundary layer height through the non-dimensional wind shear. We confirmed that this scaling is valid for the cases of various atmospheric stabilities (stable, neutral, unstable), roughness (flat smooth, rough, buildings), and flows in outdoor and indoor environments.