Turbulent coherent structures in the atmospheric surface layer: A 13-month study of dynamics based on Doppler lidar observations in mid-latitude coastal city

Turbulent coherent structures such as streaks play a crucial role in turbulent flows' behavior and pollutants' dispersion in the surface layer. This research is based on ~40,000 horizontal Doppler lidar scans recorded during a 13-month campaign in Dunkirk, France, an industrial harbor city on the North Sea. Similarly to Cheliotis et al. 1, this work is based on the results of automated classification, detailed in a companion abstract. This approach which is based on texture analysis and supervised machine learning, facilitated the distinction between organized and disorganized streaks, which appeared, respectively, mainly during the day and at dawn and dusk. There was no significant seasonal variation in the appearance of the two streak types, beyond the fact it followed the sunrise and sunset times. Organized streaks were typically associated with neutral to unstable atmospheric conditions and characterized by higher motion and thermal fluxes than their disorganized counterparts. Conversely, disorganized structures were more narrowly associated with stable conditions and higher friction velocity. The classification parameters also allowed retrieving the streaks' direction misalignment with the mean wind and their periodicity. The misalignment was more pronounced for disorganized streaks and varied between 0 and 20°. Organized streaks tended to be slightly narrower than disorganized ones, with periods of 0.48 ± 0.22 km and 0.42 ± 0.25 km, respectively (average ± 1σ standard deviation). The aspect ratio of streaks (width-to-height ratio) could only be determined during a few night-time case studies, due to difficulties in differentiating the surface and mixed layers using Doppler lidar data only. The retrieved aspect ratio of 1.64, therefore, corresponds to disorganized streaks.