Coupled-decoupled turbulence structures of stable boundary layer during heavy haze pollution events

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Heavy haze pollution events are closely related to the turbulent structure of the stable boundary layer (SBL). SBL can be divided into two regimes, weak(coupled) and strong(decoupled). The weak SBL conforms to the classical turbulent theory and couples with the surface and different heights, whereas the strong SBL does not conform and decouples. However, the thresholds between coupled and decoupled SBL have not been determined in haze pollution events. Five levels of turbulence observation data from two severe haze pollution cases observed at the 255 m meteorological tower in Tianjin were employed to address this dilemma. The first case was analyzed in detail, and the second case was used for verification. The occurrence frequency of SBL was approximately 50% during the two haze pollution cases. The wind speed thresholds distinguishing coupled and decoupled SBL are: 2.5 m s−1 at 40 m, 4.0 m s−1 at 80 m, and 5.0 m s−1 at 120 m. The decoupled SBLs accounted for >70% of SBLs in haze pollution events. The turbulent structures notably differed between coupled and decoupled SBL events. In a decoupled SBL, the scalar concentration at different heights was substantially higher, the strength and occurrence frequency of sub-mesoscale motion were larger, and turbulent barrier effects occurred. Moreover, there was an inverse gradient transport and a smaller spatiotemporal scale of turbulent eddies. Therefore, there was a weaker turbulent kinetic energy, flux, and turbulent correlation across the layers in the decoupled SBL. The results reveal the failure of Monin-Obukhov similarity theory and traditional turbulent energy spectrum theory in a decoupled SBL. Wind speed thresholds can be widely used in the severe haze pollution simulation. The results of this study also indicate that decoupled SBL requires further study and attention in haze pollution events to improve the accuracy of pollutant concentration forecasting.