The transition from unstable to stable regimes of the ABL are less studied and understood compared to other time periods. The turbulence decay period remains challenging to model due to non-stationary conditions, although important progress has been achieved (Nadeau et al. 2011). Wind speed, variances and turbulence properties evolving with time and multi-layering of the ABL can have important implications on dispersion of pollutants during the transition and the following night. There have been renewed interests in studying transition periods in recent years using field measurements (Grant 1997, Acevedo and Fitzjarrald 2001, Lothon et al. 2011), laboratory work (Cole and Fernando 1998, Kang et al. 2003) and numerical simulations (Kumar et al. 2006, Beare et al. 2006, Pino et al. 2006, Rizza et al. 2013).
The BLLAST (Boundary Layer Late Afternoon and Sunset Turbulence) field campaign, which took place in southwest France in June and July 2011, focused on the turbulence decay period and evening transition with one of its main goals to obtain an extensive description of the boundary-layer dynamical processes and its vertical structure (Lothon et al. 2011). High frequency field measurements from masts and towers covering the lowest 60 m of the boundary layer are being analysed. Tethered balloon and aircraft measurements are also analysed to gain added information and understanding of the vertical variation within the boundary layer.
This study shows how the turbulent kinetic energy and anisotropy evolve in the turbulence decay period starting at about noon until the onset of a nocturnal boundary layer in the early evening. The investigation uses high-frequency field measurements of velocity and temperature fluctuations. Analysis of turbulent fluctuations using local averaging and conditional sampling techniques give a better understanding of fluxes, variances and associated turbulence length scales throughout the ABL and its various layers during this non-stationary part of the day. This study is, therefore, complementary to a recent study about the evolution of spectra in the afternoon transition period (Darbieu et al. 2014) which is also presented at this conference. Here, the focus is placed on investigating turbulence anisotropy and how length scales are linked to turbulence structure and evolve in the turbulence decay period. We will report some of the important implications.
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