Pressure perturbations and multi-scale analysis in the atmospheric boundary layer at the afternoon and evening transition during the BLLAST campaign

Throughout the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign, which took place from 14th June to 8th July 2011 in Lannemezan (France), a large amount of observational data was collected. In this work we study the evolution of the planetary boundary layer during the transition from the diurnal convection to the nocturnal stability, especially focusing on data from a sonic anemometer and a triangular array of PAROSCIENTIFIC microbarometers. Both of them were placed near the surface and set to high temporal resolution, so that rapid fluctuations can be found. We mainly investigate days with clear weather conditions. Meteorological variables as well as turbulent parameters are studied, focusing on microbarometers records and small pressure perturbations. Wavelet analysis is also considered when wave-like motions at pressure evolution seem to be present. A multi-scale analysis is used to check which temporal scales contributions are more relevant at different times during the transition period and how these changes in the scales can characterize and influence the transition and the further development of the nocturnal boundary layer. Multi-Resolution Flux Decomposition (MRFD) is employed for vertical fluxes calculations from sonic anemometer components in order to analyse the time evolution of the turbulent and larger scales (coherent structures) present along the transition and first night-time periods.