Abstract
Abstract. We investigate the decay of planetary boundary layer (PBL) turbulence in the afternoon, from the time the surface buoyancy flux starts to decrease until sunset. Dense observations of mean and turbulent parameters were acquired during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field experiment by several meteorological surface stations, sounding balloons, radars, lidars and two aircraft during the afternoon transition. We analysed a case study based on some of these observations and large-eddy simulation (LES) data focusing on the turbulent vertical structure throughout the afternoon transition. The decay of turbulence is quantified through the temporal and vertical evolution of (1) the turbulence kinetic energy (TKE), (2) the characteristic length scales of turbulence and (3) the shape of the turbulence spectra. A spectral analysis of LES data, airborne and surface measurements is performed in order to characterize the variation in the turbulent decay with height and study the distribution of turbulence over eddy size. This study highlights the LES ability to reproduce the turbulence evolution throughout the afternoon. LESs and observations agree that the afternoon transition can be divided in two phases: (1) a first phase during which the TKE decays at a low rate, with no significant change in turbulence characteristics, and (2) a second phase characterized by a larger TKE decay rate and a change in spectral shape, implying an evolution of eddy size distribution and energy cascade from low to high wave number. The changes observed either in TKE decay (during the first phase) or in the vertical wind spectra shape (during the second phase of the afternoon transition) occur first in the upper region of the PBL. The higher within the PBL, the stronger the spectra shape changes.
Highlights
The transition from a well-mixed convective boundary layer to a residual layer overlying a stable nocturnal layer raises several issues (Lothon et al, 2014), which remain difficult to address from both modelling and observational perspectives
We found that the quality index for observed and simulated data generally remained constant until 19:00 UTC, except for a slight increase in I QLES for data above 0.6 z/zi after 18:30 UTC
The profiles show that turbulence kinetic energy (TKE) decreases within the whole depth of the planetary boundary layer (PBL) but that there is a 1-hour delay between the start of the decay at the top and the start at the bottom: at 12:30 UTC, the TKE continues to increase in the lower PBL, while it has started to decrease in the upper part
Summary
The transition from a well-mixed convective boundary layer to a residual layer overlying a stable nocturnal layer raises several issues (Lothon et al, 2014), which remain difficult to address from both modelling and observational perspectives. The well-mixed convective boundary layer with fully developed turbulence is mainly forced by buoyancy. The afternoon decrease in the surface buoyancy flux leads to the decay of the turbulence kinetic energy (TKE) and a change in the structure of the turbulence, which shows more anisotropy and intermittency. It is important to better understand the processes involved, as they can influence the dispersion of tracers in the atmosphere C. Darbieu et al.: Turbulence vertical structure of the boundary layer during the afternoon transition
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