Role of residual layer and large-scale phenomena on the evolution of the boundary layer

Mixed-layer theory and large-eddy simulations are used to analyze the dynamics of the boundary layer on two intensive operational periods during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) campaign: 1st and 2nd of July 2011, when convective boundary layers (CBLs) were observed. Continuous measurements made by several remote sensing and in situ instruments in combination with radiosoundings, and measurements done by unmanned aerial vehicles and an aircraft probed the vertical structure and the temporal evolution of the boundary layer.

Mixed layer theory (Tennekes and Driedonks, 1981) and the Dutch Atmospheric Large-Eddy Simulation model (DALES, Heus et al., 2011) are set to reproduce and analyze the dynamics of the atmosphere during these two days. The initial vertical profiles of potential temperature, specific humidity and wind, and the temporal evolution of the surface heat and moisture fluxes prescribed in the model runs are inspired by the observations taken at the two supersites that concentrated most of the instrumentation during the campaign. For DALES the initial profile (at 7 UTC on 1st July and 5 UTC on 2nd July) takes into account the existence of a residual layer above the nocturnal stable layer observed during the early morning. The mixed layer model is initialized when a well-developed convective boundary layer was observed, from 10 UTC. Due to the surface heterogeneity of the area, the models were run separately to analyze the boundary layer conditions at the two different supersites.

First, the research focuses on the role-played by the residual layer (RL) on the evolution of the boundary layer. By using DALES, we show the importance of the dynamics of the boundary layer during the previous night to the development of the boundary layer at the morning. DALES, which takes into account the residual layer, is capable to model the observed sudden increase of the boundary layer depth and of the potential temperature occurred during the morning transition. There are different sources, which can be the responsible of the overshooting for instance surface and entrainment fluxes or large-scale phenomena. Analyzing the entrainment buoyancy heat flux, a large increase is obtained by the simulation when the residual layer is incorporated in the mixed layer by the growing convective boundary layer. Consequently, a precise definition of the characteristics of the residual layer is fundamental even though it is complex because the evolution of the main variables in the residual layer during the previous night depends on different factors such as radiation divergence or advection.

Once the boundary layer is fully developed around midday, both models are used to study the dynamics of the boundary layer during the studied days. The models are able to reproduce the evolution of the boundary layer depth, potential temperature and specific humidity from late morning. They are also used to analyze the role played by subsidence and large-scale advection during the afternoon transition. During 1st of July, heat and moisture advection are negligible contributions to the heat or moisture budget, respectively, but subsidence plays an important role in the turbulence decay during the afternoon when a clear decrease of boundary layer depth was observed. On 2nd of July the main boundary layer variables follow similar evolution to the previous day, but with a lower maximum of the boundary layer depth. However, in the last part of the day, a front was approaching advecting moisture over the area and increasing the cloud cover.

To summarize, the residual layer and large-scale phenomena play a crucial role in the development of the boundary layer during some days of the BLLAST campaign.