Turbulence kinetic energy decay in the afternoon transition

We will present an investigation of the near-surface decay of turbulence kinetic energy (TKE) using observations from the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign that took place in June and July 2011 in Southern France. We investigate the turbulence decay using observations of a near-surface TKE budget (Nilsson et al. 2015a). An important new aspect of the analysis of the near-surface TKE budget is that even very near the surface the dissipation rate of TKE was found to be influenced by the boundary layer depth.

The observations were used together with idealized vertical mixed-layer profiles for TKE budget terms to build a simple model for the evolution of TKE (Nilsson et al. 2015b). There are 3 main inputs to the model: near-surface buoyancy flux, wind speed at one height in the surface layer and boundary layer depth. The model is evaluated for TKE and TKE budget terms using measurements and is then used to elucidate some new aspects on the evolution of TKE during the afternoon transition. The simple model is used to illustrate weak turbulence conditions that start to form in the upper part of the boundary layer surprisingly early, during unstable conditions in the afternoon. We refer to these conditions as a pre-residual layer.

Many different turbulence decay rates in the afternoon transition are shown to occur in situations of time-varying wind. In cases of increasing wind speed during the afternoon, the near-surface TKE can also increase depending on the amount of shear generation. These new results point out limitations associated with the concept of time-dependent exponent decay laws on the form t^-n which have often been used to describe turbulence decay.

References: Nilsson, E., Lohou, F., Lothon, M., Pardyjak, E., Mahrt, L., and Darbieu, C.: Turbulence Kinetic Energy budget during the afternoon transition – Part 1: Observed surface TKE budget and Boundary layer description for 10 Intensive Observation Period days, Atmos. Chem. Phys. Discuss., 2015a.

Nilsson, E., Lothon, M., Lohou, F., Pardyjak, E., Hartogensis, O., and Darbieu, C.: Turbulence Kinetic Energy budget during the afternoon transition – Part 2: A simple TKE model, Atmos. Chem. Phys. Discuss., 2015b.