An examination of local versus nonlocal aspects of a TKE-based boundary layer scheme in clear convective conditions

In this study, the ability of a turbulent kinetic energy (TKE)-based bounda ry layer scheme to reproduce the rapid evolution of the planetary boundary layer (PBL) observed during two clear convective days is examined together with the impact of including nonlocal features in the boundary layer scheme . The two cases are chosen from the Montreal-96 Experiment on Regional Mixi ng and Ozone (MERMOZ): one is characterized by strong buoyancy, a strong ca pping inversion, sind weak vertical wind shear; the other displays moderate buoyancy, a weaker subsidence inversion, and significant wind shear near t he PBL top. With the original local version of the turbulence scheme, the m odel reproduces the vertical structures and turbulent quantities observed i n the well-developed boundary layer for the first case. For the second case , the model fails to reproduce the rapid evolution of the boundary layer ev en though the TKE and sensible heat fluxes are greatly overpredicted. Some nonlocal aspects of the turbulence scheme are tested for these two cas es. Inclusion of nonlocal (countergradient) terms in the vertical diffusivi ty equation has little impact on the simulated PBL. In contrast, alternativ e formulations of the turbulent length scales that follow the strategy prop osed by Bougeault and Lacarrere have a greater influence. With the new turb ulent lengths, entrainment at the top of the boundary layer is enhanced so that the depth of the well-mixed layer is much larger compared to that of t he local simulations even though the turbulent sensible heat fluxes are sma ller. Comparison with observations reveals, however, that the inclusion of these modifications does not improve all aspects of the simulation. To impr ove the performance and reduce somewhat the arbitrariness in the Bougeault- Lacarrere technique, a relationship between the two turbulent length scales (mixing and dissipation) used in the turbulence scheme is proposed. It is shown that, in addition to reducing the sensitivity of the results to the p articular formulations, the simulated boundary layer agrees better with obs ervations.