Active and passive scalar statistics in turbulent atmospheric convection

We study the small-scale statistics of active and passive scalar fields, obtained from 3D large- eddy simulations of the atmospheric boundary layer turbulence. The velocity field is anisotropic and inhomogeneous, due to the action of both buoyancy and shear. We focus on scalar fields rare fluctuations dominated by the so-called fronts. Temperature, coupled to the velocity field by the Boussinesq equations, exhibits anomalous scaling and saturation of the scaling exponents to a constant value, due to the presence of thermal fronts. Altough qualitatively similar, the small- scale statistics of a passive tracer advected by the convective flow shows quantitative differences: the large fluctuations of the tracer concentration field distribute differently and appear to be less intermittent than the temperature ones. To better understand these results, the role of boundaries in this problem is discussed.