Nonlocal Turbulent Mixing in the Convective Boundary Layer Evaluated from Large-Eddy Simulation

Abstract

Large-eddy simulation is used to simulate quasi-steady state convection in a windless mixed layer over a uniform surface with constant heat flux. Different tracers are injected at each discrete height in the model to track vertical transport of tracers as a function of time. The resulting tracer source and destination information is presented in the form of transilient matrices. These matrices are asymmetric for time increments on the order of the convective time scale, t*. They show nonlocal mixing occurring over a range of wavelengths up to the mixed layer depth, some convective overturning, and the loss of nearly all of the surface layer air into thermals. Measurements of transport across finite distances exhibit skewed distributions of vertical transport velocity. The relative importance of upward versus downward transport strongly depends on both height and time, as measured by the fractional transport and mixing lengths in each direction. Process, mass, and heat transport spectra show the relatively minor contribution made by small-size eddies as compared to the medium and large scales. Favorable comparisons of these results with a variety of traditional turbulence statistics exemplify the wealth of turbulence information that is captured within a transilient matrix.