Micromixing modelling of mean and fluctuating scalar fields in the convective boundary layer

Coupling the concepts of the Lagrangian marked-particle approach and the interaction-by-exchange-with-the-conditional-mean (IECM) micromixing approach, a Lagrangian micromixing model is developed to calculate the mean and fluctuating scalar fields generated by scalar fluxes emitted into the convective boundary layer (CBL) from continuous area sources located at the top and bottom of the CBL. A continuous area source is simulated as a superposition of a series of instantaneous area sources. The scalar fields due to the continuous area sources are represented as the sum of top-down and bottom-up components with their cross-correlations included, and mixed-layer similarity is used for normalising the scalar quantities. The mixing time scale is parameterised as a linear function of time. The model-derived profile functions of top-down and bottom-up terms can be used to derive scalar concentration moments for any value of the ratio of the entrainment flux to the surface flux, including cases where a scalar material is extracted from the boundary layer (i.e. sinks). Model results on the concentration gradients; top-down and bottom-up profile functions; and scalar (temperature and humidity) variances, skewness and kurtosis are discussed; and compared with large-eddy simulation results and field and laboratory data where available. The comparison shows good performance by the Lagrangian micromixing model, considering the fact that the model used minimal flow information (i.e. the variance and third moment of the vertical turbulent velocity, and the turbulent kinetic energy dissipation rate) in the form of parameterised profiles.