Using Mixing Diagrams to Model the Mixed Layer Budget and Observationally Determine Entrainment

Mixing diagrams as introduced by Betts (1992) have been used in the past to break down the heat and moisture budgets in the daytime planetary boundary layer into vector components. They allow us to visually represent the magnitudes of advection, surface fluxes, radiative heating, and entrainment that contribute to the evolution of the heat and moisture budgets. In this study, we use Large Eddy Simulations (LES) to develop best practices for use of mixing diagrams in observations. We find that the entrainment component of the mixing diagram can be partitioned into two segments, and the magnitudes of each of these segments is highly dependent on the boundary layer depth definition. We show that near-surface values of the heat and moisture budget are not a good estimate for the entire mixed layer. We look at the differences between the morning transition, quasi steady mixed layer, evening transition, and the residual layer budget. We study the entrainment zone by comparing different boundary layer depth definitions and looking at how those definitions affect temperature, humidity, and the entrainment zone depth. In addition, we will make mixing diagrams for cases with and without surface interactions (i.e. coupled and uncoupled simulations) to see how each case changes the components of the mixing diagram. We will use these results to mutually validate mixing diagrams with observations taken from the ARM SGP site and develop best practices for observational mixing diagrams, with the goal of directly diagnosing entrainment from observations.