Describing the Structure and Evolution of the Entrainment Zone Throughout the Morning and Evening Transitions Using LES

The contribution of entrainment to the evolution of the Convective Boundary Layer (CBL) during the morning and evening transitions is uncertain. While entrainment tends to warm and dry the CBL, it could either moisten or dry the CBL during the growth of the morning transition, depending on the difference in water vapor content of the residual layer versus that of the surface layer. Whether entrainment will moisten or dry the CBL in the morning is likely dependent on land use. Additionally, Garcia and Mellado (2014) used idealized Direct Numerical Simulations to show that there is a two-layer structure within the entrainment zone, with the lower layer dominated by small scale turbulence, while the upper layer consists of overshooting thermals. One hypothesis for the moistening of the CBL through entrainment during the morning transition could be that the overshooting thermals pre-moisten the free troposphere first.

We use MicroHH simulations, a high-resolution LES that runs on the GPU, to see whether a two-layer structure is present in more complex, non-steady state boundary layers, and if so, which layer dominates during the morning and the evening transitions. We use a mixing diagram budget analysis for the entrainment zone to see how land use type impacts the contributions to heat and moisture within the entrainment zone and determine which structure dominates the entrainment zone during the morning and evening transitions.

Garcia, J. R., & Mellado, J. P. (2014). The Two-Layer structure of the entrainment zone in the convective boundary layer. Journal of the Atmospheric Sciences, 71(6), 1935–1955. https://doi.org/10.1175/jas-d-13-0148.1