Exploring Entrainment Zone Processes over a Semiarid Region: A Lidar-Based Study

The atmospheric boundary layer (ABL) is a complex integrated atmospheric system that influences many aspects of meteorology such as convection initiation, air quality, and cloud formation. Within this work, we examined the entrainment zone (EZ) processes between the ABL and free atmosphere (FA) over a semiarid region of West Texas. Semiarid areas are of interest due to their strong surface heat fluxes and intense turbulent mixing that often results in the formation of deep ABLs. Despite their importance to ABL growth, observations of EZ processes remain scarce in the existing literature. Nevertheless, due to the distinct aerosol characteristics of the ABL and FA, a layer of heightened aerosol variability often exists near the top of the ABL (z_i) that provides an opportunity for exploring EZ processes via its aerosol properties. Here, we use a newly developed Full-Width Half-Maxima method to determine the depth over which the EZ processes occur (i.e., EZ thickness; EZT) based on the aerosol variability near z_i. Exploring the relationship between the aerosol variability and turbulent kinematics within the ABL and EZ is essential for developing a comprehensive framework for parameterizing the ABL evolution. To that end, we used Doppler lidar measurements of aerosol backscatter coefficients (β) and vertical velocity (w) at Reese Center, located west of Lubbock, TX, for three cases (17 Jul 2022, 02 Sep 2022, and 08 Sep 2022). The relationship between the variability in β (σ_β,zi) and turbulent mixing observed at z_i (σ_w,zi) was deduced as it related to the EZ thickness (Δh/z_i). Each of these variables are interconnected and detail the overall ABL-to-FA exchange of mass, momentum, and energy that influences the ABL thermodynamic structure and composition. Overall, our results demonstrated a strong correlation between the Δh/z_i and σ_β,zi was observed for each case (r ≈ 0.86, 0.89, and 0.93), while a weak-to-moderate correlation was seen between σ_β,zi and σ_w,zi (r ≈ 0.33, 0.77, and 0.62). These results suggest an inherent relationship between the turbulent kinematics and the associated aerosol distribution within the EZ. More specifically, we observed that as the EZ turbulence intensified, the aerosol variability at z_i increased, suggesting enhanced exchange between the ABL and overlying FA.