Entrainment zone (EZ) processes are vital for better understanding the distribution of energy, moisture, momentum, tracers, and pollutants within the lowest part of the Earth’s atmosphere, known as the atmospheric boundary layer (ABL). EZ processes describe the ongoing turbulent exchange between the ABL and the overlying free atmosphere (FA). EZ features govern the growth and evolution of the ABL throughout the day and, thus, are paramount for accurately predicting critical atmospheric variables and processes, including near-surface meteorological conditions, air quality, and convection initiation using numerical weather prediction and dispersion models. However, EZ processes are not well-understood, and observations over diverse topographies, climatological regions, and weather phenomena remain sparse. Providing new observations over different climatological regions is pivotal for detailing the unique relationship between the surface forcing and the ABL environment. Within this study, we provide lidar observations of EZ processes over a semiarid region (West Texas in the Southern Great Plains of the US) using a newly developed retrieval method. To this end, we retrieve the EZ thickness (EZT) to explore the relationship between the aerosol vertical mixing and associated kinematics within the ABL. While exploring the relationship between the EZT and the peak value in aerosol variance at the ABL top (zi) for three cases with zi of 1660, 1000, and 1700 m AGL, we found a moderate-to-strong correlation (r ≈ -0.52, -0.82, and -0.87, respectively). Overall, the new observations and findings reported here will allow for an improved depiction of the EZ processes over a semiarid region that will help develop improved model forecasts of essential meteorological processes.

Despite many observational studies on the atmospheric boundary layer (ABL) depth z(i) variability across various time scales (e.g., diurnal, seasonal, annual, and decadal), z(i) variability before, during, and after frontal passages over land, or simply z(i) variability as a function of weather patterns, has remained relatively unexplored. In this study, we provide an empirical framework using 5 years (2014-18) of daytime rawinsonde observations and surface analyses over 18 central and southeastern U.S. sites to report z(i) variability across frontal boundaries. By providing systematic observations of front-relative contrasts in z(i) (i.e., z(i) differences between warm and cold sectors, delta(zi)=z(i)(Warm)-z(i)(Cold)) and boundary layer moisture (i.e., ABL-q) regimes in summer and winter, we propose a new paradigm to study z(i) changes across cold-frontal boundaries. For most cases, we found deeper z(i) over the warm sector than the cold sector in both summer and winter, although with significant site-to-site variability in delta z(i). Additionally, our results show a positive delta q(ABL) (i.e., frontal contrasts in ABL-q) in summer and winter, supporting what is typically observed in midlatitude cyclones. We found that a front-relative delta q(ABL) of 1 g kg(-1) often yielded at least a 100-m delta z(i) across the frontal boundary in both summer and winter. This work provides a synoptic-scale basis for z(i) variability and establishes a foundation for model verification to examine the impact of airmass exchange associated with advection on z(i). This work will advance our understanding of ABL processes in synoptic environments and help unravel sources of front-relative z(i) variability.

The thermodynamic properties of the atmospheric boundary layer (ABL) play an important role in several atmospheric processes such as convection initiation, turbulence mixing, the exchange of heat and momentum, and cloud-microphysics. Since the ABL depth (henceforth, BLD) defines the volume of the ABL, many studies consider BLD to be a key scaling parameter to understand and quantify ABL mixing processes. However, most of these studies attributed both BLD temporal and horizontal variability on various scales solely to the impact of underlying surface forcing via locally generated buoyancy fluxes and static stability. We argue that the impact of horizontal advection is often neglected yet important for a more thorough understanding of ABL thermodynamics and kinematics. Here we identified four potentially advection-dominated ABL regimes across (1) the urban-rural interface, (2) complex terrain and adjacent plains, (3) the land-sea interface where horizontal transport of marine boundary layer airmasses influences the regional ABL over coastal areas, and (4) frontal environments where mid-latitude cyclones affect ABL processes via passages of cold and warm frontal boundaries. We then introduced a conceptual framework based on observations so that ABL processes are explained not only by surface forcing but also by horizontal advection of mass, momentum, and energy. This work will help advance our understanding of ABL processes and single out potential sources that trigger drastic changes in ABL thermodynamic features including the BLDs under diverse horizontal advection environments.

In spring 2019, a catastrophic flood occurred along the Missouri and Mississippi River basins in the United States, which was characterized as the longest lasting flood since the Great Flood of 1927. The 2019 flooding resulted in extremely wet soils for 3–4 months over the Great Plains. Using rawinsonde‐derived atmospheric boundary layer depths (BLDs) and in situ soil moisture (SM) data sets at 10 sites located meridionally across the two river‐valleys, we investigated the SM controls on regional‐scale BLDs during spring 2019. The impact of spring flooding on atmospheric boundary layer dynamics is reported via regression analyses between daily SM and BLDs yielding statistically significant negative r (p < 0.0012) with substantial spatial variability (r: −0.25 to −0.70). Results suggest (1) the strengthening of the negative SM‐BLD relationship in the wake of extreme flooding and (2) positive SM anomalies of 0.05–0.12 m3 m−3 resulted in negative BLD anomalies (−100 to −400 m) compared to 8‐year means, confirming the impact of perturbed land atmosphere feedback processes (LAFP). These results offer a test bed for developing better numerical models with advanced representations of LAFP.