Empirical Evidence for the Frontal Modification of Atmospheric Boundary Layer Depth Variability over Land

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.