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

Abstract

Despite many observational studies on the atmospheric boundary layer (ABL) depth (zi) variability across time scales (e.g., diurnal, seasonal, annual, and decadal), zi variability before, during, and after frontal passages over land, or simply zi variability as a function of weather patterns, has remained relatively unexplored. In this study, we provide an empirical framework using 5-years (2014-2018) of daytime rawinsonde observations and surface analyses over 18 central and southeast US sites to report zi variability across frontal boundaries. By providing systematic observations of front-relative contrasts in zi (i.e., zi differences between warm and cold sectors, Δzi = zWarmi − zColdi) and boundary-layer moisture (i.e., ABL-q) regimes in summer and winter, we propose a new paradigm to study zi changes across cold front boundaries. For most cases, we found deeper zi over the warm sector than the cold sector in both summer and winter, though with significant site-to-site variability in Δzi. Additionally, our results show a positive ΔqABL (i.e., frontal contrasts in ABL-q) in summer and winter, supporting what is typically observed in mid-latitude cyclones. We found that a front-relative ΔqABL of 1 g kg−1 often yielded at least a 100 m Δzi across the frontal boundary in both summer and winter. This work provides a synoptic-scale basis for zi variability and establishes a foundation for model verification to examine the impact of airmass exchange associated with advection on zi. This work will advance our understanding of ABL processes in synoptic environments and help unravel sources of front-relative zi variability.

Exploring Atmospheric Boundary Layer Depth Variability in Frontal Environments Over an Arid Region

“…Thus, this experimental region provides an excellent testbed to obtain empirical evidence on the impact of frontal passages on ABL depth variability under different meteorological conditions, including the presence or absence of EMLs and drylines. Additionally, in previous studies (e.g., Seidel et al 2012;Lee and Pal 2017;Clark et al 2022), very deep z i were observed over the region. However, no distinction was made on the role of EMLs on ABL thermodynamic features including z i .…”

“…We hypothesize that horizontal advection associated with the frontal passages impact z i variability over land, since fronts extensively modify the ABL thermodynamics by advecting different airmasses on a larger region (warm-humid-southerly flows and cold-dry-northerly during prefrontal and postfrontal conditions, respectively; Sinclair et al 2010;Pal et al 2020a;Clark et al 2022). Additionally, precipitation associated with the frontal passages change LAFPs via increasing the soil moisture and hence help reduce z i .…”