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.

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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.

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