LAUSR

Atmospheric boundary layer depths (BLDs) over continental sites have long been meticulously characterized. However, a downwind‐footprint concept for BLDs over plains under the impact of seasonally and spatially changing horizontal advection of BLDs off elevated terrains has remained unexplored. For the first time, we provide observational evidence of the impact of mountains on regional BLDs using 25‐years (1991–2015) of rawinsonde‐retrieved afternoon BLDs over 22 sites located in the mountains' (Rockies and Appalachians) downstream. Results suggest that mountain‐advected air mass, elevated terrains, and wind play a significant role in modulating BLD variability “miles away” from terrains. We found significant BLD contrasts over the plains (400–1,500 m) under mountain‐advected versus flatland‐advected flows pertaining to elevated mixed layers off the mountain ranges. The BLD contrasts were higher in the downwind of Rockies than the Appalachians, and higher BLD contrasts were observed in spring and summer (900–1,500 m) than in fall and winter (100–500 m). These findings will help build advanced parameterizations in models where BLD simulations around complex terrain still remain a hurdle.