Contrasting air mass advection explains significant differences in boundary layer depth seasonal cycles under onshore versus offshore flows

The complexities in the atmospheric boundary layer depth (BLD) features over coastal regions<br>pose challenges for meteorological forecasts, air quality, greenhouse gas mixing and transport, and wind<br>energy production. Here, for the first time, we investigated afternoon BLD variability over 18 coastal sites in<br>the contiguous United States located along the Gulf of Mexico, Atlantic Ocean, and Pacific Ocean using<br>25 years of rawinsonde launches. Due to influence of shallow marine layer air via onshore flow, substantially<br>large BLD contrasts (ΔBLD, BLD under offshore minus onshore) were found. ΔBLDs over the sites varied<br>both seasonally and spatially among the coastal regions and within a region. For the sites along the Gulf of<br>Mexico and Atlantic Ocean, ΔBLDs were found to be higher in spring and summer (500–1,500 m) than in<br>winter and fall (100–450 m). Results underscore the importance of advection on BLD footprints and provide<br>observational constraints for mod