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The complexities in the atmospheric boundary layer depth (BLD) features over coastal regions pose challenges for meteorological forecasts, air quality, greenhouse gas mixing and transport, and wind energy production. Here, for the first time, we investigated afternoon BLD variability over 18 coastal sites in the contiguous United States located along the Gulf of Mexico, Atlantic Ocean, and Pacific Ocean using 25 years of rawinsonde launches. Due to influence of shallow marine layer air via onshore flow, substantially large BLD contrasts (∆BLD, BLD under offshore minus onshore) were found. ∆BLDs over the sites varied both seasonally and spatially among the coastal regions and within a region. For the sites along the Gulf of Mexico and Atlantic Ocean, ∆BLDs were found to be higher in spring and summer (500–1,500 m) than in winter and fall (100–450 m). Results underscore the importance of advection on BLD footprints and provide observational constraints for model evaluation. Plain Language Summary: The depth of the Earth's atmospheric boundary layer is an important parameter that atmospheric scientists use in weather forecast models and for understanding the height to which pollutants emitted from Earth's surface are mixed into the atmosphere. In our study, we investigated how the depth of the boundary layer varied at coastal sites along the Gulf of Mexico, Atlantic Ocean, and Pacific Ocean using 25 years of boundary layer depths obtained using weather balloon launches. We found large differences in boundary layer depths that depended on whether the wind was blowing onshore or offshore. Offshore winds resulted in much deeper boundary layers than onshore winds. The difference between boundary layer depths when the wind was offshore versus onshore was largest during the spring and summer at sites located along the Gulf of Mexico and Atlantic Ocean. Key Points: Using 25‐year rawinsonde observations, we report on the boundary layer depth variability over 18 coastal sites in the contiguous United StatesStriking differences emerged in the boundary layer depths due to interplay between marine air mass and continental air massThe offshore versus onshore contrasts in the boundary layer depths vary both seasonally and spatially among the coastal regions [ABSTRACT FROM AUTHOR]

The spatial patterns of subtropical marine stratocumulus cloud fraction variability on diurnal time scales are examined using high-temporal-resolution cloud masks that are based on 30-min, 4 km × 4 km geosynchronous infrared data for 2003-10. This dataset permits comparison of the characteristics of variability in low cloud fraction among the three subtropical marine stratocumulus regions in the northeastern (NE) Pacific, southeastern (SE) Pacific, and SE Atlantic Oceans. In all three regions, the largest diurnal cycles and earliest time of cloud breakup occur on the edges of the cloud field where cloud fractions are generally lower. The rate at which the cloud breaks up during the day is tied to the starting cloud fraction at dawn, which determines the amount of longwave cooling that is initially available to offset shortwave radiative fluxes during the day. The maximum rate of cloud breakup occurs near 1200 LT. Cloud fraction begins to increase by 1600 LT (before the sun sets) and reaches its maximum value just before dawn. The diurnal-cycle characteristics of the SE Pacific and SE Atlantic marine stratocumulus cloud decks are more similar to each other than to those in the NE Pacific. The NE Pacific cloud deck has a smaller-amplitude diurnal cycle, slower rates of cloud breakup during the day for a given cloud fraction at dawn, and a higher probability of cloud breakup overnight. [ABSTRACT FROM AUTHOR]

Measurements of formaldehyde (HCHO) were made at the Cape Verde Atmospheric Observatory between November 2006 and June 2007 using the Long-Path Differential Optical Absorption Spectroscopy (LP-DOAS) technique. Observations show that typical HCHO mixing ratios ranged between 350 and 550 pptv (with typical 2-σ uncertainties of ~110 pptv), with several events of high HCHO, the maximum being 1,885 ± 149 pptv. The observations indicate a lack of strong seasonal or diurnal variations, within the uncertainty of the measurements. A box model is employed to test whether the observations can be explained using known hydrocarbon photochemistry; the model replicates well the typical diurnal profile and monthly mean values. The model results indicate that on average 20% of HO production and 10% of OH destruction can be attributed to the mean HCHO levels, suggesting that even at these low average mixing ratios HCHO plays an important role in determining the HO (HO+OH) balance of the remote marine boundary layer. [ABSTRACT FROM AUTHOR]