Effect of Boundary Layer Thermodynamic Structures on the Formation of Low-Level Clouds over Ocean and Land

The DOE ARM Southern Great Plains (SGP, 36.61⁰N, 97.49⁰W) and Eastern North Atlantic Ocean (ENA,39.09⁰N, 28.03⁰W) sites locate almost the same latitude zone but have completely different background conditions. The airmass over the ENA site is dominated by the surrounding Atlantic Oceanic clean air with periodic episodes of polluted air advected from continents (Logan et al. 2014), whereas the ARM SGP site is located in the middle of the continent, the airmass over the site is dominated by continental air with maritime airmass advected from the Gulf of Mexico periodically.

Previous studies (Dong et al. 2006, 2013, 2014; Kennedy et al. 2013) have demonstrated that the large-scale dynamic patterns play an important role for the formation of low-level clouds, such as the subsidence from a high pressure to form inversion layer. However, the boundary layer thermodynamic structures should share the same important role as the large-scale dynamic patterns in the formation of low-level clouds over both ocean and land. Using long-term ARM ground-based observations, we found that the annual mean total cloud fractions are 51.6% and 69.4% over the ARM SGP and ENA sites, respectively. The low-level cloud fractions (cloud top height < 3 km) are 10.3% and 28.8%, respectively. That is, the low-level CF over the ENA is almost three times more than that at the SGP. With nearly the same precipitable water vapor (PWV) over these two sites, it is difficult to use water vapor sources to explain the large low-level CF difference over these two sites. In this study, we focus on exploring the boundary layer thermodynamic structures over two sites to reveal the possible mechanisms in the formation process of low-level clouds.