Spatiotemporal analysis of marine stratocumulus-topped boundary layer across the Pacific-Atacama Desert transition

A semi-permanent stratocumulus-topped boundary layer (STBL) cloud deck is advected daily from the Southeast Pacific Ocean towards the Atacama Desert, regularly producing a fog belt at the coastal mountain range. In the absence of rain (annual rainfall ~2 mm) this fog belt provides the sole water input for ecosystems and a complementary water source that can potentially be tapped by local communities. The STBL is primarily maintained by a balance of cloud-top radiative cooling, entrainment of overlying dry air forced by surface driven convection, and large-scale subsidence. This balance drives the turbulence and regulates the growth and decay of the boundary layer expressed as the tendency of its height (∂h/∂t). While these processes are well-understood over the open ocean, the STBL persistence and dilution during the ocean to inland transition (Pacific to Atacama) remains poorly understood. We aim to quantify the STBL spatiotemporal variability and the contributions of key drivers across this transition (from ~500 km offshore to ~36 km inland) in this hyper arid region (18°S–24°S).

To address this topic, we combine three years (2022-2024) of GOES satellite observations, ERA5 reanalysis data, and the ECMWF EcRAD radiation scheme in order to estimate: 1) the spatiotemporal variability of fog and low cloud cover fraction (CCF) and 2) the STBL height budget, expressed as ∂h/∂t, which we decompose into positive contributions by entrainment and cloud-top longwave radiative cooling, and negative contributions by large-scale subsidence. This approach allows us to link physical processes that control the STBL height with observed CCF variability across the ocean to inland transition.

Our findings show a clear seasonal decrease in CCF across the ocean–inland transition, from values around 0.8 over the ocean to ~0.2 inland, particularly during summer and fall. In contrast, winter and spring exhibit an almost constant CCF (~0.8) extending up to ~12 km inland (~0.4), beyond which desert influence becomes dominant. From a temporal perspective,  oceanic CCF variability is dominated by synoptic periods (7–21 days), whereas inland variability is primarily controlled by the daily cycle (24 hours), likely driven by the strong diurnal heating and enhanced entrainment over the desert. The spatiotemporal variability reflects changes in the STBL height balance. Over the ocean, this balance is close to equilibrium and slightly positive (0.42 cm s⁻¹), with radiative cooling accounting for ~52% of the total contribution. Inland, this balance is disrupted (2.77 cm s⁻¹) as entrainment becomes dominant (~69%), driven by enhanced daytime surface fluxes over the desert. These findings highlight the crucial role of the balance of physical processes controlling STBL and fog variability across the ocean–inland transition. They provide new insights into the mechanisms shaping stratocumulus persistence in coastal desert regions, with implications for ecosystem water availability and regional climate understanding.