Marine boundary layer evolution in the transition from low stratocumulus clouds to land fog in the coastal mountains of Atacama

Advection of marine stratocumulus clouds (SCu) from the South East Pacific into the Atacama Desert forms large and semi-permanent fog banks at the top of the coastal mountain range. These fog banks are the sole water input for xeric ecosystems and represent a freshwater resource to be harvested by local communities in the driest place on Earth. The fog maintenance depends on the marine boundary layer (MBL) thermal inversion, which results from the equilibrium between subsidence and ocean heat fluxes. To study these interactions, we performed a field experiment in July 2024 in North West Chile called StraToFog, to measure surface and airborne boundary layer state during the SCu-fog transition. Surface measurements of energy balance fluxes and vertical observations of MBL thermodynamics were performed over a transect following the SCu-fog transition: at the ocean, the top of the mountain range, and inland in the desert. To complement these measurements, a high-precision balance and a standard fog collector transect were installed to measure fog and dew collection. Overall, our experiment reveals two distinctive fog regimes, which depend on the interplay between MBL growth, the strength of subsidence and the development of a sea breeze that pushes the MBL with clouds onto the coastal mountain range. First regime occurs at night when inversion layer is controlled by low surface temperature, resulting in air saturation under lower humidity content (↓es), leading to fog collection ~0.5 L m^-2 h^-1. The second regime occurs after a dissipation break at noon, where MBL advection increases humidity leading to air saturation under higher air temperatures (↑e), resulting in fog collection ~3 L m^-2 h^-1. Our results show a SCu top uplift between 150 to 400 m from ocean to inland. This uplifting is explained by the abrupt topography (800 m height; 5 km long) and by the sensible heat flux increases from 32 W m^-2 over the ocean to 250 W m^-2 over land. Airborne measurements show a diurnal cycle of fog cloud formation, which show a thickness of 20 m in the morning, grow to 200 m at noon, dissipate in the afternoon, and form again up to a thickness of 100 m during the evening. In addition, a very strong inversion layer (~18 K) was observed at the top of the cloud layer at 08:00 LT. The surface soil balance experiment shows a weight increase at night (00:00-06:00 LT) under clear sky conditions, associated with dew deposition. In contrast, during foggy days, the soil weight increases in the morning (06:00-12:00 LT) under windless (<1 m s^-1) conditions, followed by a decrease in the afternoon under windy conditions (>4 m s^-1), associated with fog deposition/evaporation. Further analysis of this data, accompanied by high resolution numerical simulations, will allow us to better understand fog dynamics in drylands and its potential prediction.