Thermodynamic characterization of the boundary layer under fog and dew events in the coastal hyper-arid climate of the Atacama Desert

In the coastal Atacama Desert, fog and dew represent the main atmospheric water inputs to the surface water balance in a context of near-total absence of precipitation. Both processes originate from the advection of the marine boundary layer (MBL) over the coastal topography, strongly influencing the spatial distribution of xeric, highly adapted ecosystems. Despite advances in understanding MBL advection under fog conditions, the physical differentiation between fog and dew remains unclear due to instrumental limitations, hindering their independent quantification and the assessment of their hydrological role. This study focuses on the thermodynamic characterization of the MBL under fog and dew events in the coastal Atacama Desert and is structured around three main objectives: (1) reclassification of atmospheric water harvesting events, (2) analysis of MBL stability, and (3) assessment of moisture tendency evolution. The analysis is based on a topographic transect of meteorological stations facing the ocean, distributed between 48 and 1354 m a.s.l., using 10-minute observations collected during 2024. Event reclassification integrates visibility measurements and the Fog Low Cloud (FLC) product from the GOES satellite, enabling discrimination between fog and dew beyond the signal provided by standard fog and dew collectors (SFC and SDC). Preliminary results indicate that standard collectors fail to adequately distinguish fog from dew events, as the inclusion of visibility and satellite information increases the annual proportion of dew events from 0.5% to 3.4%, while fog events remain close to 3.4%. Analysis of vertical profiles of potential temperature (θ) and specific humidity (q) shows that fog events are associated with a thermally and moisture well-mixed MBL, whereas dew formation occurs under a stratified (stable) MBL. In particular, the vertical gradient of θ reveals distinct stability thresholds differentiating fog (∂θ/∂z < 0.0020 K m⁻¹) from dew (0.0020 K m⁻¹ < ∂θ/∂z < 0.0031 K m⁻¹) events, while vertical profiles of q do not show significant differences between event types. Finally, the analysis of moisture tendency (∂q/∂t) reveals small but significant differences between fog and dew events, with sharper moisture decreases during dew conditions, indicating stronger atmosphere–surface water exchange at dawn. This study contributes to disentangling the atmospheric processes controlling fog and dew occurrence in the driest place on Earth.