Characterizing the influence of the marine stratocumulus cloud on the land fog at the Atacama Desert

Fog in the Atacama Desert is a virtually untapped source of fresh water in the driest place on Earth. Focusing on understanding the role played by marine stratocumulus (Sc) in the development of land-fog, we analyse surface observations made along a steep transect at different heights. These observations are combined with numerical experiments performed using the Weather Research and Forecasting Model (WRF). We find two main diurnal regimes based on atmospheric thermal stability, both of which determine the formation and dissipation of fog. These are (a) a well-mixed regime characterized by low gradients of potential temperature and specific humidity, low diurnal variability, and presence of Sc cloud-fog. (b) A stratified regime characterized by high gradients of potential temperature and specific humidity, high diurnal variability, but no Sc clouds nor presence of fog. By using the parcel method, initialised with surface observations, we characterize the Sc cloud of nine typical fog events, estimating a mean cloud depth of 566 m between 740 m (±150 m) and 1307 m (±30 m). Fog observations at ground level agree with these cloud-base and cloud-top estimates, showing a liquid water mixing ratio of Sc cloud-fog in the range 0.3-0.7 g kg−1. The study reveals that the advection of marine Sc cloud and the stability of the boundary layer are key processes in the formation and dissipation of fog. Sc cloud advection over land is modulated by upwind and driven by topography and local circulation. We conclude that a realistic characterization of Sc cloud-fog is possible by combining limited surface observations and numerical experiments.