Determining Fog Frequency and Stratocumulus Cloud Vertical Structure Combining a Ground Optical Fog Observation System (GOFOS) with a Thermodynamic Characterization of the Marine Boundary Layer

Abstract:

The stratocumulus cloud (Sc), which forms fog at the coast of the Atacama Desert, is a valuable freshwater source that has the potential to improve water scarcity in this region. However, little is known about features such as fog frequency and cloud vertical structure that are essential to assess fog harvesting water potential. To investigate these features, we combined a Ground Optical Fog Observation System (GOFOS) with a thermodynamic characterization of potential temperature (θ) and specific humidity (q) along the marine boundary layer (MBL). On one hand, GOFOS directly measures local fog and low cloud spatiotemporal variability. It is composed of two optical time-lapse cameras that capture images every 10 minutes, and a series of autonomous lights that are located in an altitudinal profile, covering the entire range of regular fog presence. The cameras capture a section of the western slopes of the Coastal Cordillera, where the lights are installed. Thus, the observation of the different light sources allows to detect fog presence, the altitude of the thermal inversion cap, and the fog thickness. On the other hand, we employed the vertical gradients of θ and q, measured with traditional meteorological stations installed at different elevations in the Coastal Cordillera, to find seasonal and diurnal thresholds that define the MBL regimes related to fog formation (well-mixed MBL) and dissipation (stratified MBL); and we used the lifting-air parcel method, to estimate the location of the Sc cloud base (CB) and cloud top (CT). Our results show a seasonal variability of the vertical gradients of θ and q that suggest a nonlinearity of the thresholds that defines the MBL regimes along the year. Fog detection using the θ vertical gradients agreed in more than 84% of the time for autumn and winter, with values of more than ~75% for spring and summer. At a daily timescale, the best agreements were found between 21:00 and 02:00 local time. The CB and CT estimations had a good agreement with the GOFOS observations (r² = 0.62 and 0.77, respectively) when incorporating a mixing factor of 0.9, which represents how the surrounding air mixes with the lifted air parcel. Our findings reveal that the essential features of fog formation and its diurnal and seasonal variability can be approximated through standard meteorological observations.