Abstract
Fog is a characteristic feature of the Namib Desert and is essential to life in this fog dependent system. It is often acknowledged that advective fog from the ocean is the dominant fog type over the Namib Desert fog-zone but recent evidence suggests that other fog types occur in this area. Knowledge of the existence and spatial distribution of different fog types will enhance the mechanistic understanding of fog formation and potential changes in this region, but such knowledge is limited in literature. In this study, we investigated fog spatial variations within the Namib Desert fog-zone by applying stable isotope (δ18O and δ2H) techniques to differentiate various fog types and identify their source waters. Isotope based results showed that at least three types of fog (advective, radiation and mixed) occurred in this region and what appears as a single fog event may include all three types. Results suggest that radiation fog was the dominant fog type during our study period. The results also suggest that advective fog (with Atlantic Ocean origins) either dissipated 30–50 km inland and the residual humidity combined with locally derived moisture to form mixed fog or advective fog incorporated local moisture along its trajectory inland resulting in mixed fog. Fog in the Namib Desert was consistently depleted in 18O and 2H compared to rainfall and this was attributed to sub-cloud evaporation of the rainfall as well as different sources of fog and rainfall. Sub-cloud evaporation led to enrichment of 18O and 2H in rainfall beyond that of the first stage condensate, fog. Advective fog is often considered the architect of the fog-zone in the Namib Desert, but our results demonstrated multiple dominant fog types during the study period, suggesting knowledge of both fog frequency and fog type is needed to better predict climate change impacts on the fog-zone.
Highlights
Non-rainfall waters are the least studied hydrological components in most ecosystems
The remaining fog samples plotted between the global meteoric water line (GMWL) and local meteoric water line (LMWL), suggesting admixture of different air masses (Gat and Matsui, 1991; Martinelli et al, 1996; Liu et al, 2007), mixed fog (Kaseke et al, 2017) (Fig. 2)
There was a noticeable difference in the degree of radiative cooling and relative humidity (RH) during the fog event between the mixed fog sites and the radiation fog sites
Summary
Non-rainfall waters (e.g., fog and dew) are the least studied hydrological components in most ecosystems. These non-rainfall water components play an important role in ecosystem dynamics and are important for water-limited ecosystems (Wang et al, 2016). According to Byers (1959), it is challenging to provide an exact definition of fog because fog formation occurs under a variety of conditions and is dependent on the observer’s perspective and research interests (George, 1951; Tardif and Rasmussen, 2007). Fog formation is influenced by thermodynamic or radiative cooling, aerosol concentration, microphysical processes and surface conditions (Gultepe et al, 2007a), classification is often based on different combinations of these factors. Given the location of the Central Namib Desert (the region of interest in this study), the first three categories are of interest, we present general definitions of these categories
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