Fog in the Namib - Occurrence Dynamics Properties

This thesis is a treatise on fog in the Central Namib. In this hyper-arid area, fog represents the primary water source as rain is rare and rivers only flow underground. Fog occurs regularly at night with a distinct seasonality inland compared to the coast. At the coast, radiation, advection and inversion fog have been reported whereas inland additionally "high fog", that is cloud interception, occurs. This cloud interception fog is likely the most frequent fog type despite contrasting findings in the recent literature. The cloud interception itself is the consequence of the inland movement of a quasi-permanent stratus deck off the Namibian coast that is the result of the interaction of warm air with the cold Benguela current that leads to saturation and the large-scale subsidence that limits the vertical extent.
Notably, previous studies identified spatial variation even in the inland fog zone, that is the interception zone of the stratus. The water input from the fog, so-called fog precipitation, can vary by hundreds of millimetres per year between sites, but also by years, highlighting the immense complexity of this phenomenon. The fog climatology based on eleven stations of a distributed measurement network shows that the amount of fog precipitation is linked partially to elevation but also to the wind regime, where faster winds during a longer time tend to result in more fog precipitation. Notably, too strong winds seem to reduce the amount as well, potentially due to the flow regime around fog collectors.
An intensive observation period within the frame of the Namib fog life cycle analysis project further revealed the connection of the stratus within the fog zone and the vertical extent. Both findings confirm earlier schematics and reiterate that wind direction during fog is from a northerly sector not only on the ground but also higher up. Vertical temperature and humidity profile conducted within the period further constrain the extent of the stratus to between 200 m and 350 m above ground. The interception of the cloud base and subsequent appearance of fog is also consistent with the cloud base registered at the coast.
Non-rainfall water input measurements by microlysimeters during and after the intensive observation period showcase the challenging nature of measurements of fog water input. Whereas fog precipitation with fog collectors is the usual way, non-rainfall water input is far lower and no connection between the two amounts could be found. This may be partially due to the sampling mechanism (vertical deposition in microlysimeters vs. horizontal impact on fog collectors) but has far-reaching consequences. Fog collectors then indicate the wetness of fog and the potential of fog harvesting as a water source but do not indicate the amount of water the ecosystem receives out of fog. Ultimately, even the microlysimeters can only estimate this amount because many desert organisms in the Namib have adjusted to the regular fog with their behaviour and complex adaptations that enhance fog water collection.
Furthermore, a cloud droplet probe that sampled fog over the course of more than 1.5 years yielded insight into the patterns of fog in the Central Namib. Within the fog zone and probably as a result of intersection with different layers of the stratus, distinct distributions of fog droplets (∼2 to 50 μm) do exist. Larger droplets are pronouncedly absent at the site with higher fog precipitation. This contradiction indicates that mainly duration and wind speed may be controlling factors of fog precipitation rather than liquid water content. Collection efficiencies of the fog collectors did not express a clear connection for the site with the broad droplet size distribution and were generally below 10 %.
Ultimately, the combination of different measurement approaches demonstrates how complex the study of fog is: Firstly, where fog is officially defined as when visibility is below 1000 m, the reality is that significant deposition occurs mainly during very dense fog with a visibility below 200 m, at least in the Central Namib. In such dense fogs, up to 7.5 mm fog precipitation per event may occur, with one event during peak season roughly every second to third day. In total, up to ∼ 180 mm per year are collected compared to on average below 50 mm annually from rain. Secondly, depending on the measurement approach and the fog type, different amounts of water input are recorded, where each approach is as important as the next because each represents another physical process taking place. As expected, the research presented herein poses new questions from the performance of the devices, to the spatial and temporal variation within the fog zone to the general inter-event variation as each fog event differs slightly from the next.