Droplet size distribution, liquid water content and water input of the seasonally variable, nocturnal fog in the Central Namib Desert

In the arid Central Namib desert, rainfall is rare, but fog frequently occurs at night and exhibits a seasonal pattern. We deployed an eddycovariance setup at two different inland locations, namely Gobabeb and Vogelfederberg. The setup consisted of a cloud droplet probe and at times of up to two sonic anemometer-thermometers. It was deployed for 1.5 years, collecting data during more than 150 fog events at either one of the sites. We characterize the different fog regimes at the two stations with co-located measurements of visibility, fog collector mesh precipitation by fog collectors, and non-rainfall water input by microlysimeters for selected periods. At similar levels of visibility, fog collector mesh precipitation at Vogelfederberg is generally higher, likely due to its higher elevation and exposed location. Fog events appear front-like with a decrease of visibility and an increase of droplet numbers above 10 μm and the associated liquid water content. Fog events regularly lasted 6 h, with the first two to three hours exhibiting higher liquid water content. The droplet distribution, especially droplets above 20 μm, and thus liquid water content varied in numbers between the locations. Liquid water content correlates with fog collector mesh precipitation at Gobabeb, where the droplet distribution spans the whole resolved spectrum. While liquid water flux was bi-directional, the net sum was still a net gain for the surface. We found only a low correlation between liquid water flux and non-rainfall water input, which demonstrates that the liquid water flux can only partially connect the measured input in the fog collectors and the microlysimeters despite similar values. We therefore presume that drizzle, that is droplets out of the CDP sampling range, may play a role in the Central Namib fog, especially when fog is intermittent as fog deposition in microlysimeters continues when no droplets of the respective size range are present near the ground. Furthermore, the contrast of higher fog collector mesh precipitation at Vogelfederberg compared to Gobabeb despite lower liquid water content is potentially a result of the more numerous small droplets as a consequence of the interception with the upper parts of the stratus