First spatially continuous fog maps for the central Namib Desert reveal seasonally shifting fog belt

Résumé fourni par la source

Abstract. Fog represents the major hydrological input in the Namib Desert, yet its spatial distribution remains poorly constrained because ground-based observations are sparse, and there is no direct way to distinguish fog from elevated low clouds in satellite imagery. Here we present the first spatially continuous fog maps for the central Namib Desert derived by integrating satellite–based and ground-based observations. The method combines satellite-based fog/low cloud (FLC) detection from Meteosat SEVIRI with cloud-base height (CBH) estimates derived from observations in the FogNet station network. The regionally derived CBH is then used to separate fog from elevated low clouds. This enables spatiotemporally coherent fog mapping, which we investigate at fog event level and at monthly scales. Validation against independent station-based fog observations shows robust performance across the Namibian FogNet network, with a mean classification accuracy of about 85 % and a Heidke Skill Score of ≈0.6. The CBH constraint substantially reduces the systematic overestimation obtained by using satellite-based FLC detection alone, although some overestimation persists at inland stations during summer months when the cloud base seems to follow the topography. The resulting dataset provides the first observation-based fog maps of inland-reaching advective fog in the central Namib Desert and enables quantitative characterization of the regional fog belt. The maps reveal a fog belt extending ≈10–80 km inland from the coastline and between roughly 100 and 700 m above mean sea level (>25 fog days yr−1), within which a narrower core fog zone occurs between about 20 and 50 km inland and 150–450 m elevation (>90 fog days yr−1). The fog belt shows a pronounced seasonal shift due to the seasonality of the cloud base height. These spatially explicit fog maps provide a basis for identifying ecologically relevant fog zones and for future assessments of non-rainfall water inputs and fog-transported chemical depositions.

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