Abstract
Abstract. Fog and low clouds (FLCs) are a typical feature along the southwestern African coast, especially in the central Namib, where fog constitutes a valuable resource of water for many ecosystems. In this study, a novel algorithm is presented to detect FLCs over land from geostationary satellite data using only infrared observations. The algorithm is the first of its kind as it is stationary in time and thus able to reveal a detailed view of the diurnal and spatial patterns of FLCs in the Namib region. A validation against net radiation measurements from a station network in the central Namib reveals a high overall accuracy with a probability of detection of 94 %, a false-alarm rate of 12 % and an overall correctness of classification of 97 %. The average timing and persistence of FLCs seem to depend on the distance to the coast, suggesting that the region is dominated by advection-driven FLCs. While the algorithm is applied to study Namib-region fog and low clouds, it is designed to be transferable to other regions and can be used to retrieve long-term data sets.
Highlights
Fog is commonly perceived as a hazardous weather situation that can impact traffic systems as well as the economy (Cermak and Knutti, 2009; Egli et al, 2018)
The algorithm design uses a combination of spectral tests and contextual information in order to retrieve fog and low clouds (FLCs)
A structural similarity index (Wang et al, 2004) is computed, comparing each satellite scene with cloud-free composites to discriminate between land surfaces and FLCs
Summary
Fog is commonly perceived as a hazardous weather situation that can impact traffic systems as well as the economy (Cermak and Knutti, 2009; Egli et al, 2018). Information on FLCs from satellites is typically inferred using separate daytime (e.g., Bendix et al, 2006; Cermak and Bendix, 2008, 2011; Nilo et al, 2018) and night-time (e.g., Ellrod, 1995; Cermak and Bendix, 2007) algorithms, disrupting our view of fog development at a critical time of its life cycle, as typically, shortwave radiative heating starts the dissipation of fog shortly after sunrise (Tardif and Rasmussen, 2007; Haeffelin et al, 2010; Wærsted et al, 2017) This break in retrieval techniques has limited the applicability of satellite-based FLC observations for the analysis of entire fog life cycles.
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