Low clouds and fog along the South-Western African coast — Satellite-based retrieval and spatial patterns
Research Highlights
► This paper studies fog/low cloud frequency along the south-western African coast. ► A Meteosat-SEVIRI satellite retrieval of fog/low clouds is introduced. ► In a validation study with CALIPSO CALIOP products, good performance is found. ► A five-year study of frequencies reveals a fog/low cloud hot spot around Walvis Bay. ► The most pronounced occurrence of fog/low cloud is encountered in the summer months.
Introduction
In many regions of the world fog is seen primarily as a hazard, due to its impacts on traffic safety and air quality. In other regions, such as the coastal deserts of South America and Southern Africa, fog is a crucial source of water. The environmental and ecological importance of fog in the Namib region in particular has been highlighted in the academic literature (e.g. Seely, 1979, Shanyengana, 2002, Henschel and Seely, 2008).
Low clouds transported onshore frequently lead to fog conditions as their bases touch the terrain. Advection of low clouds along the South-Western African coast mostly occurs as a result of low pressure systems to the south of a narrow coastal cold-water upwelling zone. Warm moist air from the seaward side of the upwelling zone is transported across the latter. There, low-level clouds form, which are then transported onshore (Olivier and Stockton, 1989).
A systematic study on fog and low cloud distribution along a section of the Namibian coast was performed by Olivier (1995). Based on a labor-intensive visual analysis of one year of Meteosat data, she produced maps of the annual and seasonal distributions of fog and low cloud for 1984. Since then, the technical capabilities of geostationary satellites have improved significantly, resulting in the development of algorithms for the automated, unsupervised detection of fog and low clouds in such satellite imagery (cf. the review in Gultepe et al., 2007).
The aim of the study presented here is to provide a multi-annual evaluation of fog and low stratus distribution along the South-Western African coast. To this end, a technique for automated detection of fog and low clouds in the region is introduced. This method is largely based on the principles discussed in Cermak and Bendix (2008). The study covers a region extending roughly from 15∘ to 35∘ South and from 10∘ to 20∘ East.
Section snippets
The MSG SEVIRI system
The Spinning-Enhanced Visible and Infrared Imager (SEVIRI) on Meteosat Second Generation (MSG) is a multi-spectral instrument providing measurements in 11 channels from 0.6 to 13.4 μm and one visible broadband channel with a higher spatial resolution. With a repeat rate of 15 min it provides 96 hemispheric scans per day, at a nadir nominal spatial resolution of 3 km (Schmetz et al., 2002). The spatial and spectral resolutions present significant advances over previous geostationary systems,
Validation study
The CALIOP cloud layer product was interpreted to produce the same classes as yielded by the SEVIRI scheme introduced above: open water, bare ground, high cloud, and low cloud. Clear areas were grouped into the first two classes using geographic information on land/sea mass distribution. Clouds were only accepted in single-layer situations. A minimum optical thickness of 1 was required with a maximum reported uncertainty of 50%. Low clouds were identified with cloud top heights smaller than or
Spatial and temporal patterns of fog and low clouds
Fog/low cloud detection was performed on all available scenes from 2004 to 2009 (2127 total). On this basis, the relative monthly frequency of fog or low cloud occurrence at each time step (7:00 UTC and 14:00 UTC) was determined for each pixel. This and the other data discussed in this section are freely available in the PANGAEA data base (doi:10.1594/PANGAEA.755427).
Fig. 3 shows the annual mean frequency map for 7:00 UTC obtained as the fraction of fog/low cloud observations in all valid
Conclusions and outlook
An algorithm was developed for the detection of fog and low clouds along the South-Western African coast in geostationary satellite data. Validation with the CALIOP cloud layer product yielded good detection accuracy, with a small positive bias and overestimations of fog/low clouds mostly in the southern part of the study region. Conditions can be presumed to be slightly different there, both regarding surfaces (outside the Namib Desert region) as well as atmosphere–ocean dynamics. It therefore
Acknowledgment
Funding for satellite data evaluation was provided by Deutsche Forschungsgemeinschaft (DFG) in grant BE 1780/14-1.
References (18)
- et al.
A novel approach to fog/low stratus detection using Meteosat 8 data
Atmospheric Research
(2008) - et al.
FMet — an integrated framework for Meteosat data processing for operational scientific applications
Computers and Geosciences
(2008) - et al.
Ecophysiology of atmospheric moisture in the Namib Desert
Atmospheric Research
(2008) - et al.
Tools for atmospheric radiative transfer: Streamer and FluxNet
Computers and Geosciences
(1998) Spatial distribution of fog in the Namib
Journal of Arid Environments
(1995)Exploring fog as a supplementary water source in Namibia
Atmospheric Research
(2002)- et al.
Meteosat third generation phase A optical payload consolidation
Sensors, Systems, and Next-Generation Satellites
(2007) - et al.
A feasibility study of daytime fog and low stratus detection with TERRA/AQUA–MODIS over land
Meteorological Applications
(2006) - et al.
Detecting ground fog from space — a microphysics-based approach
International Journal of Remote Sensing
(2011)