Nuerical simulations of fog events in the Namib region (NaFoLiCa-M)
Atmospheric Science
Final Report Abstract
The Namib desert is one of the driest places on earth. Its location at the west coast of the African continent in the proximity of the cold Benguela current makes the Namib a typical coastal desert with very scarce rainfall. In contrast to that, fog occurs regularly. Fog precipitation often exceeds rainfall and thus fog water deposition is a major source of water for the ecosystem and could be a supplementary source of water for human settlements. The construction of economic fog water collection systems requires detailed knowledge of the spatio-temporal development of fog events and the processes causing this development. Due to the location of the Namib desert bounded by the cold upwelling waters of the Benguela current in the west and the Great Escarpment in the east, this knowledge may best be achieved by means of three-dimensional numerical simulations. To understand the local mechanisms leading to fog occurrence and the spatio-temporal evolution of fog events in the Namib region, the three-dimensional fog model COSMO-FOG has been developed. For this purpose, the one-dimensional fog and boundary layer model PAFOG has been implemented into a recent version of the numerical weather prediction model of the Consortium for Small Scale Modelling (COSMO), maintained by the German meteorological service (DWD). The implementation of PAFOG into COSMO has been realised by replacing the one-moment bulk microphysical parametrisation scheme of COSMO by the two-moment microphysical parametrisation of PAFOG. Idealised simulations of a horizontally homogeneous marine stratus show a clear improvement for the PAFOG microphysical parametrisation scheme in comparison to the standard scheme of COSMO. A decrease of the vertical grid spacing further improves the simulation results. COSMO-FOG has been applied to a number of case studies for inland reaching fog events which have been observed during the measurement campaign in the framework of the NaFoLiCA project. The results obtained with COSMO-FOG are evaluated with satellite retrievals and ground-based measurements. In general, COSMO-FOG captures the meteorological situation reasonably well. Especially for coastal stations the diurnal cycle and onset of the fog events are in agreement with the observations. Visual comparison of the spatio-temporal patterns of fog and low clouds (low cloud mask from satellite retrieval against vertically integrated liquid water content from model simulation) reveals that COSMO-FOG largely captures the occurrence of fog or low clouds. The comparison to long-term satellite retrievals confirms that the selected case studies are typical for Namib region fog. From the analysis of the case studies we found that low stratiform clouds are most often present above the Atlantic ocean and close to the coastline. In the evening, these clouds proceed onshore until they intercept with the terrain of the ascending Great Escarpment thus forming fog. This fog formation is the precondition for the construction of fog collection systems. The simulated cloud water content of this fog close to the ground is similar for the investigated case studies. However, the location of the landfall of low clouds varies temporally during the night and among the different fog types. In summary, it is hardly possible to clearly declare regions most suitable for fog water harvesting based on the investigated case studies. A recommendation of regions suitable for fog water harvesting would require a climatological model simulation over several years.
Publications
- COSMO-PAFOG: Three-dimensional fog forecasting with the high-resolution COSMO-model. Geophysical Research Abstracts, 19, EGU2017-5663, EGU General Assembly, Wien, Österreich, 23–28 April 2017
Maike Hacker and Andreas Bott
- Modeling the spatial and temporal variability of fog in the Namib desert with COSMO-PAFOG. Geophysical Research Abstracts, 20, EGU2018-8230, EGU General Assembly, Wien, Österreich, 8–13 April 2018
Maike Hacker and Andreas Bott
- (2019). ”Probing the fog life cycles in the Namib desert”. Bulletin of the American Meteorological Society, 100(12), 2491–2507
Spirig et al. (with: Andreas Bott, Maike Hacker and Niklas Wagner)
(See online at https://doi.org/10.1175/BAMS-D-18-0142.1) - Modeling the life cycle of fog in the Namib desert with COSMO-PAFOG. 8th International Conference on Fog, Fog Collection and Dew (2019), Taipei, Taiwan, 14–19 July 2019
Maike Hacker, Bianca Adler, Hendrik Andersen, Jan Cermak, Norbert Kalthoff, Roland Vogt, and Andreas Bott
