Dynamik, Variabilität und bioklimatische Effekte von niedrigen Wolken im westlichen Zentralafrika
Zusammenfassung der Projektergebnisse
The joint African-French German project has substantially advanced our understanding of the climatology and diurnal to interannual variability of low-level clouds in western equatorial Africa during the long dry season June–September. The low-level cloud deck is considered to be pivotal to the stability of the biodiverse rain-forest by reducing evaporation and by favoring photosynthesis through the enhanced diffuse radiation. It is thus an important question whether the current warming of the ocean and atmosphere could lead to a reduction in low-level cloud amount during the dry season, thereby threatening the stability of the rain forest in western equatorial Africa. The project compiled the most comprehensive low-level cloud climatology to date using data from surface stations during the last 50 and satellite data during the last 20 years. Climatological analysis shows that the cloud deck forms quite quickly in early June but disappears more slowly in September and even as late as October in the southwestern Republic of Congo. The cloud deck is most extensive and persistent throughout the day on the windward (e.g. western side) of the coastal low mountain ranges where mean sunshine duration in July is only between 1-2 hours. The coastal plains next to it is just a little less cloudy, whereas afternoon clearing is quite frequent over the interior plateaus and regular at the slopes to the Congo basin. Diurnally, the most thick and extensive cloud deck is observed in the early morning, with a minimum in the late afternoon. Interannual variations are manifested in a change in relatively clear and clear afternoon days. They are related to sea surface temperatures off the Gabon coast and in the eastern equatorial Atlantic; cooler ocean temperatures enhance the lower tropospheric stability and cloudiness. Surprisingly, the former is also enhanced by warmer (and often simultaneously moister) easterly flow at 3 km altitude stemming from the Congo basin, also linking the cloudiness to fluctuations of the El Niño – Southern Oscillation. Another surprise finding was that the cloudiness over western equatorial Africa can be linked to disturbances in the mid-latitudes of the Southern Hemisphere Atlantic Ocean at the intraseasonal time scale. While the 5th reanalysis from the European Centre for Medium-Range Weather Forecasts (ERA5) has a medium skill in representing day-to-day variations of the cloudiness, interannual variations are well represented. On the contrary, coupled climate models used in the last IPCC Assessment Report underestimate the cloudiness over western equatorial Africa and the adjacent Atlantic Ocean, largely due to an warm bias in the SSTs during June-September that was already documented in all previous IPCC reports. The bias in cloudiness was much improved when models were forced with observed SSTs. However, as was found in observations, the models corroborated the notion that other factors than equatorial Atlantic SSTs impact on lower-tropospheric stability and thus amount of low-level cloudiness. Case studies with two high resolution meteorological models support the largerscale picture, yet they show substantial deficiencies in the simulation of low-level clouds and missing boundary layer observation complicate the interpretation of local processes in the models. In terms of the major question regarding the ocean warming and potential disappearance of the cloud deck, the project showed that the ocean temperatures in the equatorial Atlantic are not the only mechanisms modulating their extent. Improved models and more local data in the boundary layer are necessary to achieve a more robust answer to this question.
Projektbezogene Publikationen (Auswahl)
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Satellite- and station-based climatology of low-level cloud cover during the long dry season in western Central Africa. EGU General Assembly, 19–30 April 2021, Online
Aellig R., J. Gerighausen, A. H. Fink, P. Knippertz & N. Philippon
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Characterization of Sunshine Duration in Western Equatorial Africa: In Situ Measurements versus SARAH-2 Satellite Estimates. Journal of Applied Meteorology and Climatology, 61(2), 185-201.
Philippon, N.; Ouhechou, A.; Camberlin, P.; Trentmann, J.; Fink, A. H.; Maloba, J. D.; Morel, B. & Samba, G.
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Cloud observing data of 85 stations in western Central Africa
Aellig, R., V. Moron, P. Camerblin, O. Champagne, N. Philippon, A. H. Fink & P. Knippertz
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Understanding the climatology of dry season low-level clouds over western equatorial Africa – First results from the “Dynamics, variability and bioclimatic effects of low clouds in western Central Africa (DYVALOCCA)” project. 35th Conference on Hurricanes and Tropical Meteorology, 09−13 May 2022, New Orleans, LA, USA, V18
Fink, A. H., R. Aellig, O. Champagne, N. Philippon, P. Camberlin, V. Moron, P. Knippertz, G. Sèze, J.-D. Maloba Makanga & J. L. Backita Moussounda
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Climatology of Low-Level Clouds over Western Equatorial Africa Based on Ground Observations and Satellites. Journal of Climate, 36(13), 4289-4306.
Champagne, O.; Aellig, R.; Fink, A. H.; Philippon, N.; Camberlin, P.; Moron, V.; Knippertz, P.; Seze, G. & van, der Linden R.
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Diurnal to interannual variability of low‐level cloud cover over western equatorial Africa in May–October. International Journal of Climatology, 43(13), 6038-6064.
Moron, Vincent; Camberlin, P.; Aellig, R.; Champagne, O.; Fink, A. H.; Knippertz, P. & Philippon, N.
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The representation of dry-season low-level clouds over Western Equatorial Africa in reanalyses and historical CMIP6 simulations. Climate Dynamics, 61(5-6), 2815-2837.
Camberlin, P.; Togbedji, C. F.; Pergaud, J.; Berger, A.; Aellig, R.; Fink, A. H.; Knippertz, P.; Moron, V. & Philippon, N.
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Understanding low-level Clouds in western Equatorial Africa during the long dry season. 157 pp.
Aellig, R.
