Nocturnal low-level clouds in the atmospheric boundary layer over southern West Africa: an observation-based analysis of conditions and processes
<p>During the West African summer monsoon season, extended nocturnal stratiform low-level clouds (LLCs) frequently form in the atmospheric boundary layer over southern West Africa and persist long into the following day affecting the regional climate. A unique data set was gathered within the...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2019-01-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/19/663/2019/acp-19-663-2019.pdf |
| _version_ | 1819267717583077376 |
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| author | B. Adler K. Babić N. Kalthoff F. Lohou M. Lothon C. Dione X. Pedruzo-Bagazgoitia H. Andersen |
| author_facet | B. Adler K. Babić N. Kalthoff F. Lohou M. Lothon C. Dione X. Pedruzo-Bagazgoitia H. Andersen |
| author_sort | B. Adler |
| collection | DOAJ |
| description | <p>During the West African summer monsoon season, extended nocturnal stratiform
low-level clouds (LLCs) frequently form in the atmospheric boundary layer over southern
West Africa and persist long into the following day affecting the regional climate. A
unique data set was gathered within the framework of the
Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa (DACCIWA) project, which
allows, for the first time, for an observational analysis of the processes and parameters
crucial for LLC formation. In this study, in situ and remote sensing measurements from
radiosondes, ceilometer, cloud radar and energy balance stations from a measurement site
near Savè in Benin are analyzed amongst others for 11 nights. The aim is to study LLC
characteristics, the intranight variability of boundary layer conditions and physical
processes relevant for LLC formation, as well as to assess the importance of these
processes. Based on the dynamic and thermodynamic conditions in the atmospheric boundary
layer we distinguish typical nocturnal phases and calculate mean profiles for the
individual phases. A stable surface inversion, which forms after sunset, is eroded by
differential horizontal cold air advection with the Gulf of Guinea maritime inflow, a
cool air mass propagating northwards from the coast in the late afternoon and the
evening, and shear-generated turbulence related to a nocturnal low-level jet. The
analysis of the contributions to the relative humidity changes before the LLC formation
reveals that cooling in the atmospheric boundary layer is crucial to reach saturation,
while specific humidity changes play a minor role. We quantify the heat budget terms and
find that about 50 % of the cooling prior to LLC formation is caused by horizontal cold
air advection, roughly 20 % by radiative flux divergence and about 22 % by sensible
heat flux divergence in the presence of a low-level jet. The outcomes of this study
contribute to the development of a conceptual model on LLC formation, maintenance and
dissolution over southern West Africa.</p> |
| first_indexed | 2024-12-23T21:21:36Z |
| format | Article |
| id | doaj.art-7648944e37554656891f59febeb1e217 |
| institution | Directory Open Access Journal |
| issn | 1680-7316 1680-7324 |
| language | English |
| last_indexed | 2024-12-23T21:21:36Z |
| publishDate | 2019-01-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Atmospheric Chemistry and Physics |
| spelling | doaj.art-7648944e37554656891f59febeb1e2172022-12-21T17:30:44ZengCopernicus PublicationsAtmospheric Chemistry and Physics1680-73161680-73242019-01-011966368110.5194/acp-19-663-2019Nocturnal low-level clouds in the atmospheric boundary layer over southern West Africa: an observation-based analysis of conditions and processesB. Adler0K. Babić1N. Kalthoff2F. Lohou3M. Lothon4C. Dione5X. Pedruzo-Bagazgoitia6H. Andersen7Institute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, GermanyInstitute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, GermanyInstitute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, GermanyLaboratoire d'Aérologie, Université de Toulouse, CNRS, UPS, Toulouse, FranceLaboratoire d'Aérologie, Université de Toulouse, CNRS, UPS, Toulouse, FranceLaboratoire d'Aérologie, Université de Toulouse, CNRS, UPS, Toulouse, FranceMeteorology and Air Quality Group, Wageningen University and Research, Wageningen, the NetherlandsInstitute of Meteorology and Climate Research, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany<p>During the West African summer monsoon season, extended nocturnal stratiform low-level clouds (LLCs) frequently form in the atmospheric boundary layer over southern West Africa and persist long into the following day affecting the regional climate. A unique data set was gathered within the framework of the Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa (DACCIWA) project, which allows, for the first time, for an observational analysis of the processes and parameters crucial for LLC formation. In this study, in situ and remote sensing measurements from radiosondes, ceilometer, cloud radar and energy balance stations from a measurement site near Savè in Benin are analyzed amongst others for 11 nights. The aim is to study LLC characteristics, the intranight variability of boundary layer conditions and physical processes relevant for LLC formation, as well as to assess the importance of these processes. Based on the dynamic and thermodynamic conditions in the atmospheric boundary layer we distinguish typical nocturnal phases and calculate mean profiles for the individual phases. A stable surface inversion, which forms after sunset, is eroded by differential horizontal cold air advection with the Gulf of Guinea maritime inflow, a cool air mass propagating northwards from the coast in the late afternoon and the evening, and shear-generated turbulence related to a nocturnal low-level jet. The analysis of the contributions to the relative humidity changes before the LLC formation reveals that cooling in the atmospheric boundary layer is crucial to reach saturation, while specific humidity changes play a minor role. We quantify the heat budget terms and find that about 50 % of the cooling prior to LLC formation is caused by horizontal cold air advection, roughly 20 % by radiative flux divergence and about 22 % by sensible heat flux divergence in the presence of a low-level jet. The outcomes of this study contribute to the development of a conceptual model on LLC formation, maintenance and dissolution over southern West Africa.</p>https://www.atmos-chem-phys.net/19/663/2019/acp-19-663-2019.pdf |
| spellingShingle | B. Adler K. Babić N. Kalthoff F. Lohou M. Lothon C. Dione X. Pedruzo-Bagazgoitia H. Andersen Nocturnal low-level clouds in the atmospheric boundary layer over southern West Africa: an observation-based analysis of conditions and processes Atmospheric Chemistry and Physics |
| title | Nocturnal low-level clouds in the atmospheric boundary layer over southern West Africa: an observation-based analysis of conditions and processes |
| title_full | Nocturnal low-level clouds in the atmospheric boundary layer over southern West Africa: an observation-based analysis of conditions and processes |
| title_fullStr | Nocturnal low-level clouds in the atmospheric boundary layer over southern West Africa: an observation-based analysis of conditions and processes |
| title_full_unstemmed | Nocturnal low-level clouds in the atmospheric boundary layer over southern West Africa: an observation-based analysis of conditions and processes |
| title_short | Nocturnal low-level clouds in the atmospheric boundary layer over southern West Africa: an observation-based analysis of conditions and processes |
| title_sort | nocturnal low level clouds in the atmospheric boundary layer over southern west africa an observation based analysis of conditions and processes |
| url | https://www.atmos-chem-phys.net/19/663/2019/acp-19-663-2019.pdf |
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