Nocturnal low-level clouds in the atmospheric boundary layer over southern West Africa: an observation-based analysis of conditions and processes

Bianca Adler, Karmen Babia, Norbert Kalthoff, Fabienne Lohou, Marie Lothon, Cheikh Dione, Xabier Pedruzo-Bagazgoitia, Hendrik Andersen

Research output: Contribution to journalArticleAcademicpeer-review

6 Citations (Scopus)

Abstract

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 Save 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.
Original languageEnglish
Pages (from-to)663-681
JournalAtmospheric Chemistry and Physics
Volume19
Issue number1
DOIs
Publication statusPublished - 17 Jan 2019

Fingerprint

boundary layer
cold air
advection
divergence
cooling
analysis
West Africa
heat budget
sensible heat flux
radiosonde
regional climate
air mass
energy balance
relative humidity
humidity
inflow
monsoon
thermodynamics
turbulence
dissolution

Cite this

Adler, Bianca ; Babia, Karmen ; Kalthoff, Norbert ; Lohou, Fabienne ; Lothon, Marie ; Dione, Cheikh ; Pedruzo-Bagazgoitia, Xabier ; Andersen, Hendrik. / Nocturnal low-level clouds in the atmospheric boundary layer over southern West Africa : an observation-based analysis of conditions and processes. In: Atmospheric Chemistry and Physics. 2019 ; Vol. 19, No. 1. pp. 663-681.
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abstract = "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 Save 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.",
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Nocturnal low-level clouds in the atmospheric boundary layer over southern West Africa : an observation-based analysis of conditions and processes. / Adler, Bianca; Babia, Karmen; Kalthoff, Norbert; Lohou, Fabienne; Lothon, Marie; Dione, Cheikh; Pedruzo-Bagazgoitia, Xabier; Andersen, Hendrik.

In: Atmospheric Chemistry and Physics, Vol. 19, No. 1, 17.01.2019, p. 663-681.

Research output: Contribution to journalArticleAcademicpeer-review

TY - JOUR

T1 - Nocturnal low-level clouds in the atmospheric boundary layer over southern West Africa

T2 - an observation-based analysis of conditions and processes

AU - Adler, Bianca

AU - Babia, Karmen

AU - Kalthoff, Norbert

AU - Lohou, Fabienne

AU - Lothon, Marie

AU - Dione, Cheikh

AU - Pedruzo-Bagazgoitia, Xabier

AU - Andersen, Hendrik

PY - 2019/1/17

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N2 - 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 Save 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.

AB - 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 Save 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.

U2 - 10.5194/acp-19-663-2019

DO - 10.5194/acp-19-663-2019

M3 - Article

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SP - 663

EP - 681

JO - Atmospheric Chemistry and Physics

JF - Atmospheric Chemistry and Physics

SN - 1680-7316

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ER -