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
N1 - Publisher Copyright:
© Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License.
PY - 2019/1/17
Y1 - 2019/1/17
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.
UR - https://www.scopus.com/pages/publications/85060179487
U2 - 10.5194/acp-19-663-2019
DO - 10.5194/acp-19-663-2019
M3 - Article
AN - SCOPUS:85060179487
SN - 1680-7316
VL - 19
SP - 663
EP - 681
JO - Atmospheric Chemistry and Physics
JF - Atmospheric Chemistry and Physics
IS - 1
ER -