Abstract
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 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.
Highlights
Nocturnal stratiform low-level clouds (LLCs) frequently form in the atmospheric boundary layer (ABL) over southern West Africa during the West African summer monsoon season
For the estimation of tendencies and contributions we look at the period before the onset of stratus fractus, to avoid the impact of phase changes, while we investigate the modification of the ABL conditions by the LLCs for the stratus phase only, as we expect a clearer signal from this phase
We find observational evidence for this mechanism in the temperature and wind profiles from radiosoundings: the mean profiles reveal that static stability below and in the LLCs decreases during the stratus phase compared to the jet phase (Fig. 7b and d), which we assume to represent the conditions in the cloud-free areas
Summary
Nocturnal stratiform low-level clouds (LLCs) frequently form in the atmospheric boundary layer (ABL) over southern West Africa during the West African summer monsoon season. These LLCs have typical cloud-base heights (CBHs) of only a few hundred meters above ground (Schrage and Fink, 2012; Kalthoff et al, 2018) and cover an area of about 800 000 km (van der Linden et al, 2015). The LLCs form during the night and persist long into the following day (Kalthoff et al, 2018) They affect the energy balance at the Earth’s surface, the diurnal cycle of the ABL and the regional climate (Knippertz et al, 2011; Hannak et al, 2017). A profound and accurate understanding of the processes relevant for the formation, maintenance and dissolution of the LLCs might help to identify flaws in these models
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