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Spatial distribution of relative humidity (colour-coded) and horizontal wind (arrows) at 950 hPa at 14:00 UTC (a) and 21:00 UTC (b) averaged for the months of July and August 2006. Data are from the 2.8 km simulation.
Source publication
We performed a high-resolution numerical simulation to study the development
of extensive low-level clouds that frequently form over southern West Africa
during the monsoon season. This study was made in preparation for a field
campaign in 2016 within the Dynamics-aerosol-chemistry-cloud interactions in West Africa (DACCIWA) project and focuses on...
Contexts in source publication
Context 1
... front occurs regularly when an undisturbed south-westerly monsoon flow prevails along the coast and further inland. Figure 5 shows the relative humidity at 950 hPa averaged for the 2 months. The front evolves along the coast after noon, which is re- flected by a strong gradient in relative humidity between the relatively cool maritime air mass over the Gulf of Guinea and warmer air over land (Fig. 5a). ...
Context 2
... monsoon flow prevails along the coast and further inland. Figure 5 shows the relative humidity at 950 hPa averaged for the 2 months. The front evolves along the coast after noon, which is re- flected by a strong gradient in relative humidity between the relatively cool maritime air mass over the Gulf of Guinea and warmer air over land (Fig. 5a). In general, moisture increases from south to north, which is related to strong evaporation over land. Within this large-scale moisture difference a local maximum exists along the front. This is caused by moisture convergence and upward transport of moisture from close to the surface when the monsoon flow decelerates due to surface ...
Context 3
... the coast. During the afternoon, the front is rather stationary, located about 30 km inland along the coast of eastern Ghana, Togo and western Benin. Further to the east, the front is more diffuse and is located farther away from the coast. After 16:00 UTC, the front starts pen- etrating inland and reaches SAVE at around 21:00 UTC on the average (Fig. 5b), which agrees well with the conditions of the case study (Figs. 3a, e and 4). During the subsequent hours, the front becomes more diffuse on the average, but continues to propagate ...
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This study presents the first detailed observational analysis of
the complete diurnal cycle of stratiform low-level clouds (LLC) and involved
atmospheric processes over southern West Africa (SWA). The data used here
were collected during the comprehensive DACCIWA
(Dynamics-Aerosol-Chemistry-Cloud-Interactions in West Africa) ground-based
campaign,...
A ground-based field campaign was conducted in
southern West Africa from mid-June to the end of July 2016 within the
framework of the Dynamics–Aerosol–Chemistry–Cloud Interactions in West
Africa (DACCIWA) project. It aimed to provide a high-quality comprehensive
data set for process studies, in particular of interactions between
low-level clouds (L...
Deducing realistic planetary boundary layer heights (PBLH) is crucial for weather, climate and air quality models, despite its equivocal nature. In this paper, a comparative assessment of seven PBLH estimation methods has been performed, with radiosonde profiles taken during the African Monsoon Multidisciplinary Analyses (AMMA) project campaign fro...
Citations
... Despite the numerous scientific research works undertaken with primal focus on various aspects of the WAM [17][18][19][20], with majority being model-based assessments [3,7,17,21] and a few observation-based assessments [18,19], there are still aspects of the WAM that need attention. The regional climate model (RegCM) is one of such useful models developed for simulating long-term regional climate and has been used in various regional model intercomparison projects and paleo-to future climate projections [2,3,22]. ...
In this paper, we present an analysis of summertime atmospheric simulation (June–July 2016) for southern West Africa (sWA) using the RegCM 4.7.1 regional climate model to describe the atmospheric behaviour over the region, and also engage comparisons between the modelled data and observed upper air data acquired during the DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) field campaign period. First, assessments of relative humidity and zonal wind profiles were made for selected coastal and inland stations, to infer the relative vertical and temporal atmospheric differences for both locations. Thereafter, the model’s performance was evaluated, capturing an excessive wet bias in RH profiles of the model with accompanying reduced zonal winds at the Tropical Easterly Jet (TEJ) region and thus produces excessive upper tropospheric cloud liquid water content. Also, in the lower troposphere (particularly, the monsoon layer), RegCM 4.7.1 model captures adequate spatial differences in both RH and zonal wind profiles along the coast and inland. We judge this outcome to be a valuable contribution on the path to rendering RegCM4 a good tool for simulating atmospheric and climate dynamics in sWA.
... However, the understanding of the mechanisms and processes driving the West African atmospheric boundary layer (ABL) clouds diurnal cycle has been hindered by the dearth of high-quality observations. Only a few model studies were performed to investigate the mechanisms for cloud formation, persistence and dissolution 8,9 . Therefore, a more realistic representation of clouds and convection in southern West Africa in weather forecast and regional climate models was aimed at. ...
... A number of processes expected to be relevant for the formation of LLSCs in this region have been listed by Adler et al. 9 and Kalthoff et al. 10 including large-scale advection, orographic lifting, lifting related to gravity waves, latent heat release, and vertical mixing of moisture due to shear-generated turbulence below the nocturnal low-level jet (LLJ). With respect to the latter process, shear-induced turbulence in the upper part of the LLJ and in the lower part of the African Easterly Jet (AEJ) could, depending on the position of the AEJ, mix the wet monsoon flow and the dry Saharan air layer above. ...
As part of the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) project, extensive in-situ measurements of the southern West African atmospheric boundary layer (ABL) have been performed at three supersites Kumasi (Ghana), Savè (Benin) and Ile-Ife (Nigeria) during the 2016 monsoon period (June and July). The measurements were designed to provide data for advancing our understanding of the relevant processes governing the formation, persistence and dissolution of nocturnal low-level stratus clouds and their influence on the daytime ABL in southern West Africa. An extensive low-level cloud deck often forms during the night and persists long into the following day strongly influencing the ABL diurnal cycle. Although the clouds are of a high significance for the regional climate, the dearth of observations in this region has hindered process understanding. Here, an overview of the measurements ranging from near-surface observations, cloud characteristics, aerosol and precipitation to the dynamics and thermodynamics in the ABL and above, and data processing is given. So-far achieved scientific findings, based on the dataset analyses, are briefly overviewed.
... This feature, as expected, occurred mainly because the study region was dominated by southerly to south-westerly winds, which transported more moisture over the region (see wind rose in Figure 2b). This observation also indicates the dominance of more low-level clouds within the study region during the summer months (Adler, Kalthoff, and Gantner, 2017;Aryee, Amekudzi, Quansah, et al., 2018;Baidu et al., 2017;Bessardon et al., 2018). ...
... Significant cooling (negative net potential temperature change) is observed within the lowest kilometer, which is consistent with the recent findings of Adler, Babić, et al. (2019) in Savè. This cooling is linked with the Gulf of Guinea maritime inflow (GoGMI): horizontal cold air advection related to the south-westerly monsoon flow which transports maritime air from the Gulf of Guinea northwards over SWA Adler, Kalthoff, & Gantner, 2017;Deetz et al., 2018). GoGMI has been observed about several tens of kilometers from the coast and initiated between the late afternoon and early evening. ...
... GoGMI has been observed about several tens of kilometers from the coast and initiated between the late afternoon and early evening. These propagate inland to distances beyond 100 km (Adler, Kalthoff, & Gantner, 2017;Deetz et al., 2018). ...
The presence, spatial extent, and persistence of low‐level clouds (LLCs) largely impact on the diurnal surface radiation and energy balance, as well as, the regional climate. Notwithstanding, there is a limited understanding of their evolution and processes, particularly in southern West Africa. This study assesses the development of LLCs and their dominant formative factors, as well as, their relationship with radiation and energy balance. First, ceilometer and radiosondes deployed during the dynamics‐aerosol‐chemistry‐cloud interactions in West Africa (DACCIWA) field campaign were used in identifying the LLC. Afterward, the cloud fraction was employed to characterize different LLC phases. Averagely, break‐up, dissipation, and build‐up of LLC were marked at 09:00, 12:00, and 22:00 GMT respectively. Moreover, composites of LLC diurnal evolution and their relationship with net radiation, energy storage, and surface stability showed that LLCs significantly impact on net radiation flux, by reducing downwelling shortwave radiation. Additionally, LLC onsets were characterized by a near‐steady state in net radiation flux, whereas the rate of energy storage within the lower layers marginally oscillated about equilibrium. Finally, with observations from selected intensive observation periods (IOPs), the dominant factors influencing LLC development were evaluated. Horizontal cold air advection, with enhancement by nocturnal low‐level jets, was observed to primarily influence the development of LLCs for the study period. The findings of this study are necessary for improving the understanding of LLC characteristics, formation, and interactions with surface properties, particularly over southern West Africa.
... WA is characterized by a frequent occurrence of different cloud types. Near the Gulf of Guinea in the southern part of WA, LLCs are prevalent all year round (Adler et al., 2017;Danso et al., 2019;Schrage & Fink, 2012;van der Linden et al., 2015). Farther north in the Sahelian region, these clouds are frequently observed during the WAM season from July to September (Bouniol et al., 2012). ...
... Both of these field campaigns provided an unprecedented database of surface-based observations of different cloud properties. Other studies on LLCs in the region have also been performed with satellite observations and model data (Adler et al., 2017;Hannak et al., 2017;van der Linden et al., 2015). These studies suggest that LLCs formation is linked to a number of processes including but not limited to cooling caused by horizontal cold air advection and the occurrence and strengthening of the nocturnal low-level jet (Adler et al., 2017;Babic et al., 2019b;Schrage & Fink, 2012). ...
... Other studies on LLCs in the region have also been performed with satellite observations and model data (Adler et al., 2017;Hannak et al., 2017;van der Linden et al., 2015). These studies suggest that LLCs formation is linked to a number of processes including but not limited to cooling caused by horizontal cold air advection and the occurrence and strengthening of the nocturnal low-level jet (Adler et al., 2017;Babic et al., 2019b;Schrage & Fink, 2012). ...
Since Paris Agreement in 2015, interest in projects dealing with solar photovoltaic energy implementation in West Africa (WA) has risen tremendously. Nevertheless, a notable drawback to such development in the region is the intense cloud cover which reduces incoming solar radiation when present and causes fluctuations in power production. Understanding the occurrence of clouds and their impacts on the incoming solar radiation is important for developing strategies to manage power generation. However, our understanding of clouds in WA is still relatively low. This is further complicated by the possible future impacts of climate change on the atmospheric factors that control solar power generation. Thus, this thesis seeks to address the following questions:1.How frequent and in what synoptic conditions do clouds occur and evolve in WA?2.What are the effects of the occurrence of clouds on the incoming solar radiation?3.How will climate change affect solar power generation and its atmospheric drivers?A climatology of various cloud types is first created using reanalysis and satellite-based data. In general, cloud cover in WA is characterized by a south-north gradient, with the southern areas cloudiest at all times. During the wet season, the cloud coverage is between 70% and 80% over southern WA. High- and mid-level clouds occur throughout the year over the whole region. However, low clouds, with distinct seasonal and spatial variations, are primarily confined to the south in the dry season but intensify and extend northward in the wet season. South of 15°N, at least one cloud type is present at all times, and simultaneous occurrence of all cloud types is frequent.Afterward, the synoptic conditions in which low clouds occur are investigated. During the wet season, low cloud occurrences are characterized by a high moisture flux dominated by a significant amount of background moisture and strong southwesterly winds blowing from the Guinea Coast. Furthermore, low cloud occurrence is preceded by a significant reduction of the air temperature in the lower atmosphere. Moisture flux is weaker in the dry season; however, the background moisture plays an important role in low clouds’ occurrences.Next, the direct impacts of cloud occurrence on the quantity and variability of incoming solar radiation are quantified. The attenuation of solar radiation is greatest during the summer months. In August, about 55% of solar radiation is lost on average over southern WA, while between 20% and 35% is lost over the Sahel area and northern parts of the region. In terms of the variability, the sub-daily evolution of the incoming solar radiation does not change much from one year to the other in the dry season. On the other hand, the radiation received during the rainy season is associated with higher uncertainty.Finally, the impacts of climate change on future solar power potential and its drivers are investigated with a suite of CMIP6 climate models. Solar power generation is expected to decrease in the region, with most of the models projecting about a 20% reduction over southern WA in the second part of the 21st century. All models project a decrease in surface solar radiation and an increase in the near-surface air temperature. However, the projected reduction of the future solar power potential is mainly due to decreasing solar radiation rather than rising temperatures. These results contribute to the ongoing feasibility assessments of long-term solar energy projects and have direct implications for the siting of solar farms in the region.
... This range is called the "grey zone" or "grey zone of turbulence" (Wyngaard, 2004;Honnert, 2016;Honnert et al., 2020). Although, in the grey zone, 1-D and 3-D turbulence parameterization schemes do not fit perfectly (Honnert, 2016), model simulations therein are useful (Zhou et al., 2014), so that features ranging from submesoscale (CBL heterogeneity) to mesoscale (secondary circulations) that influence convective precipitation can be appropriately represented (Chow et al., 2006;Barthlott and Hoose, 2015;Hohenegger et al., 2015;Adler et al., 2017;Gantner et al., 2017). Nevertheless, it is important to know how moist convection and precipitation depend on model grid spacing and LSR in the grey zone, as initiation and evolution of moist convection is coupled with land-surface parameters (Avissar and Chen, 1993) and their resolution (Banta, 1990;Weckwerth, 2000). ...
The impact of model grid spacing and land‐surface resolution (LSR) on convective precipitation are investigated for areas with different orographic complexities. ICOsahedral Nonhydrostatic (ICON) model simulations were performed for six days having weak large‐scale forcing using six model grid spacings (in metres): Numerical Weather Prediction (NWP) (Δ5000, Δ2500) and Large‐Eddy Simulation (LES) physics simulations (Δ1250, Δ625, Δ312, and Δ156) in a nested set‐up. Concerning LSR, we focused on simulations with LSRs of 1,250 and 5,000 m, keeping the model grid spacing at 156 m. The onset of precipitation in Δ1250 is earlier by 0.5–2 hr, while LSR modifications show a similar onset compared with Δ156. The relative percentage difference (RPD) of areal mean daily precipitation across LES physics simulations decreases consistently with model grid spacing for most of the cases. The RPD of precipitation in Δ1250 is considerably higher (75th percentile: ≈155%) than that of the LSR runs at resolutions of both 1,250 and 5,000 m, with 75th percentiles of ≈7% and ≈22%, respectively. To investigate the processes causing the differences in precipitation characteristics, like onset time and amount, the heat and moisture budgets of Δ1250 and Δ156 were compared. The results show that, at the initial stage of cloud formation, a higher number of smaller clouds are formed in Δ156 compared with Δ1250. The small clouds in Δ156 are subject to considerable evaporative cooling at their edges and shell regions, due to entrainment processes. As a result, these clouds often dissolve before they can grow deep. Later on, cloud aggregation in Δ156 also enables precipitation. The delayed onset of precipitation and reduced areas of aggregated clouds having low precipitation rates are the main reasons for less precipitation in Δ156 than in Δ1250.
... The increase in relative humidity (RH) leading to saturation and LLSC formation is due to a cooling within the monsoon layer up to around 1.5 km above ground level (a.g.l.), which mainly occurs during the stable and jet phases. The main process behind this cooling is the horizontal advection of cooler air from the Guinean coast due to the combination of a maritime inflow (MI) (Adler et al., 2017;Deetz et al., 2018) and the NLLJ (Schrage and Fink, 2012;Dione et al., 2019). The onset time and strength of the NLLJ, as well as the level of background humidity in the monsoon layer, are crucial for LLSC formation (Babić et al., 2019b). ...
Within the framework of the DACCIWA
(Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) project
and based on a field experiment conducted in June and July 2016, we analyze
the daytime breakup of continental low-level stratiform clouds in southern
West Africa. We use the observational data gathered during 22
precipitation-free occurrences at Savè, Benin. Our analysis, which
starts from the stratiform cloud formation usually at night, focuses on
the role played by the coupling between cloud and surface in the transition
towards shallow convective clouds during daytime. It is based on several
diagnostics, including the Richardson number and various cloud macrophysical
properties. The distance between the cloud base height and lifting
condensation level is used as a criterion of coupling. We also make an
attempt to estimate the most predominant terms of the liquid water path
budget in the early morning.
When the nocturnal low-level stratiform cloud forms, it is decoupled from
the surface except in one case. In the early morning, the cloud is found
coupled with the surface in 9 cases and remains decoupled in the 13
other cases. The coupling, which occurs within the 4 h after cloud
formation, is accompanied by cloud base lowering and near-neutral thermal
stability in the subcloud layer. Further, at the initial stage of the
transition, the stratiform cloud base is slightly cooler, wetter and more
homogeneous in coupled cases. The moisture jump at the cloud top is usually
found to be lower than 2 g kg−1 and the temperature jump within 1–5 K,
which is significantly smaller than typical marine stratocumulus and
explained by the monsoon flow environment in which the stratiform cloud
develops over West Africa. No significant difference in liquid water path
budget terms was found between coupled and decoupled cases. In agreement
with previous numerical studies, we found that the stratiform cloud
maintenance before sunrise results from the interplay between the
predominant radiative cooling, entrainment and large-scale subsidence at its top.
Three transition scenarios were observed depending on the state of coupling
at the initial stage. In coupled cases, the low-level stratiform cloud remains
coupled until its breakup. In five of the decoupled cases, the cloud couples
with the surface as the lifting condensation level rises. In the eight
remaining cases, the stratiform cloud remains hypothetically decoupled from
the surface throughout its life cycle since the height of its base remains
separated from the condensation level. In cases of coupling during the
transition, the stratiform cloud base lifts with the growing convective
boundary layer roughly between 06:30 and 08:00 UTC. The cloud deck breakup,
occurring at 11:00 UTC or later, leads to the formation of shallow
convective clouds. When the decoupling subsists, shallow cumulus clouds form
below the stratiform cloud deck between 06:30 and 09:00 UTC. The breakup
time in this scenario has a stronger variability and occurs before 11:00 UTC in most cases. Thus, we argue that the coupling with the surface during
daytime hours has a crucial role in the low-level stratiform cloud
maintenance and its transition towards shallow convective clouds.
... The breakup time of low-level clouds is delayed by one hour, associated with a decrease of precipitation of up to 16% . The influence of anthropogenic aerosols on low-level clouds is most significant from 16:00 to 21:00 UTC every day, which corresponds to the period of the transport of anthropogenic aerosol from the coastal urbanized area toward the north (Adler et al., 2017;Deetz et al., 2018b;Deroubaix et al., 2019). In contrast, from 09:00 to 15:00 UTC, the anthropogenic aerosols are accumulated along the coastline (Deroubaix et al., 2019), where the modeled low-level cloud cover seems to be sensitive to the anthropogenic aerosol difference between the two experiments. ...
During the West African summer monsoon, pollutants emitted in urbanized coastal areas modify cloud cover and precipitation patterns. The Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) field campaign provided numerous aircraft-based and ground-based observations, which are used here to evaluate two experiments made with the coupled WRF–CHIMERE model, integrating both the direct and indirect aerosol effect on meteorology. During one well-documented week (1–7 July 2016), the impacts of anthropogenic aerosols on the diurnal cycle of low-level clouds and precipitation are analyzed in detail using high and moderate intensity of anthropogenic emissions in the experiments. Over the continent and close to major anthropogenic emission sources, the breakup time of low-level clouds is delayed by 1 hour, and the daily precipitation rate decreased by 7.5 % with the enhanced anthropogenic emission experiment (with high aerosol load). Despite the small modifications on daily average of low-level cloud cover (+2.6 %) with high aerosol load compared to moderate, there is an increase by more than 20 % from 14:00 to 22:00 UTC on hourly average. Moreover, modifications of the modeled low-level cloud and precipitation rate occur far from the major anthropogenic emission sources, to the south over the ocean and to the north up to 11∘ N. The present study adds evidence to recent findings that enhanced pollution levels in West Africa may reduce precipitation.
... WA is characterized by a frequent occurrence of different cloud types. Near the Gulf of Guinea in the southern part of WA, LLCs are prevalent all year round (Schrage and Fink, 2012;van der Linden et al., 2015;Adler et al., 2017;Danso et al., 2019). Farther north in the Sahelian region, these clouds are frequently observed during the WAM season from July to September (Bouniol et al., 2012). ...
... Both of these field campaigns provided an unprecedented database of surface-based observations of different cloud properties. Other studies on LLCs in the region have also been performed with satellite observations and model data (van der Linden et al., 2015;Adler et al., 2017;Hannak et al., 2017). These studies suggest that LLC formation is linked to a number of processes including but not limited to cooling caused by horizontal cold-air advection and the occurrence and strengthening of the nocturnal low-level jet (Schrage and Fink, 2012;Adler et al., 2017;Babić et al., 2019b). ...
... Other studies on LLCs in the region have also been performed with satellite observations and model data (van der Linden et al., 2015;Adler et al., 2017;Hannak et al., 2017). These studies suggest that LLC formation is linked to a number of processes including but not limited to cooling caused by horizontal cold-air advection and the occurrence and strengthening of the nocturnal low-level jet (Schrage and Fink, 2012;Adler et al., 2017;Babić et al., 2019b). ...
This study focuses on daytime low-level clouds (LLCs) that
occur within the first 2 km of the atmosphere over West Africa (WA). These
daytime LLCs play a major role in the earth's radiative balance, yet their
understanding is still relatively low in WA. We use the state-of-the-art
ERA5 dataset to understand their occurrence and associated drivers as well
as their impact on the incoming surface solar radiation in the two
contrasting Guinean and Sahelian regions of WA. The diurnal cycle of the
daytime occurrence of three LLC classes namely No LCC, LLC Class-1 (LLCs
with lower fraction), and LLC Class-2 (LLCs with higher fraction) is first
studied. The monthly evolutions of hourly and long-lasting LLC (for at least
6 consecutive hours) events are then analyzed as well as the synoptic-scale
moisture flux associated with the long-lasting LLC events. Finally, the
impact of LLC on the surface heat fluxes and the incoming solar irradiance
is investigated. During the summer months in the Guinean region, LLC Class-1
occurrence is low, while LLC Class-2 is frequent (occurrence frequency around
75 % in August). In the Sahel, LLC Class-1 is dominant in the summer
months (occurrence frequency more than 80 % from June to October); however the peak occurrence frequency of Class-2 is also in the summer. In
both regions, events with No LLC do not present any specific correlation
with the time of the day. However, a diurnal evolution that appears to be
strongly different from one region to the other is noted for the occurrence
of LLC Class-2. LLC occurrence in both regions is associated with high
moisture flux driven by strong southwesterly winds from the Gulf of Guinea
and significant background moisture levels. LLC Class-2 in particular leads
to a significant reduction in the upward transfer of energy and a net
downward energy transfer caused by the release of large amounts of energy in
the atmosphere during the cloud formation. In July, August, and September (JAS), most of the LLC Class-2
events may likely be the low-level stratiform clouds that occur frequently
over the Guinean region, while they may be deep convective clouds in the
Sahel. Additionally, LLC Class-2 causes high attenuation of the incoming
solar radiation, especially during JAS, where about 49 % and 44 % of
the downwelling surface shortwave radiation is lost on average in Guinea and the Sahel, respectively.
... The stable phase starting in the late afternoon is characterised by a weak monsoon flow, vanishing thermal convection and the formation of a stable layer close to surface. The jet phase starts with the arrival of the Gulf of Guinea Maritime Inflow [Adler et al., 2017;Deetz et al., 2018a;Lohou et al., 2020] and involves the onset of the nocturnal low-level jet. In the third phase low-level clouds form and thicken, mostly as a consequence of cold air advection and to a lesser extent of radiation and flux divergence. ...
Improved atmospheric predictions across time-scales are important for the
development of greater resilience of the West African population to
hazardous weather and climate change. The EU-funded project Dynamics-
Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) produced
the most comprehensive observational dataset of the atmosphere over
densely populated southern West Africa to date. Using this dataset to foster
our understanding of atmospheric processes, and to evaluate dynamical
models and satellite data, the report highlights findings
relevant to operational meteorological services in Africa and beyond
... One can see that the inland affected area spans up to around 11°N. The LLSC coverage persists for many hours during the following day, reducing the incoming solar radiation, and impacting the surface energy budget and related processes such as the diurnal cycle of the atmospheric boundary layer (ABL) (Schuster et al., 2013;Adler et al., 2017;Knippertz et al., 2017). However, the diurnal cycle of those clouds is still poorly represented in numerical 20 weather and climate models (Hannak et al., 2017). ...
... Therefore, a better understanding of the processes behind LLSC is useful for modelling purposes. Due to the scarce weather monitoring network over West Africa, the first studies addressing the LLSC over this region were conducted with satellite images and traditional synoptic observation 25 (Schrage and Fink, 2012;van der Linden et al., 2015), as well as with numerical simulations at regional scale (Schuster et al., 2013;Adler et al., 2017;Deetz et al., 2018). They emphasized that the physical processes, spanning from local to synoptic scale such as, horizontal advection of cold air associated to WAM, lifting induced by topography, gravity waves or shear-driven turbulence, are relevant for the LLSC formation during the night. ...
... The increase of relative humidity within the ABL leading to saturation and their formation is due to the cooling which mainly occurs during the stable and the jet phases. The main process behind this cooling is the horizontal advection of cooler air from Guinea coast, due to the combination of a maritime inflow (MI) (Adler et al., 2017;Deetz et al., 2018) and the NLLJ (Schrage and Fink, 2012;Dione et al., 2019). The onset time and the strength 25 of the NLLJ, as well as the level of background humidity in the ABL, are crucial for the LLSC formation (Babić et al., 2019b). ...
Within the framework of the DACCIWA (Dynamics-Aerosol-Chemistry-Cloud-Interactions over West Africa) project, and based on a field experiment conducted in June and July 2016, we analyse the daytime breakup of the continental low-level stratiform clouds in southern West Africa. We use the observational data gathered during twenty-two precipitation-free occurrences at Savè supersite, in Benin. Our analysis, which starts since the stratiform cloud formation usually at night, focuses on the role played by the coupling between the cloud and the surface in the transition towards shallow convective clouds. It is based on several diagnostics, including Richardson number and various cloud macrophysical properties. The distance between lifting condensation level and cloud base height is used as a criterion of coupling. We also make an attempt to estimate the most predominant terms of the liquid water path budget on early morning. When the nocturnal low-level stratiform cloud forms, it is decoupled from the surface, except in one case. On early morning, the cloud is found coupled with the surface in nine cases and is remained decoupled in the thirteen other cases. The coupling, which occurs within the four hours after the cloud formation, is accompanied with a cloud base lowering and near-neutral thermal stability in the subcloud layer. Further, at the initial stage of the transition, the stratiform cloud base is slightly cooler, wetter and more homogeneous in the coupled cases. The moisture jump at cloud top is found usually around 2 g kg−1, and the temperature jump within 1–5 K, which is significantly smaller than typical marine stratocumulus, and explained by the monsoon flow environment within which the stratiform cloud develops. No significant difference of liquid water path budget terms was found between the coupled and decoupled cases. In agreement with previous numerical studies, we found that the stratiform cloud maintenance before the sunrise results from the interplay between the predominant radiative cooling, and, the entrainment and large scale subsidence at its top. Three transition scenarios were observed, depending on the state of the coupling at the initial stage. In the coupled cases, the low-level stratiform cloud remains coupled until its break up. In five of the decoupled cases, the cloud couples with the surface as the LCL is rising. In the eight remaining cases, the stratiform cloud remains decoupled from the surface all along its life cycle. In case of coupling during the transition, the stratiform cloud base lifts with the growing convective boundary layer roughly between 06:30 and 08:00 UTC. The cloud deck breakup occurring at 11:00 UTC or later leads to the formation of shallow convective clouds. When the decoupling subsists, shallow cumulus clouds form below the stratiform cloud deck between 06:30 and 09:00 UTC. The breakup time in this scenario has a stronger variability, and occurs before 11:00 UTC in most of the cases. Thus we argue that the coupling with the surface during the daytime hours has a crucial role in the low-level stratiform cloud maintenance and in its transition towards shallow convective clouds.
