A meteorological dataset of the West African monsoon during the 2016 DACCIWA campaign (original) (raw)
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Atmospheric Chemistry and Physics Discussions
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 into interactions between low-level clouds (LLCs) and boundary-layer conditions. In this region missing observations are still a major issue. During the campaign, extensive remote sensing and in-situ measurements were conducted at three supersites: Kumasi (Ghana), Savè (Benin) and Ile-Ife (Nigeria). Daily radiosoundings were performed at 06:00 UTC and 15 intensive observation periods (IOPs) were performed during which additional radiosondes were launched every 1.5 to 3 h. Remotely piloted aerial systems were also operated during the IOPs. Extended stratiform LLCs form frequently in southern West Africa during the night time and persist long into the ...
Low-level stratiform clouds and dynamical features observed within the southern West African monsoon
Atmospheric Chemistry and Physics, 2019
During the boreal summer, the monsoon season that takes place in West Africa is accompanied by low stratus clouds over land that stretch from the Guinean coast several hundred kilometers inland. Numerical climate and weather models need finer description and knowledge of cloud macrophysical characteristics and of the dynamical and thermodynamical structures occupying the lowest troposphere, in order to be properly evaluated in this region. The Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) field experiment, which took place in summer 2016, addresses this knowledge gap. Lowlevel atmospheric dynamics and stratiform low-level cloud macrophysical properties are analyzed using in situ and remote sensing measurements continuously collected from 20 June to 30 July at Savè, Benin, roughly 180 km from the coast. The macrophysical characteristics of the stratus clouds are deduced from a ceilometer, an infrared cloud camera, and cloud radar. Onset times, evolution, dissipation times, base heights, and thickness are evaluated. The data from an ultrahigh-frequency (UHF) wind profiler, a microwave radiometer, and an energy balance station are used to quantify the occurrence and characteristics of the monsoon flow, the nocturnal low-level jet, and the cold air mass inflow propagating northward from the coast of the Gulf of Guinea. The results show that these dynamical structures are very regularly observed during the entire 41 d documented period. Monsoon flow is observed every day during our study period. The socalled "maritime inflow" and the nocturnal low-level jet are also systematic features in this area. According to synoptic Published by Copernicus Publications on behalf of the European Geosciences Union. 8980 C. Dione et al.: Low-level stratiform cloud and dynamical features observed over southern West Africa 1 Introduction Clouds are an important factor of uncertainty in climate change studies. The low-level stratiform clouds (LLSCs) that develop during the West African monsoon (WAM) along the Guinean coast likely contribute to this uncertainty because they modify the Earth's energy budget in a region where the dynamics are driven by strong thermal and moisture gradients and deep convection activity (Knippertz et al., 2011). However, until recently, very little attention was paid to these clouds. Poorly represented in numerical climate models (Hannak et al., 2017), LLSCs form during the night and can extend from the Guinean coast several hundred kilometers inland; they last until midday the following day (Schrage and Fink, 2012; Schrage et al., 2007; Schuster et al., 2013). These authors emphasize the possible link between LLSC formation and persistence and the dynamical features in the region, like the monsoon flow and the nocturnal low-level jet (NLLJ). However, very few observations of low clouds and associated dynamical processes are available, which prevents studies of LLSC formation and dissolution, as well as numerical climate and weather model validation. Filling the gap of observations and studying the LLSC life cycle were therefore the primary goals of the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) project (Knippertz et al., 2015) with aircraft and groundbased campaigns (Flamant et al., 2017; Kalthoff et al., 2018) performed during summer 2016. At three supersites, Kumasi (Ghana), Savè (Benin), and Ile-Ife (Nigeria), ground-based measurements were performed. The corresponding datasets are described in Bessardon et al. (2019). According to Kalthoff et al. (2018), LLSCs form most nights at the three supersites instrumented during the DAC-CIWA field campaign. Their bases are roughly around 300 m above ground level (a.g.l.) when they form and typically rise up to 800 m a.g.l. at noon on the following day. Defining LLSCs using criteria based on a median cloud base fraction of 100 % at Savé and Kumasi, and a net longwave radiation threshold of −10 W m −2 for Ile-Ife, Kalthoff et al. (2018) noted some differences in the onset times of the LLSCs. These authors found that the onset times varied on average among the three sites: 21:00 UTC at Ile-Ife, 00:00 UTC at Kumasi, and 03:00 UTC at Savè. Beyond this general description, large variability of the LLSC characteristics is observed from one night to the next. To address this situation, it is important to consider the larger-scale context of the WAM and its dynamical features. Such work was the focus of the previous African Monsoon Multidisciplinary Analysis (AMMA) international project (Redelsperger et al., 2006). The primary dynamical feature affecting West Africa during half of the year is the monsoon flow, which is due to synoptic-scale forcing associated with a strong thermal gradient between the cold tongue over the Gulf of Guinea and dry and warm air inland in the Saharan Heat Low (Lavaysse et al., 2009). In the southern re
Low Level Cloud and Dynamical Features within the Southern West African Monsoon
Atmospheric Chemistry and Physics Discussions, 2018
During the Boreal summer, the monsoon season that takes place in West Africa is accompanied by low stratus clouds over land, that stretch from the Guinean coast several hundred kilometers inland. These clouds form during the night and dissipate during the following day. Inherently linked with the diurnal cycle of monsoon flow, those clouds still remain poorly documented and understood. Moreover, numerical climate and weather models lack fine quantitative documentation of cloud macrophysical characteristics and the dynamical and thermodynamical structures occupying the lowest troposphere. The Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) field experiment, which took place in summer 2016, addresses this knowledge gap. Low level atmospheric dynamics and low-level cloud macrophysical properties are analyzed using in-situ and remote sensing continuous measurements collected from 20 June to 30 July at Savè, Benin, roughly 180 kilometers from the coast. The macrophysical characteristics of the stratus clouds are deduced from a ceilometer, an infrared cloud camera and cloud radar. Onset times, evolution, dissipation times, base heights and thickness are evaluated. The Data from a UHF (Ultra High Frequency) wind profiler, a microwave radiometer and an energy balance station are used to quantify the occurrence and characteristics of the monsoon flow, the nocturnal low-level jet and the cold air mass inflow propagating northwards from the coast of the Gulf of Guinea. The results show that these dynamical structures are very regularly observed during the entire 41-day documented period. Monsoon flow is observed 100% of the time. The so-called 'maritime inflow' and the nocturnal low level jet are also systematic features in this area. According to monsoon flow conditions, the maritime inflow reaches Savè around 1800-1900 UTC on average: this timing is correlated with the strength of the monsoon flow. This time of arrival is close to the time range of the nocturnal low level jet settlement. As a result, these phenomena are difficult to distinguish at the Savè site. The low level jet occurs every night, except during rain events, and is associated 65% of the time with low stratus clouds. Stratus cloud form between 2200 UTC and 0600 UTC at an elevation close to the nocturnal low level jet core height. The cloud base height, 310 ± 30 m above ground level (a. g. l.) is rather stationary during the night and remains below the jet core height. The cloud top height, at 640 ± 100 m a. g. l., is typically found above the jet core. The nocturnal low level jet, low level clouds, monsoon flow and maritime inflow reveal significant day-today variability during the summer. Distributions of strength, depth, onset time, break up time, etc. are quantified here.
Atmospheric Chemistry and Physics Discussions, 2018
During the West African summer Monsoon season, extended nocturnal stratiform low-level clouds (LLC) 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 decisive 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 intra-night variability of boundary layer conditions and physical processes relevant for LLC formation, as well as to assess the importance of these processes. Typical nocturnal phases are identified and mean profiles are calculated for the individual phases revealing pronounced differences: 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 lowlevel jet. The analysis of the contributions to the relative humidity changes before the LLC formation reveals that cooling in the atmospheric boundary layer is decisive to reach saturation, while moisture changes play a minor role. We quantify the heat budget terms and find that about 50 % of the cooling prior to the 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.
Aerosol influences on low-level clouds in the West African monsoon
Atmospheric Chemistry and Physics Discussions
Low-level clouds (LLC) cover a wide area of southern West Africa (SWA) during the summer monsoon months, and have an important cooling effect on the regional climate. Previous studies of these clouds have focused on modelling and remote sensing via satellite. We present the first comprehensive set 1
Atmospheric Chemistry and Physics Discussions, 2019
Nocturnal low-level stratus clouds (LLC) are frequently observed in the atmospheric boundary layer (ABL) over southern West Africa (SWA) during the summer monsoon season. Considering the effect these clouds have on the surface energy and radiation budgets as well as on the diurnal cycle of the ABL, they are undoubtedly important for the regional climate. However, an adequate representation of LLC in the state-of-the-art weather and climate models is still a challenge, which is largely due to the lack of high-quality observations in this region. In several recent studies, a unique and comprehensive data set collected in summer 2016 during the DACCIWA (Dynamics-Aerosol-Cloud-Chemistry Interactions in West Africa) ground-based field campaign was used for the first observational analyses of the parameters and physical processes relevant for the LLC formation over SWA. However, occasionally stratus-free nights occur during the monsoon season as well. Using observations and ERA5 reanalysis, we investigate differences in the boundary layer conditions during 6 stratus-free and 20 stratus nights observed during the DACCIWA campaign. Our results suggest that the interplay between three major mechanisms is crucial for the formation of LLC during the monsoon season: (i) the onset time and strength of the nocturnal low-level jet (NLLJ), (ii) horizontal cold-air advection and (iii) background moisture level. Namely, weaker or later onset of NLLJ leads to reduced contribution from horizontal cold-air advection. This in turn results in a weaker cooling and thus saturation is not reached. Such deviation in the dynamics of NLLJ is related to the arrival of cold air mass propagating northwards from the coast called Gulf of Guinea maritime inflow. Additionally, stratus-free nights occur when the intrusions of dry air masses, originating from e.g. central or south Africa, reduce the background moisture over the large parts of SWA. Based on the backward trajectories analysis, another possible reason for clear nights is descending of air originating from drier levels above the marine boundary layer. 1 Introduction The nocturnal boundary layer (NBL) over southern West Africa (SWA) during the summer monsoon season is typically characterized with low-level stratus clouds (LLC), which cover extensive areas stretching over a region of approximately 800 000 1
Breakup of nocturnal low-level stratiform clouds during southern West African Monsoon Season
2020
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 lowlevel 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 nearneutral 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
2021
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.
The role of low-level clouds in the West African monsoon system
Atmospheric Chemistry and Physics Discussions
Realistically simulating the West African monsoon system still poses a substantial challenge to state-of-the-art weather and climate models. One particular issue is the representation of the extensive and persistent low-level clouds over southern West Africa (SWA) during boreal summer. These clouds are important in regulating the amount of solar radiation reaching the surface but their role in the local energy balance and the overall monsoon system has never been assessed. Based on sensitivity experiments using the ICON model for July 2006, we show for the first time that rainfall over SWA depends logarithmically on the optical thickness of low clouds, as these control the diurnal evolution of the planetary boundary layer, vertical stability and finally convection. In our experiments, the increased precipitation over SWA has small direct effects on the downstream Sahel, as higher temperatures due to increased surface radiation are accompanied by decreases in low-level moisture due to changes in advection, leading to almost unchanged equivalent-potential temperatures in the Sahel. A systematic comparison of simulations with and without convective parameterisation reveals agreement in the direction of the precipitation signal but larger sensitivity for explicit convection. For parametrized convection the main rainband is too far south and the diurnal cycle shows signs of unrealistic vertical mixing, leading to a positive feedback on low clouds. The results demonstrate that relatively minor errors, variations or trends in low-level cloudiness over SWA can have substantial impacts on precipitation. Similarly they suggest that the dimming likely associated with an increase in anthropogenic emissions in the future would lead to a decrease of summer rainfall in the densely populated Guinea Coastal area. Future work should investigate longer-term effects of the misrepresentation of low clouds in climate models, e.g. moderated through effects on rainfall, soil moisture and evaporation. Copyright statement. TEXT 1 Introduction Modelling the West African monsoon (WAM) system is a challenge (Roehrig et al., 2013). Climate and weather models show a considerable inter-model spread when studying for example the influence of sea surface temperatures (SST s) on the WAM