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

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

doi:10.5194/acp-19-663-2019

Cited by 35 publications

(95 citation statements)

References 28 publications

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“…4). This result is in accordance with the continuous 25 decrease in temperature within the MI from north to the south discussed by Adler et al (2018). An abrupt, large change in temperature at the front passage with constant temperature behind would have implied a very different temporal occurrence of F LF T = 1.…”

supporting

confidence: 87%

“…clouds under focus. We define 'low-level clouds' as clouds with a base height below 1500 m a. g. l.. Adler et al (2018) used a lower altitude threshold, 600 m a. g. l., which is well adapted the nocturnal stratus clouds. However, a 1500 m height limit allows us to extend our detection of LLCs to shallow convection in general and detect not only the nocturnal low base stratus deck but also the rising and fractioning cloud base during the morning growing convective boundary layer.…”

Section: Cloud Monitoringmentioning

confidence: 99%

“…Here, we use the observations of the vertical profiles for the CTH evaluation. After the despiking process, we averaged the reflectivity profiles of hydrometeors over 5 minutes and applied a threshold of -35 dBz to capture the CTH (Adler et al, 2018). Values below this threshold are considered to be related to clear air above the cloud.…”

Section: Cloud Monitoringmentioning

confidence: 99%

“…Values below this threshold are considered to be related to clear air above the cloud. Additional details about the CTH retrieval technique can be found in Babić et al (2018). This 10 algorithm enables a good estimation of the CTH, particularly when the clouds are uniform, which is true in the case of stratus clouds.…”

Section: Cloud Monitoringmentioning

confidence: 99%

“…An abrupt, large change in temperature at the front passage with constant temperature behind would have implied a very different temporal occurrence of F LF T = 1. In addition, the continuous cooling can also be explained by the turbulent mixing under the NLLJ core, which mixes the upper layer with lower layers cooled by radiative and sensible fluxes divergence (Adler et al, 2018;Babić et al, 2018).…”

mentioning

confidence: 99%

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Low Level Cloud and Dynamical Features within the Southern West African Monsoon

Dione

Lohou

Adler

et al. 2018

Preprint

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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 5 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 10 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 15 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.20The 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-to-day variability during the summer. Distributions of strength, depth, onset time, break up time, etc. are quantified here.

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supporting

confidence: 87%

Section: Cloud Monitoringmentioning

confidence: 99%

Section: Cloud Monitoringmentioning

confidence: 99%

Section: Cloud Monitoringmentioning

confidence: 99%

mentioning

confidence: 99%

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Low Level Cloud and Dynamical Features within the Southern West African Monsoon

Dione

Lohou

Adler

et al. 2018

Preprint

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An evaluation of operational and research weather forecasts for southern West Africa using observations from the DACCIWA field campaign in June–July 2016

Kniffka

Knippertz

Fink

et al. 2020

Quart J Royal Meteoro Soc

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Reliable and accurate weather forecasts, particularly those of rainfall and its extremes, have the potential to improve living conditions in densely populated southern West Africa (SWA). The limited availability of observations has long impeded a rigorous evaluation of current state-of-the-art forecast models.The field campaign of the Dynamics-Aerosol-Chemistry-Cloud Interactions in West Africa (DACCIWA) project in June-July 2016 has created an unprecedentedly dense set of measurements from surface stations and radiosondes.Here we present results from a comprehensive evaluation of both numerical model forecasts and satellite products using these data on a regional and local level. Results reveal a substantial observational uncertainty showing considerable underestimations in satellite estimates of rainfall and low-cloud cover with little correlation at the local scale. Models have a dry bias of 0.1-1.9 mm ⋅ day −1 in rainfall and too low column relative humidity. They tend to underestimate low clouds, leading to excess surface solar radiation of 43 W ⋅ m −2 . Remarkably, most models show some skill in representing regional modulations of rainfall related to synoptic-scale disturbances, while local variations in rainfall and cloudiness are hardly captured. Slightly better results are found with respect to temperature and for the post-onset rather than for the pre-onset period. Delicate local features such as the Maritime Inflow phenomenon are also rather poorly represented, leading to too cool, dry and cloudy conditions at the coast. Differences between forecast days 1 and 2 are relatively small and hardly systematic, suggesting a relatively quick error saturation. Using explicit convection leads to more realistic spatial variability in rainfall, but otherwise no marked improvement. Future work should aim at improving the subtle balance between the diurnal This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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The influence of DACCIWA radiosonde data on the quality of ECMWF analyses and forecasts over southern West Africa

Linden

Knippertz

Fink

et al. 2020

Quart J Royal Meteoro Soc

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During the DACCIWA (Dynamics–Aerosol–Chemistry–Cloud Interactions in West Africa) field campaign ∼900 radiosondes were launched from 12 stations in southern West Africa from 15 June to 31 July 2016. Subsequently, data‐denial experiments were conducted using the Integrated Forecasting System of the European Centre for Medium‐range Weather Forecasts (ECMWF) to assess the radiosondes' impact on the quality of analyses and forecasts. As observational reference, satellite‐based estimates of rainfall and outgoing long‐wave radiation (OLR) as well as the radiosonde measurements themselves are used. With regard to the analyses, the additional observations show positive impacts on winds throughout the troposphere and lower stratosphere, while large lower‐tropospheric cold and dry biases are hardly reduced. Nonetheless, downstream, that is farther inland from the radiosonde stations, we find a significant increase (decrease) in low‐level night‐time temperatures (monsoon winds) when incorporating the DACCIWA observations, suggesting a possible linkage via weaker cold air advection from the Gulf of Guinea. The associated lower relative humidity leads to reduced cloud cover in the DACCIWA analysis. Closer to the coast and over Benin and Togo, DACCIWA observations increase low‐level specific humidity and precipitable water, possibly due to changes in advection and vertical mixing. During daytime, differences between the two analyses are generally smaller at low levels. With regard to the forecasts, the impact of the additional observations is lost after a day or less. Moderate improvements occur in low‐level wind and temperature but also in rainfall over the downstream Sahel, while impacts on OLR are ambiguous. The changes in precipitation appear to also affect high‐level cloud cover and the tropical easterly jet. The overall rather small observation impact suggests that model and data assimilation deficits are the main limiting factors for better forecasts in West Africa. The new observations and physical understanding from DACCIWA can hopefully contribute to reducing these issues.

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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

Cited by 35 publications

References 28 publications

Low Level Cloud and Dynamical Features within the Southern West African Monsoon

Low Level Cloud and Dynamical Features within the Southern West African Monsoon

An evaluation of operational and research weather forecasts for southern West Africa using observations from the DACCIWA field campaign in June–July 2016

The influence of DACCIWA radiosonde data on the quality of ECMWF analyses and forecasts over southern West Africa

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