Low-level stratiform clouds and dynamical features observed within the southern West African monsoon

Cheikh Dione*, Fabienne Lohou, Marie Lothon, Bianca Adler, Karmen Babić, Norbert Kalthoff, Xabier Pedruzo-Bagazgoitia, Yannick Bezombes, Omar Gabella

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

2 Citations (Scopus)

Abstract

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. Low-level 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&thinsp;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 ultra-high-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&thinsp;d documented period. Monsoon flow is observed every day during our study period. The so-called "maritime inflow" and the nocturnal low-level jet are also systematic features in this area. According to synoptic atmospheric conditions, the maritime inflow reaches Savè around 18:00-19:00&thinsp;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&thinsp;% of the time with low stratus clouds. Stratus clouds form between 22:00 and 06:00&thinsp;UTC at an elevation close to the nocturnal low-level jet core height. The cloud base height, <span classCombining double low line"inline-formula">310±30</span>&thinsp;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 <span classCombining double low line"inline-formula">640±100</span>&thinsp;m&thinsp;a.g.l., is typically found above the jet core. The nocturnal low-level jet, low-level stratiform clouds, monsoon flow, and maritime inflow reveal significant day-to-day and intra-seasonal variability during the summer given the importance of the different monsoon phases and synoptic atmospheric conditions. Distributions of strength, depth, onset time, breakup time, etc. are quantified here. These results contribute to satisfy the main goals of DACCIWA and allow a conceptual model of the dynamical structures in the lowest troposphere over the southern part of West Africa.

Original languageEnglish
Pages (from-to)8979-8997
Number of pages19
JournalAtmospheric Chemistry and Physics
Volume19
Issue number13
DOIs
Publication statusPublished - 16 Jul 2019

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stratiform cloud
monsoon
stratus
inflow
coast
troposphere
summer
aerosol
atmospheric dynamics
cold air
microwave radiometer
profiler
air mass
energy balance
dissipation

Cite this

Dione, C., Lohou, F., Lothon, M., Adler, B., Babić, K., Kalthoff, N., ... Gabella, O. (2019). Low-level stratiform clouds and dynamical features observed within the southern West African monsoon. Atmospheric Chemistry and Physics, 19(13), 8979-8997. https://doi.org/10.5194/acp-19-8979-2019
Dione, Cheikh ; Lohou, Fabienne ; Lothon, Marie ; Adler, Bianca ; Babić, Karmen ; Kalthoff, Norbert ; Pedruzo-Bagazgoitia, Xabier ; Bezombes, Yannick ; Gabella, Omar. / Low-level stratiform clouds and dynamical features observed within the southern West African monsoon. In: Atmospheric Chemistry and Physics. 2019 ; Vol. 19, No. 13. pp. 8979-8997.
@article{328cbf9220bb48859ca99e44a188ac95,
title = "Low-level stratiform clouds and dynamical features observed within the southern West African monsoon",
abstract = "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. Low-level 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 Sav{\`e}, Benin, roughly 180&thinsp;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 ultra-high-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&thinsp;d documented period. Monsoon flow is observed every day during our study period. The so-called {"}maritime inflow{"} and the nocturnal low-level jet are also systematic features in this area. According to synoptic atmospheric conditions, the maritime inflow reaches Sav{\`e} around 18:00-19:00&thinsp;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 Sav{\`e} site. The low-level jet occurs every night, except during rain events, and is associated 65&thinsp;{\%} of the time with low stratus clouds. Stratus clouds form between 22:00 and 06:00&thinsp;UTC at an elevation close to the nocturnal low-level jet core height. The cloud base height, 310±30&thinsp;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&thinsp;m&thinsp;a.g.l., is typically found above the jet core. The nocturnal low-level jet, low-level stratiform clouds, monsoon flow, and maritime inflow reveal significant day-to-day and intra-seasonal variability during the summer given the importance of the different monsoon phases and synoptic atmospheric conditions. Distributions of strength, depth, onset time, breakup time, etc. are quantified here. These results contribute to satisfy the main goals of DACCIWA and allow a conceptual model of the dynamical structures in the lowest troposphere over the southern part of West Africa.",
author = "Cheikh Dione and Fabienne Lohou and Marie Lothon and Bianca Adler and Karmen Babić and Norbert Kalthoff and Xabier Pedruzo-Bagazgoitia and Yannick Bezombes and Omar Gabella",
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Dione, C, Lohou, F, Lothon, M, Adler, B, Babić, K, Kalthoff, N, Pedruzo-Bagazgoitia, X, Bezombes, Y & Gabella, O 2019, 'Low-level stratiform clouds and dynamical features observed within the southern West African monsoon', Atmospheric Chemistry and Physics, vol. 19, no. 13, pp. 8979-8997. https://doi.org/10.5194/acp-19-8979-2019

Low-level stratiform clouds and dynamical features observed within the southern West African monsoon. / Dione, Cheikh; Lohou, Fabienne; Lothon, Marie; Adler, Bianca; Babić, Karmen; Kalthoff, Norbert; Pedruzo-Bagazgoitia, Xabier; Bezombes, Yannick; Gabella, Omar.

In: Atmospheric Chemistry and Physics, Vol. 19, No. 13, 16.07.2019, p. 8979-8997.

Research output: Contribution to journalArticleAcademicpeer-review

TY - JOUR

T1 - Low-level stratiform clouds and dynamical features observed within the southern West African monsoon

AU - Dione, Cheikh

AU - Lohou, Fabienne

AU - Lothon, Marie

AU - Adler, Bianca

AU - Babić, Karmen

AU - Kalthoff, Norbert

AU - Pedruzo-Bagazgoitia, Xabier

AU - Bezombes, Yannick

AU - Gabella, Omar

PY - 2019/7/16

Y1 - 2019/7/16

N2 - 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. Low-level 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&thinsp;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 ultra-high-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&thinsp;d documented period. Monsoon flow is observed every day during our study period. The so-called "maritime inflow" and the nocturnal low-level jet are also systematic features in this area. According to synoptic atmospheric conditions, the maritime inflow reaches Savè around 18:00-19:00&thinsp;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&thinsp;% of the time with low stratus clouds. Stratus clouds form between 22:00 and 06:00&thinsp;UTC at an elevation close to the nocturnal low-level jet core height. The cloud base height, 310±30&thinsp;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&thinsp;m&thinsp;a.g.l., is typically found above the jet core. The nocturnal low-level jet, low-level stratiform clouds, monsoon flow, and maritime inflow reveal significant day-to-day and intra-seasonal variability during the summer given the importance of the different monsoon phases and synoptic atmospheric conditions. Distributions of strength, depth, onset time, breakup time, etc. are quantified here. These results contribute to satisfy the main goals of DACCIWA and allow a conceptual model of the dynamical structures in the lowest troposphere over the southern part of West Africa.

AB - 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. Low-level 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&thinsp;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 ultra-high-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&thinsp;d documented period. Monsoon flow is observed every day during our study period. The so-called "maritime inflow" and the nocturnal low-level jet are also systematic features in this area. According to synoptic atmospheric conditions, the maritime inflow reaches Savè around 18:00-19:00&thinsp;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&thinsp;% of the time with low stratus clouds. Stratus clouds form between 22:00 and 06:00&thinsp;UTC at an elevation close to the nocturnal low-level jet core height. The cloud base height, 310±30&thinsp;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&thinsp;m&thinsp;a.g.l., is typically found above the jet core. The nocturnal low-level jet, low-level stratiform clouds, monsoon flow, and maritime inflow reveal significant day-to-day and intra-seasonal variability during the summer given the importance of the different monsoon phases and synoptic atmospheric conditions. Distributions of strength, depth, onset time, breakup time, etc. are quantified here. These results contribute to satisfy the main goals of DACCIWA and allow a conceptual model of the dynamical structures in the lowest troposphere over the southern part of West Africa.

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DO - 10.5194/acp-19-8979-2019

M3 - Article

VL - 19

SP - 8979

EP - 8997

JO - Atmospheric Chemistry and Physics

JF - Atmospheric Chemistry and Physics

SN - 1680-7316

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