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

Abstract

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 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 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 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 clouds form between 22:00 and 06:00 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 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.

Warm Rain in Southern West Africa: A Case Study at Savè

“…At 03:30 UTC, the observed wind speed shows a maximum of more than 6 m s −1 at around 700 m altitude, with a southwesterly direction. This is the signature of the nocturnal low-level jet, with an intensity and core height typical of Savè [34,35]. At 06:00 UTC, the low-level jet has disappeared and southwesterlies of 3 to 7 m s −1 are observed up to around 2 km, corresponding to the monsoon flow.…”

What controls the formation of nocturnal low-level stratus clouds over southern West Africa during the monsoon season?

“…The number of radiosonde profiles available at 18:00 and 00:00 UTC at coastal stations, Abidjan, Accra and Cotonou, and stations inland of coastal strip, Lamto, Savè and Parakou, during stratus and stratus-free nights. ation, which is a result of a combination of days with weak wind and days with strong monsoon flow (Adler et al, 2019;Dione et al, 2019). The large wind speed standard deviations are observed at two other inland stations, Lamto and Parakou, with stronger winds at the northernmost station, Parakou.…”

“…The investigation of large-scale conditions should also elucidate where the air masses, which cause drier conditions during stratus-free nights, come from. Figure 11 shows geopotential height, wind, temperature and specific humidity differences between stratus and stratus-free nights at 18:00 UTC at the 950 hPa isobaric level, which is approximately in the middle of the ABL and also corresponds to the mean stratus layer (e.g., Kalthoff et al, 2018;Dione et al, 2019). Northwest of the DACCIWA region, in the region that corresponds to the Saharan heat low (SHL), lower geopotential heights of the 950 hPa isobaric level are evident for stratus nights, indicating a slightly stronger SHL than on stratus-free nights.…”

“…The aim of the DACCIWA ground-based campaign was to collect a comprehensive and high-quality data set (Kalthoff et al, 2018;Bessardon et al, 2019) which enables studies of LLC characteristics; the day-to-day variability of conditions in the atmo-Published by Copernicus Publications on behalf of the European Geosciences Union. spheric boundary layer (ABL) (Kalthoff et al, 2018;Adler et al, 2019;Dione et al, 2019); and physical processes relevant for formation, maintenance and dissolution of LLCs (Adler et al, 2019;Babić et al, 2019). Characteristics of the nocturnal low-level jet (NLLJ), LLCs, monsoon flow and Gulf of Guinea maritime inflow (or just maritime inflow for brevity), and their interaction during the DACCIWA campaign are analyzed in Dione et al (2019).…”

“…spheric boundary layer (ABL) (Kalthoff et al, 2018;Adler et al, 2019;Dione et al, 2019); and physical processes relevant for formation, maintenance and dissolution of LLCs (Adler et al, 2019;Babić et al, 2019). Characteristics of the nocturnal low-level jet (NLLJ), LLCs, monsoon flow and Gulf of Guinea maritime inflow (or just maritime inflow for brevity), and their interaction during the DACCIWA campaign are analyzed in Dione et al (2019).…”