The observed diurnal cycle of low-level stratus clouds over southern West Africa: a case study

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

doi:10.5194/acp-19-1281-2019

Cited by 22 publications

(51 citation statements)

References 36 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The arrival of the cooler, but not necessarily moister, mass of air more than a 100 km inland facilitates the onset of Sc clouds over land Babić et al, 2019;Dione et al, 2019). The fact that this mass of 10 air is characterized by cloudless conditions when over the sea reveals the importance of the land and other local factors on the cloud formation and maintenance Babić et al, 2019;Lohou et al, 2019). Lohou et al (2019) extended the previous work and summarized the four phases leading from cloud formation to dissipation.…”

mentioning

confidence: 79%

“…To limit the complexity of our idealized experiments and focus on the interaction of the surface and boundary-layer processes, we prescribe no advection of heat or moisture at any height. Adler et al (2019) and Babić et al (2019) found cold air advection necessary for the formation of the cloud layer. Yet 20 its relevance decreased as sunrise approached, thus justifying our assumption during our time of interest.…”

Section: Model Settings and Initial Conditionsmentioning

confidence: 99%

“…If any, they show a humidity increase above cloud layer. Such increase could be related to the previously questioned reliability of radiosonde measurements as they exit the cloud layer through their ascension (Lorenc et al, 1996;Mechem et al, 2010;Babić et al, 2019). Situations without a dry jump above 6 https://doi.org /10.5194/acp-2019-659 Preprint.…”

Section: Model Settings and Initial Conditionsmentioning

confidence: 99%

“…Possible future changes in highly sensitive 5 Sc forcings in SWA, such as anthropogenic regional aerosol increase (Boucher et al, 2013) or the global CO 2 rise (Schneider et al, 2019), further motivate a better understanding of the boundary-layer cloud dynamics over land in SWA.During the monsoon season the intertropical convergence zone shifts northward till 15 • N, facilitating the extension of the maritime air masses inland. The arrival of the cooler, but not necessarily moister, mass of air more than a 100 km inland facilitates the onset of Sc clouds over land Babić et al, 2019;Dione et al, 2019). The fact that this mass of 10 air is characterized by cloudless conditions when over the sea reveals the importance of the land and other local factors on the cloud formation and maintenance Babić et al, 2019;Lohou et al, 2019).…”

mentioning

confidence: 99%

See 3 more Smart Citations

The diurnal stratocumulus-to-cumulus transition over land

Pedruzo-Bagazgoitia

Roode

Adler

et al. 2019

Preprint

Self Cite

View full text Add to dashboard Cite

The misrepresentation of the diurnal cycle of boundary-layer clouds by large scale models strongly impacts the modeled regional energy balance in southern West Africa. In particular, recognizing the processes involved in the maintenance and transition of the nighttime stratocumulus to diurnal shallow cumulus over land remains a challenge. This is due to the fact that over vegetation, surface fluxes exhibit a much larger magnitude and variability than on the more researched marine stratocumulus transitions. An improved understanding of the interactions between surface and atmosphere is thus necessary 5 to improve its representation. To this end, the DACCIWA measurement campaign gathered a unique dataset of observations of the frequent stratocumulus to cumulus transition in southern West Africa. Inspired and constrained by these observations, we perform a series of numerical experiments using Large Eddy Simulation. The experiments include interactive radiation and surface schemes where we explicitly resolve, quantify and describe the physical processes driving such transition. Focusing on the local processes, we quantify the transition in terms of dynamics, radiation, cloud properties, surface processes and 10 the evolution of dynamically relevant layers such as subcloud layer, cloud layer and inversion layer. We further quantify the processes driving the stratocumulus thinning and the subsequent transition initiation by using a liquid water path budget.Finally, we study the impact of mean wind and wind shear at cloud top through two additional numerical experiments. We find that the sequence starts with a nighttime well-mixed layer from surface to cloud top, in terms of temperature and humidity, and transitions to a prototypical convective boundary layer by the afternoon. We identify radiative cooling as the largest factor for 15 the maintenance leading to a net thickening of the cloud layer of about 18 g m −2 h −1 before sunrise. Four hours after sunrise, the cloud layer decouples from the surface through a growing negative buoyancy flux at cloud base. After sunrise, the increasing impact of entrainment leads to a progressive thinning of the cloud layer. While the effect of wind on the stratocumulus layer during nighttime is limited, after sunrise we find shear at cloud top to have the largest impact: the local turbulence generated by shear enhances the boundary layer growth and entrainment aided by the increased surface fluxes. As a consequence wind 20 shear at cloud top accelerates the breakup and transition by about 2 hours. The quantification of the transition and its driving factors presented here sets the path for an improved representation by larger scale models.Stratocumulus (Sc) clouds play a critical role on the radiative balance of the planet given their high albedo (Hartmann et al., 1992;Chen et al., 2000) and extensive cover worldwide . These boundarylayer clouds are a common feature in the southern West Africa (SWA), and recur in the night and morning during the Monsoon season between May and Septem...

show abstract

mentioning

confidence: 79%

Section: Model Settings and Initial Conditionsmentioning

confidence: 99%

Section: Model Settings and Initial Conditionsmentioning

confidence: 99%

mentioning

confidence: 99%

See 2 more Smart Citations

The diurnal stratocumulus-to-cumulus transition over land

Pedruzo-Bagazgoitia

Roode

Adler

et al. 2019

Preprint

Self Cite

View full text Add to dashboard Cite

show abstract

“…Within the Dynamics-aerosol-chemistry-cloud-interactions in West Africa (DACCIWA) project (Knippertz et al, 2015), an extensive field campaign was conducted in June and July 2016 (Flamant et al, 2018;Kalthoff et al, 2018). 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).…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2019

Self Cite

View full text Add to dashboard Cite

Abstract. Nocturnal low-level stratus clouds (LLCs) 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 LLCs 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 and gaps in understanding of underlying processes. In several recent studies, a unique and comprehensive data set collected in summer 2016 during the DACCIWA (Dynamics-aerosol-chemistry-cloud 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 LLCs 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 a reduced contribution from horizontal cold-air advection. This in turn results in weaker cooling, and thus saturation is not reached. Such deviation in the dynamics of the NLLJ is related to the arrival of a 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, for example, central or south Africa, reduce the background moisture over large parts of SWA. Backward-trajectory analysis suggests that another possible reason for clear nights is descending air, which originated from drier levels above the marine boundary layer.

show abstract