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

Dione, Cheikh; Lohou, Fabienne; Adler, Bianca; Babić, Karmen; Kalthoff, Norbert; Pedruzo-Bagazgoitia, Xabier; Bezombes, Y.; Gabella, Omar

doi:10.5194/acp-2018-1149

Cited by 9 publications

(35 citation statements)

References 12 publications

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“…This sensitivity analysis is motivated by the recurrent winds with the shape of low-level jet, such as those in Fig. 1c, that were frequently observed during the DACCIWA campaign Adler et al, 2019;Dione et al, 2019). Failed attempts to maintain a low level jet-like wind profile together with the Sc cloud layer in preliminary experiments suggest that the jet-like wind is the result of large scale dynamics, and, thus, beyond the scope of the present study on local factors.…”

Section: Model Settings and Initial Conditionsmentioning

confidence: 91%

“…Failed attempts to maintain a low level jet-like wind profile together with the Sc cloud layer in preliminary experiments suggest that the jet-like wind is the result of large scale dynamics, and, thus, beyond the scope of the present study on local factors. The large scale 15 origin of the low level wind is also supported by more detailed observational analysis Adler et al, 2019;Dione et al, 2019). Following our idealized approach, the initial wind speed and wind direction are inspired by the observations and adapted to better study how these effects influence the Sc-Cu transition.…”

Section: Model Settings and Initial Conditionsmentioning

confidence: 94%

“…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).…”

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confidence: 99%

“…Finally, during the DACCIWA campaign a recurrent low level jet along the cloud layer was observed Dione et al, 2019), raising an additional question on the effects of wind shear on Sc and its transition d .…”

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confidence: 99%

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The diurnal stratocumulus-to-cumulus transition over land

Pedruzo-Bagazgoitia

Roode

Adler

et al. 2019

Preprint

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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...

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Section: Model Settings and Initial Conditionsmentioning

confidence: 91%

Section: Model Settings and Initial Conditionsmentioning

confidence: 94%

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confidence: 99%

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confidence: 99%

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The diurnal stratocumulus-to-cumulus transition over land

Pedruzo-Bagazgoitia

Roode

Adler

et al. 2019

Preprint

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“…Very few scientific diagnostic studies, particularly those involving the use of a mesoscale model to study the cloud properties composition that produced such a heavy downpour, are available. Research on the cloud types and its composition over this region have been studied during the last two major international field campaign namely; the African Monsoon Multidisciplinary Analyses (AMMA) [8] and Dynamics Aerosol Chemistry Cloud Interactions in West Africa (DACCIWA) [9,10] held in the Sahel and South Western African (SWA) region, respectively. The results from these fields campaign have provided representative information on the identified hotspot of the region which would help to drive global and regional model development in the region.…”

Section: Introductionmentioning

confidence: 99%

An Assessment of the Role of Ice Hydrometeor-Types in WRF Bulk Microphysical Schemes in Simulating Two Heavy Rainfall Events over Southern Nigeria

Akinola

Yin

2019

Atmosphere

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The role of ice hydrometeor-types in bulk schemes available in the Weather Research and Forecasting (WRF) model has been assessed in this study to simulate two heavy rainfall events reported over the southern part of Nigeria. This has been done with a view to provide necessary information on the convective cloud hydrometeor types and compositions in the area to improve heavy rainfall forecasts with the selection of appropriate bulk microphysical schemes. Results from the statistical validation of the simulated rainfall by different schemes showed that for the first event, the WSM5 scheme with less dense snow ice particles performed relatively better than other schemes like WSM6, Morrison with graupel (MORR_G), and Morrison with hail (MORR_H), while the WDM6 scheme performed least effectively when compared to TRMM data. Conversely, the second event showed that a WDM6 scheme with graupel as dense ice particle performed better than other schemes. Further analysis using a spatial distribution plot of simulated rainfall over the area of study shows that for both cases, almost all the schemes fail to capture the intensity and location of the heavy rainfall shown by TRMM data. In addition, the surface accumulated rainfall area average of all schemes for the first (second) event shows an underestimation (overestimation). Vertical profile plots of mass mixing ratios of different ice hydrometeor compositions showed that the WSM5 scheme contains a greater mass of snow than other type ice particles for both cases, while the hydrometeor path calculation of total mass content showed the WSM5 scheme having more snow mass content than other schemes during the period of analysis in both cases. A pressure-time plot of the differences between simulated air temperature and water vapor of the WSM5 scheme showed that WSM5 simulated the higher air temperature that was needed and water vapor at the mid and upper troposphere more than other schemes. In conclusion, results from this study has shown that less dense ice particle (e.g., snow) and high dense ice particle (e.g., graupel and hail) type-bulk schemes can both be suitable for simulating heavy rainfall events that are produced by convective system(s) that are common in the area.

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

Cited by 9 publications

References 12 publications

The diurnal stratocumulus-to-cumulus transition over land

The diurnal stratocumulus-to-cumulus transition over land

An Assessment of the Role of Ice Hydrometeor-Types in WRF Bulk Microphysical Schemes in Simulating Two Heavy Rainfall Events over Southern Nigeria

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

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