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-2018-775

Cited by 9 publications

(47 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Another challenge is the sensitive relationship of deeper clouds with the land surface and the diurnal cycle of the planetary boundary layer (PBL; Couvreux et al ., ) as well as with the widespread occurrence of extensive decks of low‐level clouds (e.g., Schrage and Fink, ; van der Linden et al ., ). During the summer monsoon these clouds typically form at night due to a combination of cold advection, long‐wave radiative cooling and turbulent mixing underneath the nocturnal low‐level jet, and then lift and dissolve in the course of the day (Schuster et al ., ; Adler et al ., ; Dione et al ., ; Adler et al ., ). Hill et al .…”

Section: Introductionmentioning

confidence: 97%

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

View full text Add to dashboard Cite

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.

show abstract

Section: Introductionmentioning

confidence: 97%

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

View full text Add to dashboard Cite

show abstract

“…The first location of the LLC appearance in southern West Africa and the way the LLC extend horizontally during the night vary a lot from one IOP to the other, but confirm that the LLC are not advected from the Guinean coast. Adler et al (2019) and Babić et al (2019a) quantify and confirm the importance of the MI and NLLJ as the main processes for the low layer cooling before the cloud formation.…”

Section: Introductionmentioning

confidence: 67%

“…In the late afternoon, when turbulence vanishes in the CBL, the flow accelerates and the convergence zone moves farther and faster northwards over land (Adler et al, 2017;Deetz et al, 2018). This phenomenon is called the Gulf of Guinea Maritime Inflow (MI) in Adler et al (2019). Schrage and Fink (2012) and Schuster et al (2013) suggested a dominant role played by the NLLJ on the LLC formation over southern West Africa because it intensifies the cold air advection and generates shear-driven turbulent mixing.…”

Section: Introductionmentioning

confidence: 99%

“…Heat budget terms during the different phases are then analyzed to study the processes involved in the LLC formation. Adler et al (2019) generalize this approach to a selection of 11 IOPs out of 15. The first location of the LLC appearance in southern West Africa and the way the LLC extend horizontally during the night vary a lot from one IOP to the other, but confirm that the LLC are not advected from the Guinean coast.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Conceptual model of diurnal cycle of stratiform low-level clouds over southern West Africa

Lohou

Kalthoff

Adler

et al. 2019

Preprint

Self Cite

View full text Add to dashboard Cite

Abstract. DACCIWA (Dynamics Aerosol Chemistry Cloud Interactions in West Africa) project and the associated ground-based field experiment, which took place during the summer 2016, provided a comprehensive dataset on the low-level stratiform clouds (LLC) which develop almost every night over southern West Africa. The LLC, inaccurately represented in the climate and weather forecasts, form in the monsoon flow during the night and break up the day after, affecting considerably the radiation budget. The DACCIWA field experiment dataset supports several published studies which give an overview of the measurements during the campaign, analyze the dynamical features in which the LLC develop, and quantify the processes involved in the LLC formation. Based on the main results of these studies and new analyses, we propose in this paper a conceptual model of the diurnal cycle of the LLC over southern West Africa. Four main phases compose the diurnal cycle of the LLC. The stable and the jet phases are the two steps during which the relative humidity increases, due to the cooling of the air, until the air is saturated and the LLC form. The horizontal advection of cold air from the Guinean coast by the maritime inflow and the nocturnal low level jet (NLLJ) represents 50 % of the total cooling. The remaining half is mainly due to divergence of net radiation and turbulence flux. The third step of the LLC diurnal cycle is the stratus phase which starts during the night and lasts until the onset of buoyancy driven turbulence on the following day. During the stratus phase, interactions between the LLC and NLLJ imply a modification of the wind speed vertical profile in the cloud layer, and a mixing of the subcloud layer by shear-driven turbulence below the NLLJ core. The breakup of the LLC occurs during the convective phase and can follow three different scenarios which depend on the intensity of the shear-driven turbulence observed during the night. The breakup time has a considerable impact on the energy balance of the Earth's surface and, consequently, on the depth of the convective boundary layer, which could vary by a factor of two from day-to-day.

show abstract

“…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

View full text Add to dashboard Cite

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.

show abstract

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

Cited by 9 publications

References 0 publications

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

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

Conceptual model of diurnal cycle of stratiform low-level clouds over southern West Africa

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

Contact Info

Product

Resources

About