A study of  large-scale vertical motion over West Africa

Three separately recorded cases of thundery weather over West Africa that occurred during the conduct of the West African Monsoon Experiment (WAMEX) of 1979, are investigated with the kinematic vertical p-velocity field. The scheme employed here is based on a least-squares-plane technique which has been desribed in Jegede and Balogun (1991), as a variant to the similar methods used by Kung (1972, 1973), and Pedder (1981). The aim in this study is to demonstrate the practicability of the kinematic method for interpreting observed surface weather. In all the three cast-s, there was some consistency noted between the precipitation patterns and the computed vertical velocity fields within the sub-region.

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On What Scale Can We Predict the Agronomic Onset of the West African Monsoon?

Monthly Weather Review ◽

10.1175/mwr-d-15-0274.1 ◽

2016 ◽

Vol 144 (4) ◽

pp. 1571-1589 ◽

Cited By ~ 6

Author(s):

Rory G. J. Fitzpatrick ◽

Caroline L. Bain ◽

Peter Knippertz ◽

John H. Marsham ◽

Douglas J. Parker

Keyword(s):

West Africa ◽

Noise Analysis ◽

West African Monsoon ◽

West African ◽

New Method ◽

The West ◽

African Monsoon ◽

Spatial Consistency ◽

Local Stakeholders ◽

Local Rainfall

Abstract Accurate prediction of the commencement of local rainfall over West Africa can provide vital information for local stakeholders and regional planners. However, in comparison with analysis of the regional onset of the West African monsoon, the spatial variability of the local monsoon onset has not been extensively explored. One of the main reasons behind the lack of local onset forecast analysis is the spatial noisiness of local rainfall. A new method that evaluates the spatial scale at which local onsets are coherent across West Africa is presented. This new method can be thought of as analogous to a regional signal against local noise analysis of onset. This method highlights regions where local onsets exhibit a quantifiable degree of spatial consistency (denoted local onset regions or LORs). It is found that local onsets exhibit a useful amount of spatial agreement, with LORs apparent across the entire studied domain; this is in contrast to previously found results. Identifying local onset regions and understanding their variability can provide important insight into the spatial limit of monsoon predictability. While local onset regions can be found over West Africa, their size is much smaller than the scale found for seasonal rainfall homogeneity. A potential use of local onset regions is presented that shows the link between the annual intertropical front progression and local agronomic onset.

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Time Series of InSAR-Measured Seasonal Surface Deformation Induced by the West African Monsoon in Sudanian West Africa

HydroShare Resources ◽

10.4211/hs.d03e53c0d3eb4c72ad43edc0b309755a ◽

2019 ◽

Keyword(s):

Time Series ◽

West Africa ◽

Surface Deformation ◽

West African Monsoon ◽

West African ◽

The West ◽

African Monsoon

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Internal Intraseasonal Variability of the West African Monsoon in WRF

Journal of Climate ◽

10.1175/jcli-d-16-0750.1 ◽

2017 ◽

Vol 30 (15) ◽

pp. 5815-5833 ◽

Cited By ~ 2

Author(s):

Ghassan J. Alaka ◽

Eric D. Maloney

Keyword(s):

East Africa ◽

Large Scale ◽

Intraseasonal Variability ◽

West African Monsoon ◽

West African ◽

Boreal Summer ◽

Kelvin Wave ◽

The West ◽

African Monsoon ◽

External Influences

The West African monsoon (WAM) and its landmark features, which include African easterly waves (AEWs) and the African easterly jet (AEJ), exhibit significant intraseasonal variability in boreal summer. However, the degree to which this variability is modulated by external large-scale phenomena, such as the Madden–Julian oscillation (MJO), remains unclear. The Weather Research and Forecasting (WRF) Model is employed to diagnose the importance of the MJO and other external influences for the intraseasonal variability of the WAM and associated AEW energetics by removing 30–90-day signals from initial and lateral boundary conditions in sensitivity tests. The WAM produces similar intraseasonal variability in the absence of external influences, indicating that the MJO is not critical to produce WAM variability. In control and sensitivity experiments, AEW precursor signals are similar near the AEJ entrance in East Africa. For example, an eastward extension of the AEJ increases barotropic and baroclinic energy conversions in East Africa prior to a 30–90-day maximum of perturbation kinetic energy in West Africa. The WAM appears to prefer a faster oscillation when MJO forcing is removed, suggesting that the MJO may serve as a pacemaker for intraseasonal oscillations in the WAM. WRF results show that eastward propagating intraseasonal signals (e.g., Kelvin wave fronts) are responsible for this pacing, while the role of westward propagating intraseasonal signals (e.g., MJO-induced Rossby waves) appears to be limited. Mean state biases across the simulations complicate the interpretation of results.

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Rainfall retrievals over West Africa using SEVIRI: evaluation with TRMM-PR and monitoring of the daylight time monsoon progression

Hydrology and Earth System Sciences Discussions ◽

10.5194/hessd-7-6351-2010 ◽

2010 ◽

Vol 7 (4) ◽

pp. 6351-6380 ◽

Cited By ~ 1

Author(s):

E. L. A. Wolters ◽

B. J. J. M. van den Hurk ◽

R. A. Roebeling

Keyword(s):

West Africa ◽

Near Infrared ◽

Rain Rate ◽

West African Monsoon ◽

West African ◽

The West ◽

Cloud Particle ◽

African Monsoon ◽

Trmm Pr ◽

Rain Rates

Abstract. This paper describes the application of the KNMI cloud physical properties – precipitation properties (CPP-PP) algorithm over West Africa. The algorithm combines condensed water path (CWP), cloud phase (CPH), cloud particle effective radius (re), and cloud-top temperature (CTT) information, retrieved from visible, near-infrared and infrared observations of the Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard Meteosat-9 to estimate precipitation occurrence and intensity. It is investigated whether the CPP-PP algorithm is capable of retrieving rain occurrence and intensity over West Africa with a sufficient accuracy, using tropical monsoon measurement mission precipitation radar (TRMM-PR) and a small number of rain gauge observations as reference. As a second goal, it is assessed whether SEVIRI is capable of monitoring both the seasonal and synoptical evolution of the West African monsoon (WAM). It is shown that the SEVIRI-detected rainfall area agrees well with TRMM-PR, having a correlation coefficient of 0.86, with the areal extent of rainfall by SEVIRI being ~10% larger than TRMM-PR. The mean retrieved rain rate from CPP-PP is about 8% higher than from TRMM-PR. The frequency distributions of rain rate reveal that the median rain rates of CPP-PP and TRMM-PR are similar. However, rain rates >7 mm h−1 are retrieved more frequently by SEVIRI than by TRMM-PR, which is partly explained by known biases in TRMM-PR. Finally, it is illustrated that both the seasonal and synoptical time scale of the WAM can be well detected from SEVIRI daytime observations. It was found that the daytime westward MCS travel speed fluctuates between 50 and 60 km h−1. Furthermore, the ratio of MCS precipitation to the total precipitation was estimated to be about 27%. Our results indicate that rainfall retrievals from SEVIRI can be used to monitor the West African monsoon.

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Aerosol influences on low-level clouds in the West African monsoon

10.5194/acp-2019-40 ◽

2019 ◽

Cited By ~ 1

Author(s):

Jonathan W. Taylor ◽

Sophie L. Haslett ◽

Keith Bower ◽

Michael Flynn ◽

Ian Crawford ◽

...

Keyword(s):

West Africa ◽

Southern Hemisphere ◽

Biomass Burning ◽

West African Monsoon ◽

Cloud Microphysics ◽

West African ◽

The West ◽

African Monsoon ◽

Low Level ◽

Local Emissions

Abstract. Low-level clouds (LLC) cover a wide area of southern West Africa (SWA) during the summer monsoon months, and have an important cooling effect on the regional climate. Previous studies of these clouds have focused on modelling and remote sensing via satellite. We present the first comprehensive set of regional, in situ measurements of cloud microphysics, taken during June – July 2016, as part of the DACCIWA (Dynamics-Aerosol-Chemistry-Clouds Interactions in West Africa) campaign, assessing spatial and temporal variation in the properties of these clouds. LLC developed overnight and mean cloud cover peaked a few hundred kilometres inland around 10:00 local solar time (LST), before clouds began to dissipate and convection intensified in the afternoon. Additional sea breeze clouds developed near the coast in the late morning, reaching a maximum extent around 12:00 LST. Regional variation in LLC cover was largely determined by the modulation of the cool maritime inflow by the local orography, with peaks on the upwind side of hills and minima on the leeward sides. In the broad-scale cloud field, no lasting impacts related to anthropogenic aerosol were observed downwind of major population centres. The boundary layer cloud drop number concentration (CDNC) was locally variable inland, ranging from 200 to 840 cm−3 (10th and 90th percentiles at standard temperature and pressure), but showed no systematic regional variations. Enhancements were seen in pollution plumes from the coastal cities, but were not statistically significant across the region. The majority of accumulation mode aerosols, and therefore cloud condensation nuclei, were from ubiquitous biomass burning smoke transported from the southern hemisphere. Consequently, all clouds measured (inland and offshore) had significantly higher CDNC and lower effective radius than clouds over the remote south Atlantic from literature. A parcel model sensitivity analysis showed that doubling or halving local emissions only changed the calculated CDNC by 13–22 %, as the high background meant local emissions were a small fraction of total aerosol. As the population of SWA grows, local emissions are expected to rise. Biomass burning smoke transported from the southern hemisphere is likely to dampen any effect of these increased local emissions on cloud-aerosol interactions. An integrative analysis between local pollution and Central African biomass burning emissions must be considered when predicting anthropogenic impacts on the regional cloud field during the West African monsoon.

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The response of land-atmosphere interactions and the atmospheric circulation across West Africa to intraseasonal variability

10.5194/egusphere-egu21-8413 ◽

2021 ◽

Author(s):

Joshua Talib ◽

Christopher Taylor ◽

Cornelia Klein ◽

Bethan L. Harris ◽

Seonaid R. Anderson ◽

...

Keyword(s):

West Africa ◽

Land Surface ◽

Rainfall Variability ◽

Regional Scale ◽

West African Monsoon ◽

Precipitation Variability ◽

West African ◽

Monsoon Circulation ◽

The West ◽

Surface Response

<p>Across West Africa rain-fed agriculture fulfils approximately 80% of the food needs of the population and employs 60% of the workforce. It is therefore critical to understand the effects of intraseasonal rainfall variability across West Africa. Previous work has shown that land-atmosphere interactions across West Africa can influence daily variability in deep convection characteristics and the impact of 10-25 day precipitation variability. Using earth observations and reanalyses, this study investigates the land surface response to 20-200 day precipitation variability and its impact on land-atmosphere interactions and the West African monsoon.</p><p>&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160;&#160; Surprisingly, even though the sensitivity of the land surface across the Sahel to strong convection is short-lived (days) and daily precipitation patterns are strongly heterogeneous, a coherent regional-scale land surface response to 20-200 day precipitation variability is observed. This sensitivity of the land surface affects land-atmosphere interactions on a regional scale and perturbs the West African monsoon circulation. For example, during sub-seasonal periods of low rainfall, soil moisture significantly decreases across the Sahel and land surface temperatures increase by up to 2&#176;C. Surface drying and warming across the Sahel is associated with an intensified heat low and a northward shift of low-level monsoon westerlies. During periods of high rainfall, the surface moistens and cools, which is associated with a high pressure tendency across the Sahel. This high pressure tendency dampens the heat low circulation across West Africa and reduces regional moisture fluxes. We show that the land surface response to 20-200 day rainfall variability across West Africa can have a significant impact on the monsoon circulation. This suggests that improving the representation of land-surface processes across West Africa has the potential to improve sub-seasonal forecast predictability and enhance early warning systems.</p>

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Soil Moisture Influence on Seasonality and Large-Scale Circulation in Simulations of the West African Monsoon

Journal of Climate ◽

10.1175/jcli-d-15-0877.1 ◽

2017 ◽

Vol 30 (7) ◽

pp. 2295-2317 ◽

Cited By ~ 19

Author(s):

Alexis Berg ◽

Benjamin Lintner ◽

Kirsten Findell ◽

Alessandra Giannini

Keyword(s):

Soil Moisture ◽

Seasonal Cycle ◽

Large Scale ◽

Monsoon Season ◽

West African Monsoon ◽

West African ◽

Monsoon Precipitation ◽

The West ◽

Large Scale Circulation ◽

Soil Moisture Variability

Prior studies have highlighted West Africa as a regional hotspot of land–atmosphere coupling. This study focuses on the large-scale influence of soil moisture variability on the mean circulation and precipitation in the West African monsoon. A suite of six models from the Global Land–Atmosphere Coupling Experiment (GLACE)-CMIP5 is analyzed. In this experiment, model integrations were performed with soil moisture prescribed to a specified climatological seasonal cycle throughout the simulation, which severs the two-way coupling between soil moisture and the atmosphere. Comparison with the control (interactive soil moisture) simulations indicates that mean June–September monsoon precipitation is enhanced when soil moisture is prescribed. However, contrasting behavior is evident over the seasonal cycle of the monsoon, with core monsoon precipitation enhanced with prescribed soil moisture but early-season precipitation reduced, at least in some models. These impacts stem from the enhancement of evapotranspiration at the dry poleward edge of the monsoon throughout the monsoon season, when soil moisture interactivity is suppressed. The early-season decrease in rainfall with prescribed soil moisture is associated with a delayed poleward advancement of the monsoon, which reflects the relative cooling of the continent from enhanced evapotranspiration, and thus a reduced land–ocean thermal contrast, prior to monsoon onset. On the other hand, during the core/late monsoon season, surface evaporative cooling modifies meridional temperature gradients and, through these gradients, alters the large-scale circulation: the midlevel African easterly jet is displaced poleward while the low-level westerlies are enhanced; this enhances precipitation. These results highlight the remote impacts of soil moisture variability on atmospheric circulation and precipitation in West Africa.

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Understanding the Mechanisms behind the Northward Extension of the West African Monsoon during the Mid-Holocene

Journal of Climate ◽

10.1175/jcli-d-16-0299.1 ◽

2017 ◽

Vol 30 (19) ◽

pp. 7621-7642 ◽

Cited By ~ 17

Author(s):

Marco Gaetani ◽

Gabriele Messori ◽

Qiong Zhang ◽

Cyrille Flamant ◽

Francesco S. R. Pausata

Keyword(s):

West Africa ◽

Radiative Forcing ◽

Numerical Models ◽

Substantial Reduction ◽

West African Monsoon ◽

West African ◽

Heat Fluxes ◽

Dust Concentration ◽

The West ◽

African Monsoon

Abstract Understanding the West African monsoon (WAM) dynamics in the mid-Holocene (MH) is a crucial issue in climate modeling, because numerical models typically fail to reproduce the extensive precipitation suggested by proxy evidence. This discrepancy may be largely due to the assumption of both unrealistic land surface cover and atmospheric aerosol concentration. In this study, the MH environment is simulated in numerical experiments by imposing extensive vegetation over the Sahara and the consequent reduction in airborne dust concentration. A dramatic increase in precipitation is simulated across the whole of West Africa, up to the Mediterranean coast. This precipitation response is in better agreement with proxy data, in comparison with the case in which only changes in orbital forcing are considered. Results show a substantial modification of the monsoonal circulation, characterized by an intensification of large-scale deep convection through the entire Sahara, and a weakening and northward shift (~6.5°) of the African easterly jet. The greening of the Sahara also leads to a substantial reduction in the African easterly wave activity and associated precipitation. The reorganization of the regional atmospheric circulation is driven by the vegetation effect on radiative forcing and associated heat fluxes, with the reduction in dust concentration to enhance this response. The results for the WAM in the MH present important implications for understanding future climate scenarios in the region and in teleconnected areas, in the context of projected wetter conditions in West Africa.

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