scholarly journals Impact of Potential Large-Scale Irrigation on the West African Monsoon and Its Dependence on Location of Irrigated Area

2014 ◽  
Vol 27 (3) ◽  
pp. 994-1009 ◽  
Author(s):  
Eun-Soon Im ◽  
Marc P. Marcella ◽  
Elfatih A. B. Eltahir
Keyword(s):  
Large Scale ◽  
West African Monsoon ◽  
West African ◽  
The West ◽  
Rainfall Distribution ◽  
Irrigation Area ◽  
Anticyclonic Circulation ◽  
African Monsoon ◽  
Low Level ◽  
The Impact

Abstract This study investigates the impact of potential large-scale irrigation on the West African monsoon using the Massachusetts Institute of Technology regional climate model (MRCM). A new irrigation module is implemented to assess the impact of location and scheduling of irrigation on rainfall distribution over West Africa. A control simulation (without irrigation) and eight sensitivity experiments (with irrigation) are performed and compared to discern the effects of irrigation location and scheduling. It is found that the irrigation effect on soil moisture could force significant changes in spatial distribution and magnitude of rainfall, depending on the latitudinal location of irrigation. In general, the large irrigation-induced surface cooling owing to anomalously wet soil tends to suppress moist convection and rainfall, which in turn induces local subsidence and low-level anticyclonic circulation. These local effects are dominated by a consistent reduction of local rainfall over the irrigated land, irrespective of its location. However, the remote response of rainfall distribution to irrigation exhibits a significant sensitivity to the latitudinal position of irrigation and the intraseasonal variation of supplied irrigation water. The low-level northeasterly airflow associated with an anticyclonic circulation centered over the irrigation area, induced at optimal location and timing, would enhance the extent of low-level convergence areas through interaction with the prevailing monsoon flow, leading to a significant increase in rainfall. As the location of the irrigation area is moved from the coast northward, the regional rainfall change exhibits a significant decrease first, then increases gradually to a maximum corresponding to irrigation centered around 20°N, before it declines again.

scholarly journals Intraseasonal Land–Atmosphere Coupling in the West African Monsoon

2008 ◽  
Vol 21 (24) ◽  
pp. 6636-6648 ◽  
Author(s):  
Christopher M. Taylor
Keyword(s):  
Soil Moisture ◽  
Sensible Heat Flux ◽  
West African Monsoon ◽  
West African ◽  
Surface Fluxes ◽  
Heat Fluxes ◽  
The West ◽  
African Monsoon ◽  
Low Level ◽  
The Impact

Abstract Via its impact on surface fluxes, subseasonal variability in soil moisture has the potential to feed back on regional atmospheric circulations, and thereby rainfall. An understanding of this feedback mechanism in the climate system has been hindered by the lack of observations at an appropriate scale. In this study, passive microwave data at 10.65 GHz from the Tropical Rainfall Measuring Mission satellite are used to identify soil moisture variability during the West African monsoon. A simple model of surface sensible heat flux is developed from these data and is used, alongside atmospheric analyses from the European Centre for Medium-Range Weather Forecasting (ECMWF), to provide a new interpretation of monsoon variability on time scales of the order of 15 days. During active monsoon periods, the data indicate extensive areas of wet soil in the Sahel. The impact of the resulting weak surface heat fluxes is consistent in space and time with low-level variations in atmospheric heating and vorticity, as depicted in the ECMWF analyses. The surface-induced vorticity structure is similar to previously documented intraseasonal variations in the monsoon flow, notably a westward-propagating vortex at low levels. In those earlier studies, the variability in low-level flow was considered to be the critical factor in producing intraseasonal fluctuations in rainfall. The current analysis shows that this vortex can be regarded as an effect of the rainfall (via surface hydrology) as well as a cause.

scholarly journals Internal Intraseasonal Variability of the West African Monsoon in WRF

2017 ◽  
Vol 30 (15) ◽  
pp. 5815-5833 ◽  
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.

scholarly journals The Impact of Aeolus wind observations on the West African Monsoon

2021 ◽  
Author(s):  
Maurus Borne ◽  
Peter Knippertz ◽  
Martin Weissmann ◽  
Michael Rennie ◽  
Alexander Cress
Keyword(s):  
Weather Prediction ◽  
West African Monsoon ◽  
West African ◽  
Boreal Summer ◽  
Horizontal Line ◽  
The West ◽  
Great Opportunity ◽  
African Monsoon ◽  
The Difference ◽  
The Impact

<p>Tropical Africa is characterized by the world-wide largest degree of mesoscale convective organisation. During boreal summer, the wet phase of the West African Monsoon (WAM), the midlevel African easterly jet (AEJ) over the Sahel allows for the formation of synoptic-scale African easterly waves (AEWs) with a maximum intensity close to the West African coast. AEWs interact with convection and its mesoscale organization through modifications in humidity, temperature and vertical wind shear, and often serve as initial disturbances for tropical cyclogenesis. In addition, rainfall can be modulated by other types of tropical waves such as Kelvin or mixed Rossby gravity waves. Upper-tropospheric conditions are dominated by the Tropical Easterly Jet (TEJ), whose variability appears to be connected to convective activity. Overall, our quantitative understanding of the WAM system is still limited. The observational network over the region is sparse and rainfall forecasts with current Numerical Weather Prediction models are hardly better than climatology.</p><p>The Aeolus satellite launched in 2018 offers a great opportunity to further investigate the WAM with an unprecedented density of free-tropospheric wind data. Assimilating Aeolus wind observations in denial experiments using the current operational system of the European Centre for Medium-Range Weather Forecasts (ECMWF) shows that the main circulation features of the WAM are greatly impacted: the AEJ and the TEJ are systematically weaker and stronger respectively by~1m/s in the analysis fields including Aeolus data. As a consequence AEWs also show a weakening in the propagation amplitude. We are currently investigating the contributions of the HLOS (horizontal line-of-sight) Rayleigh and Mie wind observations to these observed differences. Mie observations (i.e., those related to backscatter from hydrometeors and aerosol particles) seems to contribute strongly to the difference in the AEJ, which lies within a convectively active region with a high aerosol load. On the other hand, the difference seen in the TEJ appears to originate mostly in the Rayleigh (i.e., clear air) observations. Surprisingly, the ascending and descending HLOS observations contribute differently to the data impact, possibly revealing a remaining bias or model problems with the diurnal cycle. Future work will include systematic comparisons between the operational systems of DWD and ECMWF to understand the influence of different data assimilation approaches as well as the impact on forecasts.</p>

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

2019 ◽  
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.

scholarly journals Why Do Global Climate Models Struggle to Represent Low-Level Clouds in the West African Summer Monsoon?

2017 ◽  
Vol 30 (5) ◽  
pp. 1665-1687 ◽  
Author(s):  
Lisa Hannak ◽  
Peter Knippertz ◽  
Andreas H. Fink ◽  
Anke Kniffka ◽  
Gregor Pante
Keyword(s):  
Climate Models ◽  
Global Climate ◽  
West African Monsoon ◽  
High Elevation ◽  
West African ◽  
Global Climate Models ◽  
The West ◽  
Near Surface ◽  
Low Level ◽  
The Impact

Abstract Climate models struggle to realistically represent the West African monsoon (WAM), which hinders reliable future projections and the development of adequate adaption measures. Low-level clouds over southern West Africa (5°–10°N, 8°W–8°E) during July–September are an integral part of the WAM through their effect on the surface energy balance and precipitation, but their representation in climate models has received little attention. Here 30 (20) years of output from 18 (8) models participating in phase 5 of the Coupled Model Intercomparison Project (Year of Tropical Convection) are used to identify cloud biases and their causes. Compared to ERA-Interim reanalyses, many models show large biases in low-level cloudiness of both signs and a tendency to too high elevation and too weak diurnal cycles. At the same time, these models tend to have too strong low-level jets, the impact of which is unclear because of concomitant effects on temperature and moisture advection as well as turbulent mixing. Part of the differences between the models and ERA-Interim appear to be related to the different subgrid cloud schemes used. While nighttime tendencies in temperature and humidity are broadly realistic in most models, daytime tendencies show large problems with the vertical transport of heat and moisture. Many models simulate too low near-surface relative humidities, leading to insufficient low cloud cover and abundant solar radiation, and thus a too large diurnal cycle in temperature and relative humidity. In the future, targeted model sensitivity experiments will be needed to test possible feedback mechanisms between low clouds, radiation, boundary layer dynamics, precipitation, and the WAM circulation.

Agricultural impacts of large-scale variability of the West African monsoon

2005 ◽  
Vol 128 (1-2) ◽  
pp. 93-110 ◽  
Author(s):  
Benjamin Sultan ◽  
Christian Baron ◽  
Michael Dingkuhn ◽  
Benoît Sarr ◽  
Serge Janicot
Keyword(s):  
Large Scale ◽  
West African Monsoon ◽  
West African ◽  
The West ◽  
Agricultural Impacts ◽  
African Monsoon
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